Vaš brskalnik ne podpira okvirjev. Google za prikaz zadetkov uporablja okvirje. Tu lahko vidite <a href=http://www.rhyshaden.com/ethernet.htm> napotilno stran</a> za to sliko.

|

Web www.rhyshaden.com

|

8

Business Links
Earn in Your Sleep
Earn on the Web
Market on the Web
Global Domains Intnl.
Lose Kgs
Trading Links

Search Now:

Ethernet

Contents

1. Introduction

2. CSMA/CD

3. Ethernet Frame

3.1 Frame Formats
3.2 I/G and U/L
3.3 SNAP

4. Media

4.1 10Base5
4.2 10Base2
4.3 10BaseT
4.4 10BaseF
4.5 Fast Ethernet (802.3u) 100BaseTx
4.6 100BaseT4
4.7 100BaseFx
4.8 100BaseT2
4.9 100 VGAnyLAN
4.10 Gigabit Ethernet

5. Basic Terminology

5.1 Signal Quality Error
5.2 Interpacket Gap
5.3 Promiscuous Mode
5.4 Full Duplex
5.5 Half Duplex
5.6 Propagation Delay

6. Error Conditions

6.1 Runt
6.2 Long
6.3 Giant
6.4 Dribble
6.5 Jabber
6.6 FCS
6.7 Alignment Error
6.8 Broadcast Storm
6.9 Collisions
6.10 Jam

7. Transparent Bridging

8. Spanning Tree

8.1 Loop Problem
8.2 Loop Solution
8.3 Spanning Tree Initialisation
8.4 BPDU
8.5 Spanning Tree Operation
8.6 Using Spanning Tree
8.7 Steady State Operation
8.8 Port Fast
8.9 Uplink Fast
8.10 Issues With Spanning Tree

9. Translational Bridging

10. Encapsulated Bridge

11. Concurrent Routing and Bridging

12. Integrated Routing and Bridging

13. Frame Switching

13.1 Introduction
13.2 Cut Through
13.3 Store and Forward
13.4 Fragment Free

14. VLANs

14.1 Overview
14.2 Switched Network Design
14.3 Trunking Overview
14.4 ISL
14.5 Class of Service and 802.1q Trunking

1. Introduction

Back to the top

Ethernet was originally developed by Digital, Intel and Xerox (DIX) in the early 1970's and has been designed as a 'broadcast' system, i.e. stations on the network can send messages whenever and wherever it wants. All stations may receive the messages, however only the specific station to which the message is directed will respond.

The original format for Ethernet was developed in Xerox Palo Alto Research Centre (PARC), California in 1972. Using Carrier Sense Multiple Access with Collision Detection (CSMA/CD) it had a transmission rate of 2.94Mb/s and could support 256 devices over cable stretching for 1km. The two inventors were Robert Metcalf and David Boggs.

Ethernet versions 1.0 and 2.0 followed until the IEEE 802.3 committee re-jigged the Ethernet II packet to form the Ethernet 802.3 packet. (IEEE's Project 802 was named after the time it was set up, February 1980. It includes 12 committees 802.1 to 802.12, 802.2 is the LLC, 802.4 Token Bus, 802.11 Wireless, 802.12 100VG-AnyLAN etc.) Nowadays you will see either Ethernet II (DIX) (invented by Digital, Intel and Xerox) format or Ethernet 802.3 format being used.

The 'Ether' part of Ethernet denotes that the system is not meant to be restricted for use on only one medium type, copper cables, fibre cables and even radio waves can be used.

802.3 Ethernet uses Manchester Phase Encoding (MPE) for coding the data bits on the outgoing signal. The next few sections describe how Ethernet works and how Ethernet is structured.

2. CSMA/CD

Back to the top

As mentioned earlier, Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD). When an Ethernet station is ready to transmit, it checks for the presence of a signal on the cable i.e. a voltage indicating that another station is transmitting. If no signal is present then the station begins transmission, however if a signal is already present then the station delays transmission until the cable is not in use. If two stations detect an idle cable and at the same time transmit data, then a collision occurs. On a star-wired UTP network, if the transceiver of the sending station detects activity on both its receive and transmit pairs before it has completed transmitting, then it decides that a collision has occurred. On a coaxial system, a collision is detected when the DC signal level on the cable is the same or greater than the combined signal level of the two transmitters, i.e.. significantly greater than +/- 0.85v. Line voltage drops dramatically if two stations transmit at the same and the first station to notice this sends a high voltage jamming signal around the network as a signal. The two stations involved with the collision lay off transmitting again for a time interval which is randomly selected. This is determined using Binary Exponential Backoff. If the collision occurs again then the time interval is doubled, if it happens more than 16 times then an error is reported.

A Collision Domain is that part of the network where each station can 'see' other stations' traffic both unicast and broadcasts. The Collision Domain is made up of one segment of Ethernet coax (with or without repeaters) or a number of UTP shared hubs. A network is segmented with bridges (or microsegmented when using switches) that create two segments, or two Collision Domains where a station on one segment can not see traffic between stations on the other segment unless the packets are destined for itself. It can however still see all broadcasts as a segmented network, no matter the number of segments, is still one Broadcast Domain. Separate Broadcast Domains are created by VLANs on switches so that one physical network can behave as a number of entirely separate LANs such that the only way to allow stations on different VLANs to communicate is at a layer 3 level using a router, just as if the networks were entirely physically separate.

3. Ethernet Frame

Back to the top

3.1 Frame Formats

The diagrams below describe the structure of the older DIX (Ethernet II) and the now standard 802.3 Ethernet frames. The numbers above each field represent the number of bytes.

Preamble field: Establishes bit synchronisation and transceiver conditions so that the PLS circuitry synchs in with the received frame timing. The DIX frame has 8 bytes for the preamble rather than 7, as it does not have a Start Frame Delimiter (or Start of Frame).
Start Frame Delimiter: Sequence 10101011 in a separate field, only in the 802.3 frame.
Destination address: Hardware address (MAC address) of the destination station (usually 48 bits i.e. 6 bytes).
Source address: Hardware address of the source station (must be of the same length as the destination address, the 802.3 standard allows for 2 or 6 byte addresses, although 2 byte addresses are never used, N.B. Ethernet II can only uses 6 byte addresses).
Type: Specifies the protocol sending the packet such as IP or IPX (only applies to DIX frame).
Length: Specifies the length of the data segment, actually the number of LLC data bytes, (only applies to 802.3 frame and replaces the Type field).
Pad: Zeros added to the data field to 'Pad out' a short data field to 46 bytes (only applies to 802.3 frame).
Data: Actual data which is allowed anywhere between 46 to 1500 bytes within one frame.
CRC: Cyclic Redundancy Check to detect errors that occur during transmission (DIX version of FCS).
FCS: Frame Check Sequence to detect errors that occur during transmission (802.3 version of CRC). This 32 bit code has an algorithm applied to it which will give the same result as the other end of the link, provided that the frame was transmitted successfully.

From the above we can deduce that the maximum 802.3 frame size is 1518 bytes and the minimum size is 64 bytes. Packets that have correct CRC's (or FCS's) but are smaller than 64 bytes, are known as 'Runts'.

The hardware address, or MAC address is transmitted and stored in Ethernet network devices in Canonical format i.e. Least significant Bit (LSB) first. You may hear the expression Little-Endian to describe the LSB format in which Ethernet is transmitted. Token Ring and FDDI, on the other hand, transmit the MAC address with the Most Significant Bit (MSB) first, or Big-Endian, This is known as Non-Canonical format. Note that this applies on a byte by byte basis i.e. the bytes are transmitted in the same order it is just the bits in each of those bytes that are reversed! The storage of the MAC addresses in Token Ring and FDDI devices however, may sometimes still be in Canonical format so this can sometimes cause confusion. The reference to, the distribution of MAC addresses and the OUI desinations are always carried out in Canonical format.

Some discussion is warranted on the LLC field. The 802.2 committee developed the Logical Link Control (LLC) to operate with 802.3 Ethernet as seen in the above diagram. LLC is based on the HDLC format and more detail can be found by following the link. Whereas Ethernet II (2.0) combines the MAC and the Data link layers restricting itself to connectionless service in the process, IEEE 802.3 separates out the MAC and Data Link layers. 802.2 (LLC) is also required by Token Ring and FDDI but cannot be used with the Novell 'Raw' format. There are three types of LLC, Type 1 which is connectionless, Type 2 which is connection-oriented and Type 3 for Acknowledged Connections.

The Service Access Point (SAP) is used to distinguish between different data exchanges on the same end station and basically replaces the Type field for the older Ethernet II frame. The Source Service Access Point (SSAP) indicates the service from which the LLC data unit is sent, and the Destination Service Access Point (DSAP) indicates the service to which the LLC data unit is being sent. As examples, NetBIOS uses the SAP address of F0 whilst TCP uses the SAP address of 06. The Control Field identifies the type of LLC, of which there are three:

Type 1 - uses Unsequenced Information (UI) (Indicated by a Control Field value of 03) frames to set up unacknowledged connectionless sessions.
Type 2 - uses Information (I) frames and maintains the sequence numbers during an acknowledged connection-oriented transmission.
Type 3 - uses Acknowledged Connection (AC) frames in an acknowledged connectionless service.

