These addresses are defined in IEEE Std 802-2006. If the
next-to-low-order bit in the first byte of the MAC address (02-00-00-00-00-00) is set,
then the network administrator is free to use any value in the remaining 46 bits (the
low bit of the first byte is the multicast/unicast indication). This technique is not commonly
used today because of the likelihood that a configuration error will result in
unintentionally duplicated MAC addresses.
There is, unfortunately for bridges, a far more likely reason for duplicate MAC addresses.
IETF RFC 3768 defines the Virtual Router Redundancy Protocol (VRRP).
Running this protocol can result in two (or more) routers having the same MAC address,
though on different VLANs. In Figure 13.5, two routers are circled. One is on
the customer??™s dotted VLAN and one on the customer??™s solid VLAN, but both have the
same MAC address, A.
Figure 13.5 Duplicate MAC address problem
One wire, two C-VLANs
Trunk wires, two C-VLANs
carried inside one S-VLAN
A
C
B
One wire, two
C-VLANs
D
?A?
ABC
AC
B
CD
A
B
C
C
D
B
CD BD
?A?
AD
B
AB
D
Customer
bridge Y Customer
bridge Z
Customer
bridge X
Provider bridges
P, Q, R, S
A
C
A
?A?
S
P
R Q
D
A
386 Chapter 13
This duplicated MAC address is no problem for the three customer bridges, X, Y, and
Z.
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