How Stp Works - ZyXEL Communications VES-1616 Support Notes

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3. When multiple ports have the same path cost to root bridge, the port with
lowest port priority is selected as root port.
3. Select a designated port on each segment
For each LAN segment (collision domain), there is a designated port. The
designated port has the lowest cost to the root bridge. Designated ports are
normally in the forwarding state to forward and receive traffic to the segment. If
more than one port in the segment have the same path cost, the port on which
bridge has the lowest bridge ID is selected as a designated port.

How STP works

After STP determines the lowest cost spanning tree, it enables all root ports and
designated ports, and disables all other ports. Network packets are therefore only
forwarded between root ports and designated ports, eliminating any possible
network loops. STP-aware devices exchange Bridge Protocol Data Units (BPDUs)
periodically. Whenever the bridged LAN topology changes, a new spanning tree is
constructed.
Once a stable network topology has been established, all bridges listen for Hello
BPDUs (Bridge Protocol Data Units) transmitted from the root bridge. If a bridge
does not get a Hello BPDU after a predefined interval (Max Age), the bridge
assumes that the link to the root bridge is down. This bridge then initiates
negotiations with other bridges to reconfigure the network to re-establish a valid
network topology.
For example:
All contents copyright 2008 ZyXEL Communications Corporation.
VES-1616/24FA-5x Series Support Notes
94