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Chapter 4
Layer 2 Switching and the Spanning Tree Protocol (STP)
Broadcast and Multicast Frames
Remember that layer 2 switches forward all broadcasts by default. The
forwarding/filtering decision is not used in a broadcast situation because
broadcast and multicast frames do not have a destination hardware address
specified. The source address will always be the hardware address of the
device transmitting the frame, and the destination address either will be all
1s (broadcast), or it will be a combination of the network or subnet address
specified and all 1s for the host address (multicast). For example, a broadcast
and multicast in binary would be as shown in Table 4.2.
Even though we have given you an example of a multicast address, the term
multicast is most commonly used with regard to multicasting groups using
the class D IP address space.
Notice that the broadcast is all 1s and the multicast is not. They are both
a type of broadcast, except that multicasts send the frame to only a certain
network or subnet and all hosts within that network or subnet, whereas a
broadcast of all 1s sends the frame to all networks and all hosts.
When a switch receives these types of frames, the frames are then quickly
flooded out all active ports of the switch by default. To have broadcasts and
multicasts forwarded out only a limited amount of administratively assigned
ports, you create virtual LANs (VLANs), which were discussed in Chapter 3.
Loop Avoidance
Finally, the layer 2 switch is responsible for loop avoidance. It's a good idea
to use redundant links between switches. They help stop complete network
failures if one link fails. Even though redundant links are extremely helpful,
T A B L E 4 . 2
Broadcast and Multicast Example
Binary
Decimal
Broadcast
11111111.11111111.11111111.11111111
255.255.255.255
Multicast
10101100.00010000.11111111.11111111
172.16.255.255
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