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Chapter 7Ethernet Lan Switching concepts

Although using 10BASE-T with a hub improved Ethernet as compared to the older

standards, several drawbacks continued to exist, even with 10BASE-T using hubs:

Any device sending a frame could have the frame collide with a frame sent by any otherdevice attached to that LAN segment.

Only one device could send a frame at a time, so the devices shared the (10-Mbps)bandwidth.

Broadcasts sent by one device were heard by, and processed by, all other devices on theLAN.

Ethernet bridges were created to solve some of the performance issues. Bridges solved the

growing Ethernet congestion problem in two ways:

They reduced the number of collisions that occurred in the network.They added bandwidth to the network.Shows the network after it has been segmentedusing a bridge. The bridge creates two

separate collision domains.

LAN switches perform the same basic core functions as bridges, but with many enhanced

features. Like bridges, switches segment a LAN into separate parts, each part being aseparate collision domain. Switches have potentially large numbers of interfaces, withhighly optimized hardware, allowing even small Enterprise switches to forward millions of

Ethernet frames per second. By creating a separate collision domain for each interface,

switches multiply the amount of available bandwidth in the network. And, as mentioned in

Chapter 3, if a switch port connects to a single device, that Ethernet segment can usefullduplex logic, essentially doubling the speed on that segment.

A switchs effect of segmenting an Ethernet LAN into one collision domain per interface issometimes called microsegmentation.

The IEEE defines three general categories of Ethernet MAC addresses:

Unicast addresses: MAC addresses that identify a single LAN interface card.Broadcast addresses: A frame sent with a destination address of the broadcast address(FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the

frame.

Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset ofdevices on a LAN to communicate.

The IP protocol supports the multicasting of IP packets. When IP multicast packets are sent

over an Ethernet, the multicast MAC addresses used in the Ethernet frame follow thisformat: 0100.5exx.xxxx, where a value between 00.0000 and 7f.ffff can be used in the last

half of the address.

bridges perform three actions:

1. Deciding when to forward a frame or when to filter (not forward) a frame, based on the

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destination MAC address

2. Learning MAC addresses by examining the source MAC address of each framereceived by the bridge

3. Creating a (Layer 2) loop-free environment with other bridges by using Spanning TreeProtocol (STP)

To decide whether to forward a frame, a switch uses a dynamically built table that lists

MAC addresses and outgoing interfaces. Switches compare the frames destination MAC address to this table to decide whether the switch should forward a frame or simply ignoreit.

A switchs MAC address table is also called the switching table, or bridging table, or eventhe Content Addressable Memory (CAM), in reference to the type of physical memory

used to store the table.

Switches build the address table by listening to incoming frames and examining thesourceMAC address in the frame. If a frame enters the switch and the source MAC address is not

in the MAC address table, the switch creates an entry in the table. The MAC address is

placed in the table, along with the interface from which the frame arrived.

that occurred when there were no entries in the MAC address table? As it turns out, when

there is no matching entry in the table, switches forward the frame out all interfaces (except

the incoming interface). Switches forward these unknown unicast frames (frames whose

destination MAC addresses are not yet in the bridging table) out all other interfaces, withthe hope that the unknown device will be on some other Ethernet segment and will reply,allowing the switch to build a correct entry in the address table.

The process of sending frames out all other interfaces, except the interface on which theframe arrived, is calledflooding. Switches flood unknown unicast frames as well asbroadcast frames. Switches also flood LAN multicast frames out all ports, unless the switchhas been configured to use some multicast optimization tools

Switches keep a timer for each entry in the MAC address table, called aninactivity timer.The switch sets the timer to 0 for new entries. Each time the switch receives another frame

with that same source MAC address, the timer is reset to 0. The timer counts upward, so

the switch can tell which entries have gone the longest time since receiving a frame fromthat device. If the switch ever runs out of space for entries in the MAC address table, the

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switch can then remove table entries with the oldest (largest) inactivity timers.

STPThe third primary feature of LAN switches is loop prevention, as implemented by

Spanning Tree Protocol (STP). Without STP, frames would loop for an indefinite period of

time in Ethernet networks with physically redundant links. To prevent looping frames, STP

blocks some ports from forwarding frames so that only one active path exists between anypair of LAN segments (collision domains). The result of STP is good: frames do not loop

infinitely, which makes the LAN usable. However, although the network can use some

redundant links in case of a failure, the LAN does not load-balance the traffic.To avoid Layer 2 loops, all switches need to use STP. STP causes each interface on aswitch to settle into either a blocking state or a forwarding state.Blocking means that the

interface cannot forward or receive data frames.Forwarding means that the interface can

send and receive data frames. If a correct subset of the interfaces is blocked, a single

currently active logical path exists between each pair of LANs.

Larry sends a single unicast frame to Bobs MAC address, but Bob is powered off, so noneof the switches has learned Bobs MAC address yet. Bobs MAC address would be an

unknown unicast address at this point in time. Therefore, frames destined for Bobs MAC address are forwarded by each switch out every port. These frames loop indefinitely.

