Thursday, October 9, 2008

Spanning Tree Timers

With the same topology as depicted in the Spanning Tree post there is a requirement to influence how long switch 2 takes to failover to its redundant paths to switch 1.

There are a few timers that can be tweaked. The spanning-tree forward-time and max-age timers allow them to be altered on a per VLAN basis.

Max-age is the time a switch waits if no configuration BPDUs are received from the root before entering the listening and then learning states. The default here is 20 seconds. Forward-time is 15 seconds by default for the listening and learning states.

The time it takes a redundant port to become active is equal to max-age plus the listening and learning forward-times i.e 20 + 15 + 15 = 50 seconds.

The minimum max-age is 6 seconds, and the minimum forward-time is 4 seconds. Hence the fastest spanning-tree convergence time that can be achieved by tweaking the timers is 14 seconds.

If faster convergence is required then tweaking the vlan spanning-tree timers is not going to help. In this instance for switch 2 where the are redundant paths to an upstream switch the spanning-tree uplink-fast feature can abe considered. This is not configurable on a per vlan basis but applies to all vlans on the switch.

With uplink-fast enabled if switch 2 detects a problem on the root port it can transition the redundant blocked interface to the forwarding state without waiting for the listening and learning timers. Typically this takes only 3-5 seconds.

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