Second,
this behavior is normal and desirable when a bridge has only two ports for a given
VLAN, as is the case for most customers in most bridges, even when the customer??™s
EVC has more than two ports. (In Figure 13.12, for example, only one bridge in the
entire network has more than two ports for the customer illustrated.) Third, such behavior
on a continued basis is very unusual; it requires that a unicast ???conversation???
between two customer addresses be purely one-way, or at least that frames in one direction
are emitted no more often than once every 5 minutes, the default timeout period
for MAC addresses. Over the many years of employing bridges in enterprise networks,
network administrators, protocol designers, and users have come to understand the
undesirability of one-way bridged traffic and learned to avoid it.
So why is this behavior used by bridges? Consider the alternative:
?– If a frame with an unknown destination MAC address is not flooded, the only real
alternative is to discard it, until the address can be discovered.
?– Discovering which bridge every customer MAC address is attached to is not trivial.
Although there are special cases, e.
Pages:
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909