ZyXEL Communications VES-1616 - Manual (Page 93)

ZyXEL VES-1616/24FA-5x Series Support Notes
All contents copyright 2008 ZyXEL Communications Corporation. 92
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.
z
Host sends a unicast frame to a 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 the MAC address of the host on Port 2.
z 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.
z 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.
z Switch B can't forward the frames properly because the instability of mapping
MAC address to Port.
How STP Works
Spanning Tree provides a loop-free network. When a switch supporting STP
recognizes a loop in the network topology, it blocks one or more redundant ports.