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Answers to the Chapter 4 Q&A Section 721

for any frame that enters an interface. That is one of the reasons that LAN switches, which
have a much larger number of interfaces than traditional bridges, need to have optimized
hardware and logic to perform table lookup quickly.

Define the term collision domain.

A collision domain is a set of Ethernet devices for which concurrent transmission of a
frame by any two of them will result in a collision. Bridges, switches, and routers separate
LAN segments into different collision domains. Repeaters and shared hubs do not
separate segments into different collision domains.

When a bridge or switch using Spanning-Tree Protocol first initializes, who does it assert
should be the root of the tree?

Each bridge/switch begins by sending CBPDUs claiming itself as the root bridge.

Name the three reasons why a port is placed in forwarding state as a result of Spanning
Tree.

First, all ports on the root bridge are placed in forwarding state. Second, one port on each
bridge is considered its root port, which is placed in forwarding state. Finally, on each
LAN segment, one bridge is considered to be the designated bridge on that LAN; that
designated bridge's interface on the LAN is placed in forwarding state.

Define the difference between broadcast and multicast MAC addresses.

Both identify more than one device on the LAN. Broadcast always implies all devices on
the LAN, whereas multicast implies some subset of all devices. Multicast is not allowed
on Token Ring; broadcast is allowed on all LAN types. Devices that intend to receive
frames addressed to a particular multicast address must be aware of the particular
multicast address(es) they should process. These addresses are dependent on the
applications used. Read RFC 1112, the Internet Group Message Protocol (IGMP), for
related information about the use of Ethernet multicast in conjunction with IP multicast.
For example, the broadcast address is FFFF.FFFF.FFFF, and one sample multicast address
is 1000.5e00.0001.

Excluding the preamble and starting delimiter fields, but including all other Ethernet
headers and trailers, what is the maximum number of bytes in an Ethernet frame?

1518 bytes. See Figure 4-5 on page 141 for more detail.

Define the term broadcast domain.

A broadcast domain is a set of Ethernet devices for which a broadcast sent by any one of
them should be received by all others in the group. Unlike routers, bridges and switches
do not stop the flow of broadcasts. Two segments separated by a router would each be in
a different broadcast domain. A switch can create multiple broadcast domains by creating
multiple VLANs, but a router must be used to route packets between the VLANs.

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