User guide

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Spanning Tree Protocol (STP)
In order to build a robust network, it is necessary
to include certain levels of redundancy within the
interconnections between switches. This will help to
ensure that a failure of one link does not lead to a
complete failure of the whole network.
The danger of multiple links is that data packets, especially
multicast packets, become involved in continual loops as
neighbouring switches use the duplicated links to send and
resend them to each other.
To prevent such bridging loops from occurring, the
Spanning Tree Protocol (STP), operating at layer 2, is
used within each switch. STP encourages all switches
to communicate and learn about each other. It prevents
bridging loops by blocking newly discovered links until it
can discover the nature of the link: is it a new host or a
new switch?
The problem with this is that the discovery process can
take up to 50 seconds before the block is lifted, causing
problematic timeouts.
The answer to this issue is to enable the portfast variable
for all host links on a switch. This will cause any new
connection to go immediately into forwarding mode.
However, take particular care not to enable portfast on
any switch to switch connections as this will result in
bridging loops.