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New Member

Spanning tree issue

I have two Core Switches and all the access switches are connected to both the core switces. Per VLAN spanning tree is configured where in certain VLAN goes through Core switch 1 and Certain VLAN go through Core Switch 2. When i reboot core switch 1, then i could see only momentary disconnection, but when i reboot core switch 2 the connectivity is going down and coming back after the core switch 2 comes back up. The traffic is not switching to core switch 1. The Spanning tree summary is fine and having the correct information for the VLAN. Now how can i troubleshoot to identify the problem?

Thanks in Advance


Re: Spanning tree issue

Hi Friend,

Can you confirm that the vlans whose root bridge is Core 2 are also carried/allowed in the trunk ports between the access switch and Core 1?

I know you must have done that but just to recheck once.

Also is it possible for you to paste the config of spanning tree root bridge of core 1 & core 2 and also do you have a trunk lin kbetween core 1 and core 2?



New Member

Re: Spanning tree issue


Thanks for your mail. I am not sure whether this is a natural behaviour, but in the access switches when i give sh spantree vlan no, ideally i should get one port in blocking and one port in forwarding and for some vlan i could see 30 ports in forwarding state. Those are the ports that are configured for that particular vlan where end systems are connected. This is not common for all the vlan. Certain vlan shows only one in blocking and one port in forwarding.

Thanks in Advance


Re: Spanning tree issue

Hi janakiraman,

Yes itzzz natural behaviour, Since u told that 30 ports are connected to end systems it will be in forwarding nature only. For those end systems that is the only way to reach ur other part of the network. So it is quite natural that all of them are in forwaring state.

Rate if it does,


Re: Spanning tree issue

Focus on a unique host in a vlan that is experiencing loss of connectivity as a result of switch 2 going down.

Tell us:

- to where connectivity is lost (ie is the host you're trying to reach attached directly to switch 2 for instance).

- what are the state and roles for the ports on this vlan when the problem occurs (looking into an access switch where the problem is occurring is enough, + information on switch 1 for this vlan).