3.2 I/G and U/L within the MAC address

With an Ethernet MAC address, the first octet uses the lowest significant bit as the I/G bit (Individual/Group address) only and does not have such a thing as the U/L bit (Universally/Locally administered). The U/L bit is used in Token Ring A destination Ethernet MAC address starting with the octet '05' is a group or multicast address since the first bit (LSB) to be transmitted is on the right hand side of the octet and is a binary '1'. Conversely, '04' as the first octet indicates that the destination address is an individual address. Of course, in Ethernet, all source address will have a binary '0' since they are always individual.

The first 3 octets of the MAC address form the Organisational Unique Identifier (OUI) assigned to organisations that requires their own group of MAC addresses. A list of OUIs can be found at OUI Index.

3.3 Subnetwork Access Protocol (SNAP)

The SNAP protocol was introduced to allow an easy transition to the new LLC frame format for vendors. SNAP allows older frames and protocols to be encapsulated in a Type 1 LLC header so making any protocol 'pseudo-IEEE compliant'. SNAP is described in RFC 1042. The following diagram shows how it looks:

As you can see, it is an LLC data unit (sometimes called a Logical Protocol Data Unit (LPDU)) of Type 1 (indicated by 03). The DSAP and SSAP are set to AA to indicate that this is a SNAP header coming up. The SNAP header then indicates the vender via the Organisational Unique Identifier (OUI) and the protocol type via the Ethertype field. In the example above we have the OUI as 00-00-00 which means that there is an Ethernet frame, and the Ethertype of 08-00 which indicates IP as the protocol. The official list of types can be found at Ethertypes. More and more vendors are moving to LLC1 on the LAN but SNAP still remains and crops up time and time again.

Have a look at the document IPX for further discussion of 802.3 and 802.5 headers (SNAP etc.) in an IPX environment.

4. Media

Back to the top

4.1 10Base5

Traditionally, Ethernet is used over 'thick' coaxial cable (Normally yellow in colour) called 10Base5 (the '10' denotes 10Mbps, base means that the signal is baseband i.e. takes the whole bandwidth of the cable (so that only one device can transmit at one time on the same cable), and the '5' denotes 500m maximum length). The minimum length between stations is 2.5m.

The cable is run in one long length forming a 'Bus Topology'. Stations attach to it by way of inline N-type connections or a transceiver which is literally screwed into the cable (by way of a 'Vampire Tap') providing a 15-pin AUI (Attachment Unit Interface) connection (also known as a DIX connector or a DB-15 connector) for a drop lead connection (maximum of 50m length) to the station. The segments are terminated with 50 ohm resistors and the shield should be grounded at one end only.

5-4-3 Rule

The segment could be appended with up to a maximum of 4 repeaters, therefore 5 segments (total length of 2,460m) can be connected together. Of the 5 segments only 3 can have devices attached (100 per segment). A total of 300 devices can be attached on a Thicknet broadcast domain.

4.2 10Base2

It was common to see the Thick coax used in Risers to connect Repeaters which in turn provide 'Thin Ethernet' coaxial connections for runs around the floors to up to 30 workstations. Thin ethernet (Thinnet) uses RG-58 cable and is called 10Base2 (The '2' now denoting 200m maximum length, strictly speaking this is 185m). The minimum length between stations is 0.5m. Following is a table detailing various types of coaxial cable:

RG-58 /U - solid copper core (0.66mm or 0.695mm), 53.5 ohms.
RG-58 A/U - stranded copper core (0.66mm or 0.78mm), 50 ohms.
RG-58 C/U - military version of RG58 A/U (0.66mm), 50 ohms.
RG-59 - broadband transmissions e.g. cable TV.
RG-6 - higher frequency broadband transmissions. A larger diameter than RG-59.
RG-62 - ArcNet.
RG-8 - Thicknet, 50 ohms.

Each station connects to the thinnet by way of a Network Interface Card (NIC) which provides a BNC (British Naval Connector). At each station the thinnet terminates at a T-piece and at each end of the thinnet run (or 'Segment') a 50-ohm terminator is required to absorb stray signals, thereby preventing signal bounce. The shield should be grounded at one end only.

A segment can be appended with other segments using up to 4 repeaters, i.e. 5 segments in total. 2 of these segments however, cannot be tapped, they can only be used for extending the length of the broadcast domain (to 925m). What this means is that 3 segments with a maximum of 30 stations on each can give you 90 devices on a Thinnet broadast domain.

(There is also a little used 10Broad36 standard where 10 Mbps Ethernet runs over broadband up to 3.6km. With broadband, a number of devices can transmit at the same time using multiple basebands e.g. multiple TV stations each with its own baseband signal frequency on one wire).

4.3 10BaseT

Nowadays, it is becoming increasingly important to use Ethernet across Unshielded Twisted Pair (UTP) or Shielded Twisted Pair (STP), this being called 10BaseT (the 'T' denoting twisted pair). For instance, Category 5 UTP is installed in a 'Star-wired' format, with runs recommended at no greater than 100m (including patch leads, cable run and flyleads) and Ethernet Hubs with UTP ports (RJ45) centrally located. It has been found though that runs of up to 150m are feasible, the limitations being signal strength. Also, there should be no more than a 11.5dB signal loss and the minimum distance between devices is 2.5m. The maximum delay for the signal in a 10Mbps network is 51.2 microseconds. This comes from the fact that the bit time (time to transmit one bit) is 0.1 microseconds and that the slot time for a frame is 512 bit times.

The wires used in the RJ45 are 1 and 2 for transmit, 3 and 6 for receive.

In order to connect to ethernet in this 'Star Topology', each station again has a NIC which, this time, contains an RJ45 socket which is used by a 4-pair RJ45 plug-ended droplead to connect to a nearby RJ45 floor or wall socket.

Each port on the hub sends a 'Link Beat Signal' which checks the integrity of the cable and devices attached, a flickering LED on the front of the port of the hub tells you that the link is running fine. The maximum number of hubs (or, more strictly speaking, repeater counts) that you can have in one segment is 4 and the maximum number of stations on one broadcast domain is 1024.

The advantages of the UTP/STP technology are gained from the flexibility of the system, with respect to moves, changes, fault finding, reliablity and security.

The following table shows the RJ45 pinouts for 10BaseT:

RJ45 Pin | Function | Colour
1 | Transmit | White/Orange
2 | Transmit | Orange/White
3 | Receive | White/Green
4 | | Blue/White
5 | | White/Blue
6 | Receive | Green/White
7 | | White/Brown
8 | | Brown/White

If you wish to connect hub to hub, or a NIC directly to another NIC, then the following 10BaseT cross-over cable should be used:

The 4 repeater limit manifests itself in 10/100BaseT environments where the active hub/switch port is in fact a repeater, hence the name multi-port repeater. Generally, the hub would only have one station per port but you can cascade hubs from one another up to the 4 repeater limit. The danger here of course, is that you will have all the traffic from a particular hub being fed into one port so care would need to be taken on noting the applications being used by the stations involved, and the likely bandwidth that the applications will use.

There is a semi-standard called Lattisnet (developed by Synoptics) which runs 10MHz Ethernet over twisted pair but instead of bit synchronisation occurring at the sending (as in 10BaseT) the synchronisation occurs at the receiving end.

4.4 10BaseF

The 10BaseF standard developed by the IEEE 802.3 committee defines the use of fibre for ethernet. 10BaseFB allows up to 2km per segment (on multi-mode fibre) and is designed for backbone applications such as cascading repeaters. 10BaseFL describes the standards for the fibre optic links between stations and repeaters, again allowing up to 2km per segment on multi-mode fibre. In addition, there is the 10BaseFP (Passive components) standard and the FOIRL (Fibre Optic Inter-Repeater Link) which provides the specification for a fibre optic MAU (Media Attachment Unit) and other interconnecting components.

The 10BaseF standard allows for 1024 devices per network.

4.5 Fast Ethernet (802.3u) 100BaseTx

Fast Ethernet uses the same frame formats and CSMA/CD technology as normal 10Mbps Ethernet. The difference is that the maximum delay for the signal across the segment is now 5.12 microseconds instead of 51.2 microseconds. This comes from the fact that the bit time (time to transmit one bit) is 0.01 microseconds and that the slot time for a frame is 512 bit times. The Inter-Packet Gap (IPG) for 802.3u is 0.96 microseconds as opposed to 9.6 microseconds for 10Mbps Ethernet.

