Problem with switches in MST domain

Answered Question
Apr 19th, 2012
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Dear Experts,


I just created a scenario in Switches where few switches are in MST domain



I make one switch ROOT-A as primary for vlan from 1 to 200 and switch ROOT-B as prmary for all other


Is it normal that a PC in access switch connected with VLAN 108 uplink to ROOT-B will not forward the traffic to GW router


regards

Haris P

Correct Answer by Jan Hrnko about 5 years 4 months ago

Hi Haris,


I believe that I might know the root of your problem.


The thing is that your MST tree is built independently of how your trunks are configured. What I mean is that altough you disallow some VLANs on trunk - MST can still select these links for forwarding.


Switch ROOT-A is for VLAN 108. That's fine.

So I believe that the problem in your network lies in this:

The link from ROOT-A to ROOT-B is picked for forwarding frames for instance 1 according to MST. But VLAN 108 is dissalowed on this link.

MST blocks link from ROOT-B to Switch beneath GW because of higher cost of link from ROOT-B to ROOT-A (I suppose you have all links of the same cost and in this scenario ROOT-B would need to go through the unnamed Switch beneath GW and then to ROOT-A which costs more than is the cost of direct link). You always choose the lowest cost route towards root bridge.

But you have disallowed VLAN 108 on the link that is in MST unblocked.


So you have to either reconfigure MST according to what you want to achieve

OR

allow VLAN 108 on link between the Switches ROOT-A and ROOT-B.

This is the link that MST for instance 1 (where VLAN 108 resides) haven't blocked.


The other problem you have - with VLAN 350 is the same problem as desribed above but from the other side. Also - allow VLAN 350 onlink between the Switches ROOT-A and ROOT-B.

In this case MST tree for instance 2 has blocked the link between ROOT-A and unnamed switch. The reason is same as for VLAN 108.


Best regards,

Jan

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Hitesh Vinzoda Thu, 04/19/2012 - 04:54
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Hi,


Do all the downstream switches and the switch connected to gateway router agrees on the root placement? perform below command verify if everyone agrees on the root for each instance and mark the blocked ports in your topology and then trace visually the path from end host to the gateway. This should provide you about whats going on.



Thanks

Hitesh


Please rate useful posts.

Correct Answer
Jan Hrnko Thu, 04/19/2012 - 07:07
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  • Silver, 250 points or more

Hi Haris,


I believe that I might know the root of your problem.


The thing is that your MST tree is built independently of how your trunks are configured. What I mean is that altough you disallow some VLANs on trunk - MST can still select these links for forwarding.


Switch ROOT-A is for VLAN 108. That's fine.

So I believe that the problem in your network lies in this:

The link from ROOT-A to ROOT-B is picked for forwarding frames for instance 1 according to MST. But VLAN 108 is dissalowed on this link.

MST blocks link from ROOT-B to Switch beneath GW because of higher cost of link from ROOT-B to ROOT-A (I suppose you have all links of the same cost and in this scenario ROOT-B would need to go through the unnamed Switch beneath GW and then to ROOT-A which costs more than is the cost of direct link). You always choose the lowest cost route towards root bridge.

But you have disallowed VLAN 108 on the link that is in MST unblocked.


So you have to either reconfigure MST according to what you want to achieve

OR

allow VLAN 108 on link between the Switches ROOT-A and ROOT-B.

This is the link that MST for instance 1 (where VLAN 108 resides) haven't blocked.


The other problem you have - with VLAN 350 is the same problem as desribed above but from the other side. Also - allow VLAN 350 onlink between the Switches ROOT-A and ROOT-B.

In this case MST tree for instance 2 has blocked the link between ROOT-A and unnamed switch. The reason is same as for VLAN 108.


Best regards,

Jan

Haris P Sun, 05/06/2012 - 11:18
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Dear Jan ,


Thanks for the Info .


Actually over the trunk ,i should allow all VLANS and it solved the issue



regards

Haris P

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