ZyXEL Communications ES-2048 - Manual (Page 55)

ES-2000 Series Switch Support Notes
All contents copyright (c) 2006 ZyXEL Communications Corporation.
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2. Filtering Database Instability:
When multiple copies of a frame arrive at different ports of a switch, the MAC
entry instability in Filtering Database will occur.
1. Host sends an unicast frame to Router (source MAC address is
Host's MAC, destination MAC address is Router's MAC). Both
Switch A and Switch B will receive this frame and learn MAC
address of Host on Port 2.
2. Switch A has not yet learned the MAC address of Router. So Switch
A will flood a copy of the received frame to Segment B.
3. When the copy of the frame from Switch A arrives at Switch B,
Switch B will remove the first entry (Host MAC address on Port 2) in
Filtering Database and add a new mapping of Host MAC address
on Port 1. Switch B incorrectly learn Host MAC address on Port 1.
Switch B can't forward frames properly because the instability of
mapping MAC address to Port.