Fast Ethernet is the most popular of the newer standards and is an extension to 10BaseT, using CSMA/CD. The '100' denotes 100Mbps data speed and it uses the same two pairs as 10BaseT (1 and 2 for transmit, 3 and 6 for receive) and must only be used on Category 5 UTP cable installations with provision for it to be used on Type 1 STP. The Copper physical layer being based on the Twisted Pair-Physical Medium Dependent (TP-PMD) developed by ANSI X3T9.5 committee. The actual data throughput increases by between 3 to 4 times that of 10BaseT.

Whereas 10BaseT uses Normal Link Pulses (NLP) for testing the integrity of the connection, 100BaseT uses Fast Link Pulses (FLP) which are backwardly compatible with NLPs but contain more information. FLPs are used to detect the speed of the network (e.g. in 10/100 switchable cards and ports).

The ten-fold increase in speed is achieved by reducing the time it takes to transmit a bit to a tenth that of 10BaseT. The slot-time is the time it takes to transmit 512 bits on 10Mbps Ethernet (i.e. 5.12 microseconds) and listen for a collision (see earlier). This remains the same for 100BaseT, but the network distance between nodes, or span, is reduced. The encoding used is 4B/5B with MLT-3 wave shaping plus FSR. This wave-shaping takes the clock frequency of 125MHz and reduces it to 31.25MHz which is the frequency of the carrier on the wire.

The round trip signal timing is the critical factor when it comes to the distance that the signal can run on copper UTP. The cable has to be Category 5 and the distance must not exceed 100m.

The IEEE use the term 100BaseX to refer to both 100BaseTx and 100BaseFx and the Media-Independent Interface (MII) allows a generic connector for transceivers to connect to 100BaseTx, 100BaseFx and 100BaseT4 LANs.

There is no such thing as the 5-4-3 rule in Fast Ethernet. All 10Base-T repeaters are considered to be functionally identical. Fast Ethernet repeaters are divided into two classes of repeater, Class I and Class II. A Class I repeater has a repeater propagation delay value of 140 bit times, whilst a Class II repeater is 92 bit times. The Class I repeater (or Translational Repeater) can support different signalling types such as 100BaseTx and 100BaseT4. A Class I repeater transmits or repeats the incoming line signals on one port to the other ports by first translating them to digital signals and then retranslating them to line signals. The translations are necessary when connecting different physical media (media conforming to more than one physical layer specification) to the same collision domain. Any repeater with an MII port would be a Class I device. Only one Class I repeater can exist within a single collision domain, so this type of repeater cannot be cascaded. There is only allowed one Class I repeater hop in any one segment.

A Class II repeater immediately transmits or repeats the incoming line signals on one port to the other ports: it does not perform any translations. This repeater type connects identical media to the same collision domain (for example, TX to TX). At most, two Class II repeaters can exist within a single collision domain. The cable used to cascade the two devices is called and unpopulated segment or IRL (Inter-Repeater Link). The Class II repeater (or Transparent Repeater) can only support one type of physical signalling, however you can have two Class II repeater hops in any one segment (Collision Domain).

4.6 100BaseT4

100BaseT4 uses all four pairs and is designed to be used on Category 3 cable installations. Transmit is on pairs 1 and 2, receive is on pairs 3 and 6, whilst data is bidirectional on 4 and 5 and on 7 and 8. The signaling is on three pairs at 25MHz each using 8B/6T encoding. The fourth pair is used for collision detection. Half-Duplex is supported on 100BaseT4.

4.7 100BaseFx

100BaseFx uses two cores of fibre (multi-mode 50/125um, 60/125um or single-mode) and 1300nm wavelength optics. The connectors are SC, Straight Tip (ST) or Media Independent Connector (MIC). The 100BaseT MAC mates with the ANSI X3T9.5 FDDI Physical Medium Dependent (PMD) specification. At half-duplex you can have distances up to 412m, whereas Full-duplex will give 2km.

There is also a proposed 100BaseSx which uses 850nm wavelength optics giving 300m on multi-mode fibre.

The encoding used is 4B/5B with NRZ-I wave shaping with a clock frequency of 125MHz.

4.8 100BaseT2

This little known version of Fast Ethernet is for use over two pairs of Category 3 cable and uses PAM-5 for encoding. There is simultaneous transmission and reception of data in both pairs and the electronics uses DSP technology to handle alien signals in adjacent pairs.

100BaseT2 can run up to 100m on Category 3 UTP.

4.9 100VG-AnyLAN

Based on 802.12 (Hewlett Packard), 100VG-AnyLAN uses an access method called Demand Priority. The 'VG' stands for 'Voice Grade' as it is designed to be used with Category 3 cable. This is where the repeaters (hubs) carry out continuous searches round all of the nodes for those that wish to send data. If two devices cause a 'contention' by wanting to send at the same time, the highest priority request is dealt with first, unless the priorities are the same, in which case both requests are dealt with at the same time (by alternating frames). The hub only knows about connected devices and other repeaters so communication is only directed at them rather than broadcast to every device in the broadcast domain (which could mean 100's of devices!). This is a more efficient use of the bandwidth. This is the reason why a new standard was developed called 802.12 as it is not strictly Ethernet. In fact 802.12 is designed to better support both Ethernet and Token Ring.

The encoding techniques used are 5B/6B and NRZ.

All four pairs of UTP are used. On Cat3 the longest cable run is 100m but this increases to 200m on Cat5.

The clock rate on each wire is 30MHz, therefore 30Mbits per second are transmitted on each pair giving a total data rate of 120Mbits/sec. Since each 6-bits of data on the line represents 5 bits of real data due to the 5B/6B encoding, the rate of real data being transmitted is 25Mbits/sec on each pair, giving a total rate of real data of 100Mbits/sec. For 2-pair STP and fiber, the data rate is 120Mbits/sec on the transmitting pair, for a real data transmission rate of 100Mbits/sec.

4.10 Gigabit Ethernet

Although the functional principles of Gigabit Ethernet are the same as Ethernet and Fast Ethernet i.e. CSMA/CD and the Framing format, the physical outworking is very different. One difference is the slot time. The standard Ethernet slot time required in CSMA/CD half-duplex mode is not long enough for running over 100m of copper, so Carrier Extension is used to guarantee a 512-bit slot time.

1000BaseX (802.3z)

802.3z is the committee responsible for formalising the standard for Gigabit Ethernet. The 1000 refers to 1Gb/s data speed. The existing Fibre Channel interface standard (ANSI X3T11) is used and allows up to 4.268Gbps speeds. The Fibre Channel encoding scheme is 8B/10B.

Gigabit Ethernet can operate in half or full duplex modes and there is also a standard 802.3x which manages XON/XOFF flow control in full duplex mode. With 802.3x, a receiving station can send a packet to a sending station to stop it sending data until a specified time interval has passed.

There are three media types for 1000BaseX. 1000BaseLX, 1000BaseSX and 1000BaseCX.

With 1000BaseSX, 'S' is for Short Haul, and this uses short-wavelength laser (850nm) over multi-mode fibre. 1000BaseSX can run up to 300m on 62.5/125um multimode fibre and up to 550m on 50/125um multimode fibre.

Using 1300nm wavelength, Gigabit Ethernet (1000BaseLX where the 'L' is for Long wavelength laser, or Long Haul) can run up to 550m on 62.5/125um multi-mode fibre or 50/125um multi-mode fibre. In addition, 1000BaseLX can run up to 5km (originally 3km) on single-mode fibre using 1310nm wavelength laser.

1000BaseCX is a standard for STP copper cable and allows Gigabit Ethernet to run up to 25m over STP cable.

There is currently an issue as many multimode fibre installations using 62.5/125um fibre and so 220m is often the limit for the backbone when it should be 500m to satisfy ISO 11801 and EIA/TIA 568A.

1000BaseT (802.3ab)

Many cable manufacturers are enhancing their cable systems to 'enhanced Category 5' standards in order to allow Gigabit Ethernet to run at up to 100m on copper. The Category 6 standard has yet to be ratified, and is not likely to be due for a while.

In order to obtain the 1000Mbps data bit rate across the UTP cable without breaking the FCC rules for emission, all 4 pairs of the cable are used. Hybrid circuits at each end of each pair are used to allow simultaneous transmission and reception of data (full-duplex) by separating the transmission signal from the receiving signal. Because some transmission signal still manages to couple itself to the receiving side there is an additional echo canceller built in, this is called a NEXT canceller. This system minimises the symbol rate.

Encoding is carried out with PAM-5.

5. Basic Terminology

Back to the top

5.1 Signal Quality Error (SQE)

The SQE test or 'heartbeat' is a test signal generated on the cable after every transmission to assess the ability of the transceiver to detect collisions. The test is a very short frame that is too short to look like a collision. Ethernet 1.0 did not have this in its standard and 802.3 says that repeaters must not connect to a transceiver that generates the SQE test because of the Jam signal that is designed to prevent redundant collisions from occurring. The option is normally available to turn off SQE test for this reason.

