Monday, January 31, 2011

CCNP - ENTERPRISE - Spanning-tree Protocol Link Convergence Tools

The following tools allow for faster link convergence in relation to STP:
1) Portfast - Applied on access switch ports linked to host devices (not other switches). Transitions the port to a forwarding state straight away instead of transition through the STP states Listening (15 secs), then Learning (15 secs), and finally forwarding. Not to be used on trunk links as it could result in loops occurring.

Applied globally via:
sw1(config)#spanning-tree portfast default

Or per interface:
sw1(config-if)#spanning-tree portfast

Portfast is also applied using the macro interface command:
sw1(config-if)#switchport host

2) UplinkFast - Applied to access switches, allows fast fail-over when dual uplinks are connected to a distribution layer switch. Can incorporate multiple redundant links to an uplink switch (not just 2 links), where there are more than 2 redundant links to a given switch the link with the next lowest root path cost is unlocked  immediately.

UplinkFast bypasses the Listening and Learning stages.

Note that the command is not allowed on a Root bridge as UplinkFast ensures that the local switch does not become the Root bridge. UplinkFast raises the Bridge Priority to 49,152 therefore it is unlikely the switch would become the Root bridge.

Applied globally via:
sw1(config)#spanning-tree uplinkfast

3) BackboneFast - Enabled on ALL switches in the topology, allows for fast convergence to the network backbone or core after an STP topology change. It shortens the STP convergence times by actively determining whether an alternate path exist to the Root bridge when a link goes down. It uses Root Link Query (RLQ) protocol to detect if the upstream switches have a connection to the Root bridge.

BackboneFast shortens the Max-age timer which in turn can reduce convergence times from 50 secs to approx 30 secs.

Applied on All switches in the topolgy using:
sw2(config)# spanning-tree backbonefast

Verify using;
sw2#sh spanning-trree backbonefast

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