While many bridges are able to break
the network into segments and to translate VLAN IDs as frames move from segment
to segment, such techniques are prone to misconfiguration and provide only a modest
increase in the number of VLANs effectively available. See ???Backbone Bridges??”MACin-
MAC,??? for a more complete solution to the VLAN problem.
Spanning Tree Convergence Time As network size increases, the time required for
the spanning tree algorithms to converge can increase. In particular, a ring topology,
which is often the topology imposed on a provider by the available physical links, is
slow to converge. The basic problem is the ???counting to infinity??? problem inherent in
any distance vector routing algorithm such as RIP (IETF RFC2453, STD0056) or the
Multiple Spanning Tree Protocol. Rings almost invariably invoke the ???chatty timer???
that suspends the operation of the protocol for 1 sec. This allows the spanning tree
protocol to converge more quickly, with less network disruption than otherwise would
happen, but the invocation of this timer necessarily results in a 1-sec interruption in
connectivity.
Furthermore, topology changes in networks with rings often trigger ???sync events,???
where each bridge must cut off connectivity with each of its neighbors until the two
Ethernet Bridging 395
agree that they understand the new network topology.
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