5.2 InterPacket Gap (IPG)

The IPG is the fixed time gap between Ethernet Frames. For 802.3 (10Mbps Ethernet) This is set at 9.6 micro seconds. Sometimes this is called the Inter-Frame Gap (IFG).

5.3 Promiscuous Mode

This mode is used by special network adaptors used in devices such as network analysers and transparent bridges. What happens is that the network controller passes ALL frames up to the upper layers regardless of destination address. Normally the frames are only passed up if they have that particular device's address, the destination address is checked and if it does not match that of the adapter then the rest of the frame is ignored. Network Analysers are interested in seeing all frames, regardless of the destination address so special adapters can be installed that run in Promiscuous mode and allow all frames to be sent to the buffer for capture and analysis.

5.4 Full-Duplex

Ethernet can exist between switch ports only and uses one pair of wires for transmit and one pair for receive. NICs for 10BaseT, 10BaseFL, 100BaseFX and 100BaseT have circuitry within them that allows full-duplex operation and bypasses the normal loopback and CSMA/CD circuitry. Collision detection is not required as the signals are only ever going one way on a pair of wires. In addition, Congestion Control is turned on which 'jams' further data frames on the receive buffer filling up.

5.5 Half-Duplex

Half-Duplex allows data to travel in only one direction at a time. Both stations use CSMA/CD to contend the right to send data. In a Twisted Pair environment when a station is transmitting, its transmit pair is active and when the station is not transmitting it's receive pair is active listening for collisions.

5.6 Propagation Delay

Propagation Delay, or Latency, is the time taken for a frame to traverse the media from the sending station to the receiving station. A 64 byte frame takes 51.2 microseconds to travel between stations, a 512 byte frame takes 410 microseconds and a 1518 byte frame takes 1214 microseconds, provided that there are no other devices between the stations. This marries with the fact that 10,000 bits traverse the network in 1 second. A bridge would typically add 300 microseconds to the latency to the network.

The Path Delay Value is the time it takes an Ethernet frame to travel the furthest distance across the network. It is made up of the sum of the Link Segment Delay Values (LSDV) plus the repeater and DTE delays and maybe some safety margin.

6. Error Conditions

Back to the top

6.1 Runt

A Runt is a frame that is shorter than 64 bytes (512 bits), which is the smallest allowable frame, and may have a corrupted FCS. It can be caused by a collision, dodgy software or a faulty port/NIC.

6.2 Long

This is a frame that is between 1518 and 6000 bytes long. Normally it is due to faulty hardware or software on the sending station.

6.3 Giant

This is a frame that is more than 6000 bytes long. Normally it is due to faulty hardware or software on the sending station.

6.4 Dribble

A frame that is defined as a 'dribble' is one that is greater than 1518 bytes but can still be processed. This could point to a problem where the IPG is too small or non-existent such that two frames join together.

6.5 Jabber

This is when a device is having problems electrically. Ethernet relies on electrical signalling to determine whether or not to send data, so a faulty card could stop all traffic on a network as it sends false signals causing other devices to think that the network is busy. This shows itself as a long frame (longer than 1518 bytes) with an incorrect FCS or is an alignment error. A NIC that is jabbering will send out a frame and then follow it with A's and 5's, i.e. 101010101010... or 0101010101..., which are preamble bits indicating a falsely busy network.

6.6 Frame Check Sequence (FCS) Error, or CRC error

This defines a frame which may or may not have the right number of bits but they have been corrupted between the sender and receiver, perhaps due to interference on the cable. The IEEE 802.3 says that there should be no more than 10-8 errors, i.e. 1 in 82 x 106.

6.7 Alignment Error

Frames are made up of a whole number of octets. If a frame arrives with part of an octet missing, and it has a Frame Check Sequence (FCS) error, then it is deemed to be an Alignment Error. This points to a hardware problem, perhaps EMF on the cable run between sender and receiver.

6.8 Broadcast Storm

An incorrect packet broadcast onto a network that causes multiple stations to respond all at once, typically with equally incorrect packets which causes the storm to grow exponentially in severity. When this happens there are too many broadcast frames for any data to be able to be processed. Broadcast frames have to be processed first by a NIC above any other frames. The NIC filters out unicast packets not destined for the host but multicasts and broadcasts are sent to the processor. If the broadcasts number 126 per second or above then this is deemed to be a broadcast storm. An acceptable level of broadcasts is often deemed to be less than 20% of received packets although many networks survive well enough on higher levels than this. The performance lower-specified workstations may be impacted by as little as 100 broadcasts/second. Some broadcast/multicast applications such as video conferencing and stock market data feeds can issue more than 1000 broadcasts/sec.

6.9 Collisions

Collisions are a normal occurrence on an Ethernet network. The more devices there are within a segment (Collision Domain) the more collisions are likely. A badly cabled infrastructure can cause unnecessary collisions due to a device being unable to sense a carrier and transmitting anyway.

If a collision rate is high then it may be worth while considering segmenting the network by way of a bridge or router. This reduces the chance of a collision occurring on each of the segment thereby releasing more bandwidth for real traffic.

A good guide to use is collisions should not total more than 1% of the frames transmitted. The following table gives a guide as to recommendations of collision levels with respect to bandwidth utilisation of a segment:

%age Utilisation | Maximum %age Collisions
less than 20 | 1
20-49 | 5
over 50 | 15

A Late Collision occurs when two devices transmit at the same time without detecting a collision. This could be because the cabling is badly installed (e.g. too long) or there are too many repeaters. If the time to send the signal from one end of the network to the other is longer than it takes to put the whole frame on to the network then neither device will see that the other device is transmitting until it is too late. The transmitting station distinguishes between a normal and a late collision by virtue that a late collision is detected after the time it takes to transmit 64 bytes. This means that a late collision can only be detected with frames of greater size than 64 bytes, they still occur for smaller frames but remain undetected and still take up bandwidth. Frames lost through late collisions are not retransmitted.

Excessive Collisions describe the situation where a station has tried 16 times to transmit without success and discards the frame. This means that there is excessive traffic on the network and this must be reduced.

For normal Ethernet traffic levels, a good guideline is if the number of deferred transmissions and retransmissions together make up for less than 5% of network traffic, then that is considered healthy.

A transmitting station should see no more than two collisions before transmitting a frame.

6.10 Jam

On detection of a collision, the NIC sends out a Jam signal to let the other stations know that a collision has occurred. A repeater, on seeing a collision on a particular port, will send a jam on all other ports causing collisions and making all the stations wait before transmitting. A station must see the jam signal before it finishes transmitting the frame, otherwise it will assume that another station is the cause of the collision.

Jamming is a term used to describe the collisions reinforcement signal output by the hub/repeater to all ports. The Jam signal consists of 96 bits of alternating 1s and 0s. The purpose is to extend a collision sufficently so that all devices cease transmitting.

Jamming is used when dealing with congestion. It is an to attempt to eliminate frame loss within the switch by applying "back pressure" to those end stations or segments that are consuming the switch buffer capacity. One way of accomplishing this is for the switch to issue an Ethernet "jam" signal when buffers fill beyond a design threshold level. Jam signals normally are the result of collision detection. When the sending end systems on the segment receive the jamming signal, they will back off for a random time period before attempting a retransmission.

Each transmitting node monitors its own transmission, and if it observes a collision (i.e. excess current above what it is generating, i.e. > 24 mA) it stops transmission immediately and instead transmits a 32-bit jam sequence. The purpose of this sequence is to ensure that any other node which may currently be receiving this frame will receive the jam signal in place of the correct 32-bit MAC CRC, this causes the other receivers to discard the frame due to a CRC error.

7. Transparent Bridging

Back to the top

A Bridge connects distinct segments allowing traffic to pass between the segments. The maximum size of the network can be extended both in distance, repeater and station count. A bridge can be used to split up a large segment into two smaller ones. The benefit of this is that there are less chances for collisions on the smaller segments leaving more bandwidth for real data. There are some protocols such as Local Area Transport (LAT), Maintenance Operation Protocol (MOP) and native NetBIOS that do not have any network layer addressing and so these protocols cannot be routed across a large network, they have to be bridged. There are implementations of NetBIOS that run over IP and IPX thereby allowing NetBIOS to be routed in that way.

A Transparent or Learning Bridge learns the MAC addresses of the stations on all the attached segments since it receives and examines every frame transmitted on the attached networks. This is called Promiscuous Mode and the ports of the bridge operate in Learning Mode as they build the Forwarding Tables. This source address list is called the Forwarding Table. If a frame arrives at a bridge port destined for a station connected to the same port, then the bridge does not forward that frame out of any port. If the frame's destination address is held in the forwarding table then the bridge knows on which port the destination device is connected and the bridge forwards the frame out of that port only. If the destination is unknown, then the bridge forwards the frame out of all the ports, that is it Floods the frame out of all ports except the one that the frame arrived on. This whole process is known as Transparent Bridging because the bridges are 'transparent' to the stations, they just see one large segment, the bridge's MAC address is invisible to the frames and the frames are never altered in any way.

