3. When switch B receives a copy of the frame through switch A on segment 2, switch B
also forwards a copy of the frame to segment 1 if it cannot locate an entry in the MAC
address table for Router Y.
4. Router Y receives a copy of the same frame for the second time.
A loop-avoidance mechanism eliminates this problem by preventing one of the four
interfaces from transmitting frames during normal operation, thereby breaking the loop.
MAC Database Instability
MAC database instability results when multiple copies of a frame arrive on different ports
of a switch. This subtopic describes how MAC database instability can arise and explains
what problems can result.
Figure 2-20 illustrates this problem: switch B installs a database entry, mapping the MAC
address of host X to port 1. Sometime later, when the copy of the frame transmitted through
switch A arrives at port 2 of switch B, switch B removes the ?¬?rst entry and installs an entry
that incorrectly maps the MAC address of host X to port 2, which connects to segment 2.
Server/Host X Router Y
Unicast
Segment 1
Segment 2
Unicast
Unicast
Switch A Switch B
Improving Performance with Spanning Tree 49
Figure 2-20 MAC Database Instability
Depending on its internal architecture, the switch in question may or may not cope well
with rapid changes in its MAC database.
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