Spanning Tree Basics

mikedelafield · ‎02-29-2008

For a basic Spanning-Tree setup is this a good starter...

Designated root bridge commands;

spanning-tree mode pvst

spanning-tree vlan xx priority 4096

And all other (non-root) bridges commands;

spanning-tree mode pvst

spanning-tree extend system-id

I know other things like UDLD and BPDU Guard are useful, but for a basic mesh setup are the above commands enough to be going on with and avoid Loops?

Jon Marshall · ‎02-29-2008

Hi Mike

I generally do the following.

1) Select 2 switches to be spanning tree root primary and spanning tree root secondary.

2) On primary root -

spanning-tree vlan xx priority 8192

3) On secondary root

spanning-tree vlan xx priority 16384 - where vlan xx is the same vlan number as in 2).

If all your switches support Rapid-pvst I would run this instead of pvst.

If you use the extended system-id use it on all the switches including the root and secondary.

HTH

Jon

mikedelafield · ‎02-29-2008

thanks for your reply.

i'm still not sure on the extended system-id. exactly what its for and relates to?

practically speaking; when is it used and required?

i have read the page below but i'm still not sure.

http://www.ciscosystems.com/en/US/docs/switches/lan/catalyst4500/12.2/25ew/configuration/guide/spantree.html#wp1044354

Edison Ortiz · ‎02-29-2008

The system-id, along with the switch priority, and the allocated spanning-tree MAC address, enables to make the bridge ID unique for each Vlan in order to avoid a tie between 2 switches when electing the root for a Vlan.

For instance, if you configure a switch as a root for a Vlan and another switch as a root for the same Vlan, the switch with the lower MAC address will win the election process due to its system-id information.

HTH,

__

Edison.