These forwarding tables can grow very large so some manufacturers apply a process called Aging whereby the oldest addresses (typically 300 seconds) are removed to free up memory.

Transparent bridges connect LANs that use the same protocols at the physical and data link layer and they do all the work with regards to tracking which station sits on which network. There is no route discovery process or route selection process with transparent bridges.

A problem with the bridge is that it adds 20 to 30% latency in a network for acknowledgement-oriented protocols or 10 to 20% for sliding window type protocols, plus it relies on redistributing broadcasts and multicasts from a particular segment to all other segments (since the source address for broadcasts does not exist and is not in the forwarding table). These segments therefore see more broadcasts than they would if the segments were totally isolated. There is therefore a greater risk of broadcast storms occurring.

A Remote Bridge has a LAN interface on one side and a WAN interface on the other. Another remote bridge at the other end of the WAN completes the connection. This allows WAN connections without having to use layer 3 devices (routers).

8. Spanning Tree (802.1d)

Back to the top

Spanning Tree applies to both bridged networks and switched networks, so bear this in mind as a switched network is basically made up of multiple LAN segments (Collision Domains). Consider the following network:

8.1 Loop Problem

In the above setup (without Spanning Tree), there is a possibility that a bridging loop can occur as Sonny tries to talk to Jim for the first time. The following events occur in the communication process:

A frame from Sonny arrives at interface X on both Bridge A and B. They both learn Sonny's MAC address and add it to their forwarding tables.
Neither bridge yet knows about Jim so Bridge A floods the frame out of interface Y and Bridge B floods the interface Y. These bridges would also flood the frame out of other interfaces if they had any.
Bridge C receives the frame on interface Y and creates an entry for Sonny in its forwarding table to point to interface Y.
Bridge C does not yet know about Jim so it floods the frame out on interface X.
Bridge C also learns about Sonny from interface X since it learnt this from Bridge A.
Jim receives the same frame from both Bridge B and Bridge C.
When Jim sends a frame to Sonny, Bridge B knows to send anything to Sonny via it's interface X and Bridge C knows to send anything to Sonny out on it's interface X, oh dear! What about interface Y?
Oops! A loop has occurred! Bridge C will keep learning about Sonny on both interfaces X and Y, sometimes X, sometimes Y, depending on the latency of the LANs in between. This continual flooding results in broadcast storms which create an unneccesary heavy load on the bridges or switches and the network becomes extremely sluggish, if it is operating at all.

This sequence of events is based on the operation of Transparent Bridging i.e. broadcasts and unknown unicasts are forwarded out of all ports, bar the incoming port.

8.2 Loop Solution

To get around this problem, within Spanning Tree, only one bridge/switch is allowed to be the Root Bridge. All traffic for the whole Spanning Tree network of bridged/switched LANS goes through the root bridge. In addition, only one link is allowed between devices, and on each LAN segment, only one device becomes the Designated Bridge, through which ALL that particular LANs traffic must go. The other links remain dormant. The designated bridge has a port that leads to the root bridge of the bridged network of multiple LAN segments. The designated bridge is determined by the bridge with the smallest path cost to the root bridge. The path cost is determined by the cumulative port costs on the path to the root bridge. A 100Mbps port has a cost of 19 for example.

There are three main steps that Spanning Tree takes when building the loop free Spanning Tree network:

Root bridge election for the whole network.
Root port election on each bridge/switch.
Designated port election on each LAN segment.

8.3 Spanning Tree Initialisation

On initial setup, all participating bridges declare themselves to be the root, they then exchange Hello (or Configuration) Bridge Protocol Data Units (BPDU) containing specific bridge information. The BPDUs are sent to the well-known multicast address 0x0180c2000000. There are two types of BPDUs, Configuration BPDUs used for bridge and port elections, and there are Topology Change Notification (TCN) BPDUs which are sent back up towards the root when there is a topology change. BPDUs are sent to a well-known multicast address.

8.4 Bridge Protocol Data Unit (BPDU)

These BPDUs are in the following format:

Protocol ID - indicates that the packet is a BPDU.
Version - the version of the BPDU being used.
Message Type - the stage of the negotiation.
Flags - two bits are used to indicate a change in topology and to indicate acknowledgement of the TCN BPDU.
Root ID - the root bridge priority (2 bytes) followed by the MAC address (6 bytes).
Root Path Cost - the total cost to from this particular bridge to the designated root bridge.
Bridge ID - the bridge priority (2 bytes) followed by the MAC address (6 bytes), lowest value wins! The default bridge priority is 0x8000 (3276810).
Port ID - the ID of the port from which are transmitted the BPDUs, a root port, this is made up of the configured port priority and the bridge MAC address.
Message Age - timers for aging messages (only has effect on the network if the root bridge is configured with this parameter).
Maximum Age - the maximum message age before information from a BPDU is dropped because it is too old and no more BPDUs have been received. (only has effect on the network if the root bridge is configured with this parameter). The default value for this is 20 seconds.
Hello Time - the time between BPDU configuration messages sent by the root bridge (only has effect on the network if the root bridge is configured with this parameter). The default value for this is 2 seconds.
Forward Delay - this temporarily stops a bridge from forwarding data to give a chance for information of a topology change to filter through to all parts of the network. This means that ports that need to be turned off in the new topology have a chance to be switched off before the new ports are turned on (only has effect on the network if the root bridge is configured with this parameter). The default value for this is 15 seconds.

The timers are based on the assumption that the diameter of the network is no more than 7 bridges/switches away from the root (including the root bridge).

The following port costs are the current IEEE defaults:

10Gbps : 1
1Gbps : 4
622Mbps : 6
155Mbps : 14
100Mbps : 19
45Mbps : 39
16Mbps : 62
10Mbps : 100
4Mbps : 250

8.5 Spanning Tree Operation

Whilst the BPDUs move between links on the network, the bridge ports are in Listening Mode as the bridges listen to the BPDUs and decide which is the root bridge, and which bridges are the designated bridges. When a bridge sees a BPDU from a bridge with a lower Bridge ID it stops sending its own BPDUs. Eventually, one of the bridges is determined as the Root Bridge, it is the one with the lowest ID value (the bridge priority is user-defined and is looked at first, followed by the MAC address).

Once the the root bridge has been determined, all the other bridges work out a least cost path to the root bridge. The root bridge sends a BPDU with an initial value for the Root Path Cost of zero. Each bridge receives the BPDU and adds the cost of the particular interface on which the BPDU was received to the Root Path Cost and sends its own generated BPDU out of all its interfaces. Fast Ethernet ports default to a port cost of 19. The cumulative Root Path Cost is used to determine which interface a particular bridge received the lowest cost BPDU and this interface becomes the Root Port.

Then the other bridges designated to be part of the tree, determine whether any ports are not root ports. These non-designated ports are then placed into the following conditions in order:

Blocking Mode - where no frames are forwarded and just Hello BPDUs are listened to, this lasts for 20 seconds, which is the Maximum Age Time.
Then ports change from Blocking Mode to Listening Mode where they remain for 15 seconds (this is called the Forward Delay Time), where no data frames are forwarded, they are just listened to, BPDUs however, are both received AND sent.
Then the ports change to Learning Mode for another 15 seconds (again using the Forward Delay time), where still no data frames are forwarded, and addresses are being learned as the bridge/switch builds the forwarding tables.
Finally, the designated ports move to Forwarding Mode, where data frames are forwarded now as well as addresses still being learned.

After this process, if a port fails, then it is placed into a Disabled State.

From this sequence, it can be seen that the worst case scenario is if a link is up but it fails to see any more BPDUs, there will be a 20 second wait before the information from the last BPDU ages out. At this point, the port goes into listening mode for 15 seconds and then learning mode for 15 seconds before it starts forwarding data frames again. This adds up to 50 seconds with the default timers.

8.6 Using Spanning Tree

In the case of the above diagram, Bridge A has the lowest ID so it becomes the root bridge. The cost from Sonny to Jim is 4 via Bridge A and 5 via Bridges B and C. Bridge C is taken out of the tree leaving Bridges A and B to be the designated bridges. If Bridge A or B failed, then Bridge C would come back on line and the Spanning Tree re-shaped.

A good network design should ensure that the root bridge is as close to the centre of the network as possible in order to facilitate faster convergence and also to make sure that the network is not running sub-optimally where traffic is taking less direct routes thereby doubling LAN bandwidth usage. Rather than rely on Spanning Tree using the default settings, it is often a good idea to shape the tree by setting bridge IDs and port costs.

Default timers are advertised by the root bridge, the default for the Forward Delay Timer is usually 15 seconds which gives enough time for the transition stage to complete before traffic is forwarded. On some vendor's equipment you can come across a mechanism called Port Fast (LAN traffic) or Uplink-Fast (Dial-up traffic) that allows you to bypass the Forward Delay Timer and forward data immediately rather than wait for a BPDU and all the delays between the modes mentioned earlier. This is useful only on point-to-point links such as trunk ports acting in a redundant manner in Spanning Tree where you know that you are in no danger of creating a loop.

