g., when every customer station logs on to a
bridge using IEEE Std 802.1X or when all MAC addresses are known, customer
MAC addresses cannot, in general, be discovered by a provider.
?– Even if the MAC addresses can be reliably learned (this is not impossible, if all of
the stations are running the IP, they would have to be passed around in control
protocols if not learned. This imposes a significant added burden to the bridge??™s
control plane processing capability.
Put simply, it has proven more economical for bridges to set up a single path for any
given VLAN, so MAC addresses can be learned in hardware, from the data in flight,
404 Chapter 13
rather than going to the trouble of discovering all MAC addresses, passing them around
among the bridges in control protocols, and installing those MAC addresses in the tables
through software. Remember that MAC addresses are a flat address space, and that
unlike IP host addresses and subnets, no summarization can be performed on MAC
addresses to lighten the burden on the control plane. From a practical standpoint, enterprise
users have found that the excess bandwidth consumed by flooding is dampened
by learning faster than software can update forwarding tables from addresses passed
in the control plane.
Pages:
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910