LAN switches — extending coverage and managing traffic in LAN networks

The popularity of LANs in the working environment grew rapidly in the 1980s and 1990s.

But while a 10 Mbit/sshared medium might suffice among a small number of users, sooner or later the limits of a single LAN are reached, and one of the following problems arises:

• user traffic demand exceeds the aggregate 10 Mbit/s total capacity of the LAN;

• the required geographical coverage exceeds the maximum cabling lengths of a single collision domain;

• the number of users exceeds the maximum allowed in a collision domain or the number of physical ports available on the ethernet hubs.

For all three of these problems there is a simple solution: split the single LAN (single collision domain) into two or more smaller, interconnected LANs. In this way the above problems are solved, but a new one arises: how can I interconnect the new smaller LANs to one another,

so that the stations and their end-users can continue to intercommunicate with one another?

The subject of LAN interconnection we shall return to later in the chapter. In the meantime we discuss how one of the above problems (that of meeting traffic demand) may nowadays be most easily be solved by the use of aLAN switch orethernet switch.

In the shared port or single collision domain configuration of an ethernet LAN, any two of the end-user stations may communicate with one another at up to 10 Mbit/s half duplex (i.e., they may only communicate in one direction at a time). At first glance, the 10 Mbit/s bit rate may appear to offer fantastically fast data transfer — and it does, provided only a few users share the LAN. But when the number of stations gets nearer the limit of 100 allowed by 10baseT ethernet LANs, and all the users are active at once (as they typically are in an office network), the situation looks quite different: if each user wants to send and receive data in equal volumes, then the equivalent bit rate available for each is the equivalent of 50 kbit/s

‘full duplex’. This is only the equivalent of a dial-up ISDN line, and as any frequent Internet

‘surfer’ will tell you: it can take a frustatingly long time to download large datafiles at this speed. Worse still, the theoretical maximum aggregate throughput capacity of an ethernet LAN comes nowhere near the nominal 10 Mbit/s transmission rate of the bus itself. Particularly at high traffic loadings, much of the bus capacity is lost due to collisions of the packets.

Full- and limited-availability (full-mesh and partial-mesh) switches

In contrast to ashared medium, which only allows one of the end stations to transmit at any one time, a switch allows multiple paths between different end-stations to be established at the same time. This multiplies accordingly the capacity of the LAN. Figure 4.11a illustrates a full availability 6-port switch configuration. The full availability (i.e., full-mesh and non-blocking) switch matrix of Figure 4.11a allows all six of the stations to be simultaneously communicating: A with C; B with E and D with F. In this configuration, the maximum

Figure 4.11 Full availability (full-mesh) and limited availability (partial mesh) switches.

LAN switches — extending coverage and managing traffic in LAN networks 147 throughput of the network has been multiplied to 3×10 Mbit/s=30 Mbit/s, reflecting the three simultaneous paths which may be established across thebackplane of the switch. The switch of Figure 4.11a is termed a full availability matrix, since all of the ports can be configured to communicate at once (assuming that the desired destination port is free).

Figure 4.11b shows alimited availabilityswitch matrix. In a limited availability (orpartial mesh) matrix, not all of the ports can communicate at once, since insufficient paths are available. The number of available paths (in our example, two) is less than half the number of ports (in our case 6 ports/2=3 paths are required forfull availability). When all the available paths of a limited availability switch are already in use, then any attempts to set up further communications will beblocked, irrespective of whether the destination port is free or busy.

Thus, for example, it is not currently possible to establish communication between the free ports A and C in Figure 4.11b, since all the available switch paths are already in use.

How an ethernet switch operates — the creation of the source address table (SAT)

Like most other modern data networking components, ethernet switches are designed to be able to administer themselves. When you first plug-in the end-user equipment (i.e., the DTE or station) to an ethernet switch (which mostly nowadays are based upon 10/100baseT), it starts the initial period of auto-sensing orauto-negotiation (also calledNWAY) on each port.

By so doing, the switch is able to configure each of its ports to the correct bit rate, to half-duplex (HDX) or full duplex (FDX) and (as appropriate) to the correct ethernet or fast ethernet technology (10baseT, 100baseTX, 100baseT4 or 100baseT2). Now the switch is ready to support communication between the different ports.

Since the switch initially knows none of the MAC addresses of the attached DTE devices, it starts operating in the same manner in which a hub would operate. Any packets received from any of the attached devices are simplyflooded to all of the attached ports. In this way, the switch can be sure that the packet will reach the correct MAC destination (provided of course that the destination device is connected). But meanwhile, the switch learns from its experience. It learns where the MAC address of the device which originated the packet is.

