Spanning Tree

Unanswered Question
Jan 4th, 2010

I have two 4000 switches currently are configured as a root bridge for 10 vlans.  VLAN 100,200,6,106 etc.  I will be connecting a 6509 switch with 2 Port Channels (4 physical ports) to (2) 4000 switches as Layer II trunks, and will also configure the above VLANs in 6509.  6509 will act as an internim switch for couple of months since we are in process of redesigning our data center.  How do I  make sure that 6509 doesn't become a root bridge for the above VLANs to avoid spanning tree loop.  As far as I am concerned, the 6509 will not become a root bridge if the priority for the above VLANs are lower on 4000 switches.

What do I need to configure on Port channel ports to make (1) port channel connecting to 4000 (A) switch as a forwarding path, and the other Port Channel connecting to 4000(B) switch as a backup path.


I have this problem too.
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Marwan ALshawi Mon, 01/04/2010 - 23:59

lets say you have vlan 100 and 200

switch A is the root for these vlans ( lower priority)

switch B is the secondary next lower priority

you can do

switch A

spanning-tree vlan 10,200 root primary

switch B

spanning-tree vlan 10,200 root secondary

in the switch 6500 make the priority of the above vlans to the highest in this case you will make sure that this switch will not become a root as long as Switch A or switch B is up

also int the L2 channels add the fllowing command in Switch A and Switch B going to the 6500 switch

spanning-tree guard root

this will disable the interface of switch A and B to the 6500 in case of switch 6500 is elected as root for any vlan

once you have Switch A the root for the required vlans and switch B  the secondary root

the path will be through switch A and through switch B will be in blocking ( back up ) from the 6500 switch prespective,   this is default and you do not need any change to STP costs

good luck

if helpful Rate

KARUPPUCHAMY MA... Tue, 01/05/2010 - 00:05


As far as I am concerned, the 6509 will not become a root bridge if the priority for the above VLANs are lower on 4000 switches.  --No. It is not correct.

The lowest priority wil become the root bridge.In your scenario, we have to increase the priority value of 6509 switch and ot should be greater that 4500 that 6509 switch will not become a root bridge.

Have you inter-connected these 4500(A) and 4500(B).Need more info.

Because, if you have connected these 4500(A) and 4500(B), then make 4500(A) as a root bridge for all the 10 vlans then, the port-channel from 6500 to 4500(A) will be primary path and 4500(B) will be secondary path.

Karuppuchamy CCIE(R&S),CCSP

abbas.ali Tue, 01/05/2010 - 00:26

Thanks guys!  Yes the two 4000 switches are connected together as a Layer II.


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