Because the switches never learn Bobs MAC address (remember, hes powered off and

can send no frames), they keep forwarding the frame out all ports, and copies of the framego around and around.

Internal processing on Cisco Switches.Because the destination MAC address occurs very early in the Ethernet header, a switch

can make a forwarding decision long before the switch has received all the bits in the frame

With cut-through processing, the switch starts sending the frame out the output port as

soon as possible. Although this might reduce latency, it also propagates errors. Because theframe check sequence (FCS) is in the Ethernet trailer, the switch cannot determine if the

frame had any errors before starting to forward the frame. So, the switch reduces theframes latency, but with the price of having forwarded some frames that contain er rors.

Fragment-free processing works similarly to cut-through, but it tries to reduce the numberof errored frames that it forwards. One interesting fact about Ethernet carrier sense multiple

access with collision detection (CSMA/CD) logic is that collisions should be detected

within the first 64 bytes of a frame.

LAN switches provide the following benefits:Switch ports connected to a single device microsegment the LAN, providing dedicatedbandwidth to that single device.

Switches allow multiple simultaneous conversations between devices on different ports.

Switch ports connected to a single device support full duplex, in effect doubling theamount of bandwidth available to the device.

Switches support rate adaptation, which means that devices that use different Ethernetspeeds can communicate through the switch (hubs cannot).

Switches use Layer 2 logic, examining the Ethernet data-link header to choose how toprocess frames. In particular, switches make decisions to forward and filter frames, learn

MAC addresses, and use STP to avoid loops, as follows:

Step 1 Switches forward frames based on the destination address:a. If the destination address is a broadcast, multicast, or unknown destination

unicast (a unicast not listed in the MAC table), the switch floods the frame.b. If the destination address is a known unicast address (a unicast address found

in the MAC table):

i. If the outgoing interface listed in the MAC address table is differentfrom the interface in which the frame was received, the switch forwards

the frame out the outgoing interface.ii. If the outgoing interface is the same as the interface in which the frame

was received, the switch filters the frame, meaning that the switch

simply ignores the frame and does not forward it.Step 2 Switches use the following logic to learn MAC address table entries:

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a. For each received frame, examine the source MAC address and note the

interface from which the frame was received.b. If they are not already in the table, add the address and interface, setting theinactivity timer to 0.

c. If it is already in the table, reset the inactivity timer for the entry to 0.

Step 3 Switches use STP to prevent loops by causing some interfaces to block,meaning that they do not send or receive frames.

Collision Domains: As mentioned earlier, a collision domain is the set of LAN interfaceswhose frames could collide with each other, but not with frames sent by any other devicesin the network.

The switch on the right separates the LAN into different collision domains for each port.Likewise, both bridges and routers also separate LANs into different collision domains

(although this effect with routers was not covered earlier in this book). Of all the devices inthe figure, only the hub near the center of the network does not create multiple collision

domains for each interface.

Broadcast domain: The term broadcast domain relates to where broadcasts can be

forwarded. A broadcast domain encompasses a set of devices for which, when one of thedevices sends a broadcast, all the other devices receive a copy of the broadcast.

Broadcasts sent by a device in one broadcast domain are not forwarded to devices inanother broadcast domain. the router does not forward a LAN broadcast sent by a PC on

the left to the network segment on the right.

A collision domain is a set of network interface cards (NIC) for which a frame sent byone NIC could result in a collision with a frame sent by any other NIC in the samecollision domain.

A broadcast domain is a set of NICs for which a broadcast frame sent by one NIC isreceived by all other NICs in the same broadcast domain.

For a single collision domain:

The devices share the available bandwidth.The devices may inefficiently use that bandwidth due to the effects of collisions,particularly under higher utilization.

Virtual LAN: Although you can think about and define the term LAN from many

perspectives, one perspective in particular will help you understand VLANs:

A LAN consists of all devices in the same broadcast domain.

Without VLANs, a switch considers all interfaces on the switch to be in the same broadcast

domain. In other words, all connected devices are in the same LAN. (Cisco switchesaccomplish this by putting all interfaces in VLAN 1 by default.) With VLANs, a switch

can put some interfaces into one broadcast domain and some into another based on some

simple configuration. Essentially, the switch creates multiple broadcast domains by puttingsome interfaces into one VLAN and other interfaces into other VLANs. These individual

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broadcast domains created by the switch are called virtual LANs.

there are many motivations for using VLANs, including the following:

To create more flexible designs that group users by department, or by groups that worktogether, instead of by physical location

To segment devices into smaller LANs (broadcast domains) to reduce overhead causedto each host in the VLAN

To reduce the workload for STP by limiting a VLAN to a single access switchTo enforce better security by keeping hosts that work with sensitive data on a separateVLAN

To separate traffic sent by an IP phone from traffic sent by PCs connected to the phonesCampus LAN design terminology

Access: Provides a connection point (access) for end-user devices. Does not forwardframes between two other access switches under normal circumstances.

Distribution: Provides an aggregation point for access switches, forwarding framesbetween switches, but not connecting directly to end-user devices.

Core: Aggregates distribution switches in very large campus LANs, providing veryhigh forwarding rates.