ZyXEL Communications ES-2024PWR, ES-2048, ES-2024A How STP works, Switch a Switch B Switch C MAC =

ES-2000 Series Switch Support Notes

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 lowest bridge ID is selected as a designated port.

1. 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. When 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:

Switch A:			Switch B:			Switch C: MAC =
MAC = 00A0C5111111,			MAC = 00A0C5222222,			00A0C5333333
Priority = 32768			Priority = 32768			Priority = 1
	Port 1	Port 2		Port 1	Port 2		Port 1
Cost	19	100	Cost	19	100	Cost	19
Priority	128	128	Priority	128	128	Priority	128

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