8.7 Steady State Operation

Configuration BPDUs are sent regularly by the root bridge along with the age of the message. These are resent by each designated bridge in the Spanning Tree. The receiving bridges store this limited age. If the information times out for a particular port, then the bridge will try to become the designated bridge for that LAN. If root bridge information times out due to a better root port being found, or a root bridge failure then the bridge tries to become a root bridge itself and the election process restarts.

If a designated bridge (a bridge with active ports pointing to the root bridge) fails, or is removed from the network, or the root port fails, a directly connected bridge in the LAN segment detects that it is not receiving Configuration BPDUs from that bridge. This is because the information from the last BPDU times out according to the Maximum Age timer (default 20 seconds). It then sends a TCN BPDU to its designated bridge/switch that is destined for the root bridge/switch. The designated bridge receiving this TCN BPDU sends back a Configuration BPDU containing an acknowledgement as well as sending another TCN BPDU on towards the root bridge. The root bridge, on receipt of the TCN BPDU, sends a modified Configuration BPDU to all bridges in the network indicating that a toplogy change has occurred by setting the Topology Change Flag. Any directly connected bridges on the same segment, receiving the configuration change BPDU, create their own BPDUs and send those out. They also age out their forwarding tables according to the Forward Delay timer (15 seconds) rather than use the default time of 300 seconds. This fast aging lasts until the root bridge resets the Topology Change Flag once enough time has elapsed for the configuration change notification to have propagated throughout the tree. This time is determined by the formula Max Age + Forward Delay which equals 20 + 15 = 35 seconds by default. This speedy aging out is important as a topology change could mean that a particular device is learned via another port now and we want to avoid sending data traffic into a black hole whilst the normal aging time of 300 seconds times out.

It is quite normal for TCNs to occur in a network e.g. ports change state as users switch machines off. TCNs DO NOT in themselves cause a recalculation of Spanning Tree, although they could be a symptom of STP recalculation. Spanning Tree recalculation occurs when priorities are administratively changed, or configuration BPDUs fail to reach a Designated Bridge i.e. If a network segment in the spanning tree fails and a redundant path exists. So this will happen if any bridge/switch is added or removed or parameter changes occur.

8.8 Port Fast

A feature on some manufacturers' equipment called Port Fast allows certain ports that are say connected to key devices such as servers to still play a part in Spanning Tree but rather than the port go through the process of waiting to go through the Spanning Tree states as described above, the port can jump straight from blocking mode to forwarding mode. This is only of use for ports connecting to end devices since you could end up creating loops if this 'port fast' technique was applied to trunk links to other switches. This is an important feature to prevent host machines and servers from timing out their network connections when they initially connect.

8.9 Uplink Fast

There is another technique called Uplink Fast that can be used on access switches that have multiple paths to the root bridge/switch. An uplink group is created containing the root port and all alternate ports. On a failure of the root port link frames are immediately forwarded on the alternate link and Spanning Tre converges in a second or two instead of 35 seconds. A problem with this is that the forwarding tables are temporarily incorrect thereby causing dropped frames. Cisco use a proprietary multicast to cause relevant switches to update their forwarding tables. Uplink fast is all very well but it applies to all VLANs rather than on a per VLAN basis so it is no good in environments where you want to load share traffic.

8.10 Issues with Spanning Tree

The trouble with Spanning Tree is that it necessitates the need for some links to remain dormant, thus wasting network bandwidth (e.g. an expensive serial link between two remote bridges). In addition, no load sharing can take place using the standard 802.1q VLAN specification although Cisco has a proprietary implementation of 802.1q which does allow Per VLAN Spanning Tree (PVST), plus its own Inter-Switch Link (ISL) which also allows PVST. The issues with PVST occur when connecting to non-Cisco devices running 802.1q trunking.

Spanning Tree allows for a maximum of 7 concatenated bridges (assuming 1 second for a default Hold Time that a bridge holds a frame before discarding it) A frame should be delivered no more than 7 seconds after initial transmission. This is important as the bridge timers need to be kept in synch at the extremities of the network, and there needs to be a limit to the accumulated forwarding delay between stations.

There are three types of Spanning Tree. The originally there was a version from DEC and one from IBM. The IBM version was the basis for the current standard form IEEE 802.1d. These different versions are not compatible with one another.

9. Translational Bridging

Back to the top

Where LANs using different protocols at the Physical and Data Link layers are to be connected, a Translational Bridge can be used. Examples include a bridge connecting an Ethernet network to a Token Ring network. Bridges cannot support messages of different lengths when converting between frame formats, so the end devices must be capable of configuring the messages to be of the same lengths. Nowadays you will only likely to be seeing this in an environment where FDDI is converted to Ethernet.

10. Encapsulated Bridge

Back to the top

It is common to see an environment where identical LAN environments are connected together via a dissimilar LAN environment. An example of this is two Ethernet LANs separated by a FDDI LAN or a Serial link. An encapsulated bridge does not change the frame headers as the Translational Bridge does, instead the central LAN protocol (e.g. FDDI) encapsulates the connected LAN transparently bridged frames (e.g. Ethernet) across its backbone and dismantles the encapsulation at the other end.

11. Concurrent Routing and Bridging (CRB)

Back to the top

Traditionally, if a protocol is configured on a router then if network layer addressing is available, the protocol is routed rather than bridged. If no network layer addressing is available then the protocol is bridged. In the past, at no time could a packet be both routed and bridged.

With Concurrent Routing and Bridging, one router can both route a protocol through one interface and bridge that same protocol through another interface at the same time, but not through the same interface. Routed protocols have to be routed out of routing interfaces and bridged protocols have to be bridged out of bridging interfaces.

12. Integrated Routing and Bridging (IRB)

Back to the top

This allows you to both route and bridge a given protocol through the same interface (this is an extension of Concurrent Routing and Bridging).

An example of its use is when you are migrating from a bridged network to a routed network where you want to connect bridged segments to routed networks. With IRB you can route between routed interfaces and bridge groups, or you can route between bridge groups. In addition, you can still bridge non-routable traffic between bridge interfaces within a bridge group.

You can conserve layer 3 logical addresses by assigning one layer 3 address to a bridge group and just bridge local traffic.

The implementation of IRB is made possible with the concept of the Bridge-Group Virtual Interface (BVI). The BVI acts as a routed interface that does not perform bridging but represents the bridge group that is used to send bridged traffic to a routed network. The BVI interface number is the bridge group number of the bridge group assigned to that interface. The BVI takes the MAC address of one of the bridged interfaces in the particular bridge group. The network layer address of the BVI must be in the same network as the routed hosts.

For host A to talk to host B, host A must have its default gateway set as the IP address of the BVI. The MAC address of the BVI is the bridged interface and is learned by host A via ARP. At this point, the destination MAC address of the packet is the BVI's MAC address and the source MAC address is that of host A.

The bridging software on the bridged interface looks at the packet and decides whether the packet is to be routed or bridged. If the destination MAC address is one of the router's interfaces (in this case it is, the BVI) and if the layer 3 protocol is configured on that interface, the bridging software makes the packet look like it has come from the BVI rather than the bridge group (10 in this case), the packet is sent to the routing engine and routed out of that interface.

If the packet is destined to a host without any layer 3 addressing and within the same bridge group as the BVI, then the bridging software sees that the destination MAC address is not the BVI but a host device. The packet is then bridged to the bridge group if the MAC address of the destination is known, or flooded out all bridge group interfaces if the destination MAC address is unknown.

13. Frame Switching

Back to the top

13.1 Introduction

Bridging and Frame Switching are practically one and the same technology. Frame Switching is Bridging that has been speeded up. Bridging has always been software-based and normally a bridge would just have two ports used to connect the two LANs being bridged. Switching is hardware-based and has many ports but all the rules that apply to bridging also apply to switching and more besides. The MAC address is always left unchanged in bridging (barring the bit ordering change in Translational bridging!).

LAN Frame switches can include FDDI, Token Ring or Ethernet switches. Effectively, the switch provides single Collision Domains per switch port and each port acts as a bridge port to the rest of the network. Forwarding tables are kept per port, different media, different speeds etc. can be configured on a port by port basis. The speed enhancement to the network is achieved through the 'microsegmentation' of the large Collision Domain into many smaller ones. Each port on an Ethernet switch is effectively a very fast bridge port. The switch itself has its own MAC address e.g. 0800.a300.df00, and then each of its ports is given a MAC address, commonly in port order; port 1 has address 0800.a300.df01, port 2 has address 0800.a300.df02 etc. If this particular switch is the root bridge, then the MAC address 0800.a300.df00 is advertised as the root bridge, however the BPDUs originate from whatever MAC address is assigned to the port from which the BPDU emanates.