How? Because each packet contains a MAC header, which contains both the destination and thesource MAC address. In this way, the switch is able to relate the source MAC address to the port where the packet originated. When subsequently a packet is sent to the switch with this address as itsdestinationMAC-address, the switch knows already which port the packet must be forwarded to.

Over time, the switch is able to build a completesource address table (SAT)of all the MAC addresses of devices connected to it and their respective port numbers. In other words, the source address table (SAT) is determined by observing source addresses, and all entries in the table are the MAC addresses of DTEs connected to local switch ports. With the SAT to hand, the switch no longer needs toflood all packets to each of the ports, but instead, can direct them only to the relevant port. As a result, multiple simultaneous paths can be established between different pairs of ports, as we saw in Figure 4.11.

Apart from the benefit of increasing network traffic capacity, an ethernetswitchalso offers more data security than an ethernet hub, since the packets are switched directly between only the relevant two communicating ports and not usually broadcast or flooded to all the devices in the LAN.

The individual ports of an ethernet LAN switch may be used either to connect individual DTEs (data terminal equipment), such as a single PC, to connect whole collision domains, or to connect other switches. It does not matter, as far as the operation of the switch is con-cerned, whether one or more individual MAC-source addresses are assigned to each port.

Note: *Risers are inter-floor channels designed into modern office buildings by architects to provide a passageway or conduit for inter-floor telecommunications and electrical cabling, as well as piping and other building services.

Figure 4.12 Typical office LAN based on ethernet switches.

Thus switches can be used either as the ‘star-point’ of individual LANs, or for interconnect-ing different switchedsubnetworks or collision domains (i.e., subnetworks usingLAN hubs).

Figure 4.12 illustrates the huge flexibility of modernswitched LANs., showing a typical office LAN comprising a backbone network with an enterprise switch at the centre of it and fast ethernet (100baseFX) fibre cabling trunking it to separateworkgroup switches on each floor.

The workgroup switches are normally placed beside the patch panel in the wiring cabinet of the office floor, from where the UTP (unshielded twisted pair) cabling (typically Cat 5) leads out to the sockets installed in each office.

End-user PCs are typically connected directly to the switch, though it is also possible to install small hubs or desktop switches to create small collision domains for interconnected local printers, scanners and other peripheral equipment. The maximum number of DTEs (i.e., end-stations) a switch port can support is known as the end-station density (ESD) or port address support (PAS). The value of the ESD or PAS is set by the switch designer, when deciding how much capacity to build-into the SAT memory space. Differing values of ESD or PAS are the main distinguishers betweenenterprise, workgroup anddesktopswitches.

Because a LAN switch is able to ‘learn’ about the devices connected to it and the network around it, it is easy to add further devices at will to the switch later. In addition, it is possible to unplug devices, or move them from one location to another (and thus from one switch port to another). Thus ethernet switches are ‘plug-and-play’ devices — you connect them together to configure the network topology you want, and they self-administer themselves to make it work.

The source address table (SAT) can be kept up-to-date even when devices are unplugged from a switch by associating anageing time(typically 300 seconds) with each of the entries

Other types of LAN (token ring and token bus) 149 in the table. If no traffic is received from a particular address in the SAT within the ageing time window, then the entry for this address is deleted from the SAT. A 300 second ageing time may seem a little short, and some devices allow the human LAN administrator to adjust the value, but the ageing time should not be set too long. Long ageing times hamper the ease with which devices can be moved around an office (i.e., from one switch port to another) and lead to the potential of misdirected traffic. Users complain of problems, and the human administrator cannot figure out what is going wrong — all the wire connections appear to be alright, but the communication doesn’t work!Ageing is a useful way of ensuring that routing tables automatically remain updated, no matter how the network topology changes! But ageing times should not be set too short, for this reduces the efficiency of the network with the extra volume of packet flooding and slows up the switch by keeping it permanently busy updating SAT entries.

Mirrored ports — for measuring traffic in switched ethernet lans

Before LAN switches emerged, humanLAN administratorsused to monitor the traffic volumes and flows in LANs by the use of LANprobes orsniffers. These are devices which, in the case of ethernet collision domains, monitor the source and destination addresses of all the packets broadcast over the ethernetbus. Once switches appeared, most of the packets within the LAN were no longer broadcast or flooded to all the ports. As a result, it became difficult to monitor either the traffic in the LAN as a whole or the traffic flows to and from a given DTE (data terminal equipment). For this reason, switches usually offer a mirrored port facility. When a port has been mirrored a second port is set up in parallel with the first. It receives all the traffic that the first port receives, and any packets originated by it are handled as if they had been originated by the first. The mirror port is useful for the connection of aprobe orsniffer for analysing the traffic to the mirrored port.