Sunday, November 20, 2011

Spanning Tree Design



Optimizing STP topology.

In the first topology an Access Layer switch is designated as a root bridge for the entire network.

Choosing a good root bridge should be based on:

  • Performance
  • Redundancy
  • Packets should travel from an access layer switch to a distribution layer switch to a core layer switch without the packet traversing more then one of each switch at the same layer.
  • Should be at or near the network center

The RB is elected by the lowest BID which in turn is determined by the Brdige Priority and the lowest MAC address. This election is not always optimal.


In this picture PC-6 is pinging DC-1. Switch ASW-1 is the RB. It is also an Accesss Layer switch.

The pachet will follow a normal path to DC-1. As it is, this behaviour is ok but what happens when the link ASW-2 to DSW-1 fails ?


Here we have a problem.

Because DSW-2 has no direct connectivty to DSW-1 nor to CORE-2 (it’s ports are in blocking state), the only path that a packet could take is via ASW-1, which is not very efficient.

Without a doubt the Root Bridge has to change.

How to find out the RB in the network:

Since, STP is running on all switches it does not matter on which switch you will issue this command:

switch#show spanning-tree

    Spanning tree enabled protocol ieee
    Root ID Priority 32769
      Address 0005.5E0B.7123
      Cost 19
      Port 1(FastEthernet0/1)

This confirms that the RB is ASW-1.


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