Some switches allow you to implement a Backpressure scheme whereby, on a particular port, jamming frames can be sent to reduce traffic coming into the switch. This stops one port hogging the backplane on a switch thereby effecting other users. Obviously, you would not wish to implement this on a server port, since this will affect many people and you would wish to keep as much of the switch processing capability for the attached servers. This is why so much play is made of the backplane capability of a particular manufacturer's switch.

13.2 Cut-through

A Cut-through switch first reads the Destination address of a frame and then sends the frame straight to the destination before the rest of the frame has arrived at the switch. The first 20 to 30 bytes of the frame need to be read to make sure that the frame is not a collision fragment. If the destination address remains unknown, then the switch temporarily stores the frame. Cut-through switching is fine for fixed speed networks such as all 10BaseT, and it is very fast, however if the switch has mixed speed ports such as 10/100 autosensing ports, then there is a bottle neck when packets are moving across the switch fabric from a 100BaseT segment to a 10BaseT segment. Some switches, although they forward the frame as soon as they read the destination address they still read the frame up to the CRC and if there are a certain level of errors, they can be configured to automatically change to a Store and Forward mode.

13.3 Store and Forward

A Store and Forward switch, or 'buffered switch', stores each frame frame in a buffer before forwarding it on to the appropriate port. This gets around the underflow or overflow situation that could happen in a mixed speed environment.

13.4 Fragment-free Switching

This is similar to Cut Through Switching but here the frame is checked a little further than the destination address to the Length field in order to weed out collision fragments, before it is forwarded.

Latency of a network increases as the network gets busier. On a busy network, the backoffs (retransmits) that could occur with Cut-through switches increase, thereby increasing latency. A Store and Forward switch on 10Mbps LAN delays a frame by one frame time, obviously increasing latency, ut there are no backoffs.

14. VLANs

Back to the top

14.1 Overview

Virtual LANs have been made possible as the switching infrastructure has replaced the traditional shared media LANs. An individual switch port can be assigned to a logical LAN, the next switch port can be easily assigned to a completely different logical LAN. This is made possible by 'tagging' the frames entering the port so that these frames are identified as belonging to a particular logical LAN whilst they travel along the switch fabric of the box. Once these frames are sent out of their logical LAN ports, the tag is removed from the frame. In the past, proprietary frame tagging has been implemented by Cisco (Inter Switch Link or ISL) and Bay Networks (Lattis Span), but the standard is now defined by 802.1q and may be the one that you go for in order to allow interoperability between different manufacturers boxes.

More recently frame tagging has been applied to trunk ports as links to other switches and routers within the wider network, carry multiple VLANs. This 'tagging' is very important since it enables the VLANs to spread 'Enterprise-wide' as the backbones take the bulk of network traffic and VLAN information. This is why VLANs need to operate across high-bandwidth trunked FDDI (802.10), Fast Ethernet (ISL and 802.1q) and ATM links (ATM LANE).

One key difference between ISL and 802.1q is that ISL allows multiple instances of Spanning Tree (i.e. one per VLAN) to exist on a trunk link whereas 802.1q only allows one instance of Spanning Tree on a trunked link. Which trunking method you use influences the network design somewhat particularly if you are wishing to make the most of the network connections and not have part of the network not being used because Spanning Tree has blocked some ports. ISL allows you to loadshare VLANs across parallel links so you don't have to have network ports not being used. This does not mean that you are completely tied in to using Cisco equipment only since you can 'map' ISL VLANs to 802.1q VLANs before entering a load-sharing section of the network and also other manufacturers such as Lucent support ISL anyway.

The use of switches has meant that microsegmentation has increased the number of Collision Domains thereby minimising the number of collisions that occur across this 'flat' network. This in turn frees up more bandwidth for data but at the cost of increasing the amount of broadcast traffic. VLANs are the next step where multicast and broadcast traffic is restricted to each of the VLANs and reduces the possibility of broadcast storms.

On a large network one Spanning Tree would take a very long time to converge. At the best of times even a RIPv1 network converges more quickly than a Spanning Tree network. For this reason, due to the greater likelihood of changes occurring the bigger the network is, it is a good thing to have one instance of Spanning Tree for each VLAN which decreases the calculation time, improves scalability and accommodates changes more efficiently.

VLANs allow you to connect any user to any logical LAN. The benefit here is that the user could be anywhere in the building or even another building. A particular department does not have to have all its employees physically situated in the same place. In addition, security is easily maintained since the only way to communicate between the virtual LANs is by routing between them, either by way of a router (slow) or via a layer 3 switch. VLANs also simplify moves, adds and changes plus give opportunities to load share traffic. Ideally, you should look to design your logical networks such that at least 80% of LAN traffic is maintained with the LAN, and a maximum of 20% of traffic routed between LANs. Note here that Layer 3 switching makes this less of an imperative. Ultimately, layer 3 switches will completely replace routers as far as intraLAN routing is concerned, the routers will remain as edge devices, doing the job that they are best at.

In addition, multiple logical networks can be multiplexed through one physical connection, provided that the connection is between two boxes that use the same Multi-Link Trunking (MLT) standard.

Originally, VLANs were simply based on port ID i.e. different ports being assigned to different VLANs. This is fine if the different groups are local, but not flexible enough to accommodate campus-wide VLANs. 'Port-centric' VLANs require no lookup table and are easy on the processor especially if an ASIC is taking care of the switching, plus there is a high level of security as packets are very unlikely to 'leak' into other VLANs. These type of VLANs are often referred to as Static VLANs as they are manually configured.

Membership of a VLAN can be determined by the MAC address. The user can move departments and the MAC address is remembered by the switch and the user remains part of the same logical LAN. VLANs can also be based on protocol or even application. These methods enable Moves and Changes to be implemented with little effort. These types of VLANs are called Dynamic VLANs since the ports can automatically determine their VLAN assignments on a per packet basis. The problem with Dynamic VLANs is that a lookup table has to be produced with all the known MAC addresses mapped to the relevant VLANs. Not only is this table likely to be very large, but it is also going to change frequently as new devices are added to the network and old ones are removed. If an organisation has a lot of movement within the building environment, then the Dynamic VLAN may be the best design.

Frame filtering is often used by switches to aid in minimising LAN traffic. Tables are kept and frames are compared to the table entries to varying levels of frame depth. The deeper into the frame the switch has to go, the greater the switch latency. Also, the larger the table; the more the latency and these tables need to be synchronised with other switches.

14.2 Switched Network Design

A good general network design is shown below:

All networks have the Core, Distribution and Access layer functions within them even if two or more of the functions are dealt with by one box. Generally, it is a good idea to have the servers connected to the Distribution switch and let the distribution switch deal with the VLANs. The routers could either be separate 'routers on a stick' or built in to the switch and these should deal with access policies and filtering. The Core switches need to be left purely to switch packets and not worry about policies, routing or server traffic.

One thing to be aware of when upgrading networks is the capability of the servers. It is all very well upgrading the client links and the backbone links, but if the servers can so easily become the bottleneck in a network. You need to make sure that the server, be it based on NT or Unix, is capable of fast network technology such as Duplex operation and Gigabit Ethernet. Not only do you need to make sure that the server can cope with fibre-based cards for 1000BaseSX or 1000BaseLX, you also need to be sure that the servers internal components such as the hard disk, processor and the bus can cope with a fast network. A duplex gigabit card can completely swamp the whole PCI bus in a standard PC which can cause problems since this PCI bus is meant to be share by the other cards in the machine.

Examples

The following diagram illustrates a typical flat network.

The trouble with this network is that it is just one collision domain and one broadcast domain so it is prone to high collision rates and alot of the bandwidth on the network is going to be given over to broadcasts. The problem with broadcast traffic is that each station on the network be it a server or a client, will have to process the broadcast packets. This processing has to be carried out by the CPU of the computer and can have quite a large effect on the processing capability of the computer.

A chassis-based switch can replace the hubs and provide a far more efficient network:

Using the switch, not only is more bandwidth available to each client because each client has it's own collision domain, but also one can configure VLANs to separate certain groups within the organisation and thereby reduce broadcast traffic freeing up even more bandwidth. The router can either remain a separate device or become integrated into the electronics of the switch. You will notice that in this small network, the core, distribution and access layers can be satisfied by one box even though the functions are still distinct.

The following design extends the previous one for a larger network:

Here, the clients are fed from 'access' or 'workgroup' switches perhaps located in different closets elsewhere in the building. In this case, they are linked via 100BaseT switched links which could be copper (100BaseT) or fibre (100BaseFX). Alternatively, they could be ATM or even FDDI links depending on the capabilitites of the main distribution and access switches. This is because ATM LAN Emulation or FDDI 802.10 can be used to maintain the VLANs across the trunk. The trunk to the local router could be ISL (which would allow load sharing of VLANs ) or 802.1q, this local router could be contained within the switch, making the switch a layer 3 switch.

