Tag Archives: backbone fast

Spanning Tree enhancements (Backbone Fast)

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Last time I look at the spanning tree enhancment I covered uplink fast, this is for detecting when a directly connected root port fails and switching over to a back up in the shortest time possible. But what happens if the link that fails is not directly connected. When a switch loses its link back to the root and needs to find an alternate path back. In the digram below switch B is blocking its port to Switch A to prevent loops.

The question is what happens if the link between Switch A and the Root fails? Well with out backbone fast the following sequince takes place.

When the link fails Switch A will no longer be receiving BPDU’s from the root, the direct link is down and the port on switch B is blocking so not forwarding BPDU’s.

Switch A will assume it is the new root and start to send BPDU’s towards Switch B declaring it is the root. However Switch B will see these are inferior BPDU’s to the on it has stored for the port connected to Switch A and ignore them.

This will continue to happen until the BPDU on the port times out, after which the port will go in to the listing and learning state before starting to forward. This is 20 seconds (max age timer) plus 2 x 15 seconds for the listing and learning stage. so a total of 50 seconds.

The idea behind Backbone fast is to cut this by 20 seconds by bypassing the max age timer. The idea is that if Switch B can confirm it still has a link back it’s current known root switch, then it can ignore the max age timer and start the listing and learning process on a port immidatly it receives a inferior BPDU.

Once backbone fast is enabled, when a switch receives a inferior BPDU on one of its ports, it will send a RLQ (root link query) packet out all it’s non designated ports including its root port (so all ports that lead back to the root). If it receives a RLQ response (these are sent from the bridge) then it knows it still has a link to root. It can then age out the port it is receiving the inferior BPDU’s on and start the listing learning stages. If it does not receive any responses then the switch has lost connectivity to the rest of the network and needs to start recomputing the whole STP.

Either way the max age time has been eliminated and 20 seconds have been shaved of the re convergence / fail over time.

Just like Uplink fast Backbone fast is configured on a switch level with the following command.

Switch(config)#spanning-tree backbonefast

and it needs to be configured on all switches on the network.

CISCO’s document HERE explains it in much more details and more examples.

DevilWAH

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Spanning Tree enhancements (Uplink fast)

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In my last job, I jumped straight in to configuring Rapid spanning Tree, I mean what is the point of running Standard STP with its 50second fail over times, when you can enable Rapid-STP and gain sub second fail over??

Well if you want to pass your CCNP SWITCH you need to know it, and you need to know how to configure the enhancements. Actually having read through them and labed them up. They do help in understanding how STP works and how the original protocol was improved in a number of way, before CISCO took all the enhancements and came up with Rapid-STP.

Over the next few post I will be covering all of the basic enhancements, including uplinkfast, backbone fast, portfast, loopguard etc..

Uplinkfast.

This is normaly configured on access switchs that have two links back to the root, in these cases after the initial STP algrothem has run, one of the ports (lowest priority back to the root bridge) will be designated as the root port, while the other will be blocked. See digram below.

Now with standard STP, if the active link fails, the switch sees the root port link has fail and as it is receiving root BPDU’s on the backup blocked port it starts to bring this up. However with out uplink fast enabled this requires the port to go through the listening and learning stages. By default this is 30 seconds of outage, and even with best STP tuning it still results in a 14 second outage.

However with uplink fast configured the switch keeps track of the blocked ports that point back to bridge and forms them in to an “uplink group”. Now if the primary link goes down the switch can pick the next best root port and immediately places it in the forwarding mode as this will not be creating a loop. This creates an almost instant fail over of the primary link. However switch CAM tables will now be out of sync, which could result in frames being sent down the wrong links. To sort this out, the switch creates dummy frames with source address from its CAM table, and destination of multicast address. this updates the other switches on the network.

Now when the link comes back up, the switch waits twice the forward delay + 5 seconds before it switches back over. This allows the core switch at the other end of the link to have time to run through STP and start forwarding on the port.

And that’s Uplink fast. Providing a method to allow instant fail over of directly redundant links towards the root.

Configuration is very simple and is carried out in global config mode.

Switch(config)Spanning-tree uplinkfast

DevilWAH

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