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 first entry and installs an entry that incorrectly maps the MAC address of host X to port 2, which connects to segment 2.

Figure 2-20 MAC Database Instability

Server/Host X

Unicast

Server/Host X

Unicast

Port 1 Switch A

Port 2

Router Y

Port 1

Segment 1

Unicast

Switch B ^

Port 2

Segment 2

Depending on its internal architecture, the switch in question may or may not cope well with rapid changes in its MAC database. Again, a loop-avoidance mechanism eliminates this problem by preventing one of the four interfaces from transmitting frames during normal operation, thereby breaking the loop.

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