In the diagram below, the network has been extended further to an Enterprise level:

The ATM switch takes on the role of the core switch and links to other sites which also have core switches. The backbone within in site could be Gigabit Ethernet, again depending on the capabilities of the access and distribution switches. The server farm and the routing is carried out by the distribution switch, it is best to leave the core switches to high speed switching and, as described before, the distribution switches should take on the burden of access policies. If 100BaseT is to be fed to the desktop it becomes more important to upgrade the bandwidth between the access and distribution switches to either Gigabit Ethernet or a multiple link such as 4 x 100BaseT links forming a 400BaseT channel (when operating in duplex mode this can effectively allow a maximum throughput of 800Mb/s).

In a large campus environment, multiple switch/routers at the distribution level can provide the opportunity to form resilient links to the core via the use of HSRP or VRRP. These protocols can provide alternative paths for specified VLANs.

The Virtual Router redundancy protocol gives the possibility of load sharing traffic across the routers by setting up separate groups with the aim of roughly half the local traffic going through one router and half through another by way of two or more virtual default gateways.

14.3 Trunking Overview

The trunking protocol used can be important in deciding the structure of the network. Although 802.1q is the industry standard it has a limitation in that it only allows one instance of Spanning Tree in a trunk link even if there are a number of VLANs in the trunk. Cisco's ISL, however, allows an instance of Spanning Tree per VLAN and the advantage of this is that trunk ports can remain available for some VLANs if blocked for other VLANs and hardware need not remain dormant as it would if 802.1q was implemented.

Trunking protocols such as Cisco's ISL could be used in the link to the servers. The advantage of this is that Intel manufacture ISL capable NICs and these NICs allow the server to have a number of different VLANs, and therefore completely different networks, to be served by the server as if the server was made up of completely different devices. The NIC cards themselves take care of the processing of frames so the CPU of the server does not have to take the load! Another advantage of using ISL-aware NICs is that the traffic does not have to be dealt with by a router and this therefore aids to keeping the network fast.

14.4 Cisco's Inter-Switch Link (ISL)

Cisco use a proprietary tagging method called Inter-Switch Link (ISL) which takes a different approach to tagging the Ethernet frame. Instead of increasing the frame size by inserting fields, ISL encapsulates the Ethernet frame.

Cisco's Inter-Switch Link (ISL) allows Per VLAN Spanning Tree (PVST) so multiple VLANs can exist across a trunk link. Multiple Spanning Trees allow load sharing to occur at layer 2 by assigning different port priorities per VLAN. 802.1q only allows Mono Spanning Tree (MST) i.e. one instance of Spanning Tree trunk.

ISL only runs on point-to-point links on Fast Ethernet (copper or fibre) and Token Ring (ISL+). Although ISL will operate over 10Mbps links it is not recommended! ISL runs between switches, from switches to routers and from switches to Intel and Xpoint Technologies NICs which understand ISL, thereby allowing servers to distinguish between VLANs.

With ISL the data frame is not touched but is encapsulated according to the following process:

The frame enters the switch and is stored in the port's buffer.
The SAINT/SAGE encapsulates the ISL on a trunk port.
The encapsulation has 30 bytes of information, 26 bytes for the header (VLAN ID and port number) and 4 bytes for the FCS.
The frame is switched to the destination port(s).
The SAINT/SAGE dencapsulates the frame before it is sent out of a normal port, or leaves it alone if the port is a trunk port.

The following diagram details the ISL frame tagging format:

ISL Multicast Address - this reserved address is 0x01000C0000 (40 bits).
Type Field - this identifies the type of frame that is encapsulated, 0000 is Ethernet, 0001 is Token Ring, 0010 is FDDI, 0011 is ATM.
User Field - 0000 means Normal Priority, 0001 means Priority 1, 0010 means Priority 2 and 0011 means High Priority. Similar to the Class of Service field in 802.1p.
Source address - this is the MAC address of the frames source.
Length - this is the length of the frame excluding the fields up to AAAA03 and also excluding the FCS.
AAAA03 - indicates that the ISL frames use SNAP LLC.
OUI - this is the Organisational Unique Identifier of the source of the frame i.e. the first three bytes of the Source Address.
VLAN ID - Cisco use the lowest 10 bits of the 15 to give a possible 1024 different VLANs although only 250 VLANs can be active at any one time. The Catalyst 3000 supports 64 VLANs and the 7000 series routers support 255 VLANs.
B - when set to '1' this bit indicates whether the frame is a BPDU, CDP or VTP frame and the frame is sent straight to the NMP for processing.
Index Field - indicates the port number of the source port.
R - this is set to 0x0000 for Ethernet frames but for Token Ring or FDDI frames the AC or FC fields are placed here e.g. for FDDI an FC of 0x12 would mean 0x0012 is placed in the R field.
Original frame - can be up to 24575 bytes in length.
FCS - This is the extra FCS added by ISL.

14.4 Class of Service and VLANs (802.1p & 802.1q)

Quality of Service (QoS) is becoming more important as data networks begin to carry more time sensitive traffic such as real time voice and video. At layer 2 this is referred to as Class of Service (CoS).

The 802.1 group have been working on an extension to the MAC layer that takes into account CoS. 802.1p is a standard for traffic prioritisation where network frames are tagged with one of eight priority levels, where 7 is high and 0 is low. Switches and routers that are 802.1p compliant can give traffic that is time-sensitive such as voice traffic, preferential treatment if the priority tag has been set to a higher value than other traffic.

In order to accommodate tagging an Ethernet frame a new field has been introduced called the Tag Control Info (TCI) field between the Source MAC address and the Length field of the Ethernet frame. This is illustrated below:

Tagged Frame Type - this indicates the type of tag, for Ethernet frames this is currently always 0x8100.
802.1p Priority - this ranges from binary 000 (0) for low priority to binary 111 (7) for high priority. This maps to the Quality of Service (QoS) values used in the TOS field IP precedence values.

000 (0) - Routine
001 (1) - Priority
010 (2) - Immediate
011 (3) - Flash
100 (4) - Flash Override
101 (5) - Critical
110 (6) - Internetwork Control
111 (7) - Network Control

Canonical - this is always 0.
802.1q VLAN ID - this identifies the VLAN number when trunking VLANs.

Although the frame illustrated is an 802.3 frame, 802.1p/802.1q can also be applied to the Ethernet frame where the TCI is inserted just before the Type field and just after the Source MAC Address.

You will note the similarity between the 802.1p priority field and the Precedence field in the Diff Serv Code Point of the IP datagram. This makes mapping between IP layer 3 and MAC layer priorities much easier.

You will note that the Ethernet frame becomes 'oversized' i.e. grows from the standard maximum size of 1518 bytes to 1522 bytes. This sometimes called a Baby Giant. Consequently these frames may be dropped by some network equipment, although most vendors now support 802.1p and 802.1q.

When applying Layer 2 Priority Queueing within a trunk, commonly two priority levels (low and high) are implemented, although as we have seen there is scope though to increase to eight. This is because each priority has to have its own queue. This is implemented in hardware and is therefore expensive so most manufacturers currently build in two queues per port, a low priority queue for priority levels 0 to 3 and a high priority queue for priority levels 4 to 7. Prioritisation is determined on the outbound packets from a switch, therefore they are already ordered on the inbound ports of the next switch so that prioritisation need not be implemented on the inbound ports, unless the ports are using buffering. Low priority frames or frames without an 802.1p tag are treated with 'best effort' delivery. As time goes on more manufacturers will include separate queues for each priority level to give more granularity and as the applications begin to demand it.

On a 802.1q trunk, one VLAN is NOT tagged. This VLAN is called the Native VLAN, and must be configured the same on each side of the trunk. This way, we can deduce to which VLAN a frame belongs when we receive a frame with no Tag, otherwise the frame will remain in whatever tagged VLAN it arrived on even if it is the wrong one. When a switch trunks a frame, it inserts the Tag and then recomputes the FCS.

The 802.1q standard implements Spanning Tree on the Native VLAN, and this applies to all the trunked VLANs, this is called Mono Spanning Tree (MST). Cisco have adapted 802.1q and use a tunnelling mechanism to provide what is called Per VLAN Spanning Tree Plus (PVST+) with VLAN numbers up to 1005. This gives the same benefits that ISL gives.

The native VLAN configured on each end of a 802.1q trunk must be the same. A switch receiving a non-tagged frame will assign it to the native VLAN of the trunk. If one end is configured for Native VLAN 1 and the other to Native VLAN 2, a frame sent in VLAN 1 on one side will be received on VLAN 2 on the other. You are then merging VLAN 1 and 2.

Home

Copyright © Rhys Haden 1998-2004

Dial-up Accelerator     Trading Links     Network Marketing
Wireless Service     Great Domainia Domain Names

All rights reserved. All trademarks, logos, and copyrights are property of their respective owners.