Question re MST and Trunks

[srowles]

Hi

I am configuring a LAN which consists of 6500´s at the core and 3750´s at the access layer. I am using MST as the spanning tree protocol.

I have just read the following recommendation in a Cisco document with relation to MST.

"Ensure that trunks carry all of the VLANs mapped to an instance or do not carry any VLANs at all for this instance."

I was intending to clear unused VLANs from trunk links using the "switchport trunk allowed vlan " command.

Can anyone confirm that this should or should not be done when using MST.

Thanks in advance.

[Kevin Dorrell]

That can be done, but carefully.

You can clear any of the completely unused VLANs.

You can also clear used VLAN that you don't want on that trunk, BUT if you do so, you should clear all the VLANs in that MST instance. That is, you should never have the situation where a trunk carries only some of the VLANs in that particular instance. You should clear the whole instance set from the trunk, or not at all.

Hope that helps.

Kevin Dorrell

Luxembourg

[srowles]

Hi Kevin

Thanks for your reply.

Just to confirm my understanding - In the network that I am configuring there are around 100 VLANs and 2 MST instances. I have configured 2 instances of MST to allow load balancing across 2 uplinks that are connected to each stack of access-layer switches. Each stack however only serves approximately 10 VLANs. From what I understand I should not clear any of the VLANs from the uplinks (which are trunk links) as this would mean that only a subset of the VLANs for a particular instance would be allowed across each trunk. I therefore need to allow all VLANs on all trunk links even though only 10 are actually required for each stack.

My MST config actually looks like this:

Instance Vlans mapped

-------- -------------------------------------------

0 1-7,201-207,251-399,403-410,412-499,700-706,751-799,851-4094

1 8-200,208-250,400-402,411

2 500-699,707-750,800-850

-----------------------------------------------------

I am currently allowing all VLANs on all trunks except for VLAN 1 as I have set the native VLAN to 999 for security purposes. Taking into consideration your comments, I guess that I should actually not allow all VLANs that are in instance 0 as I am breaking the rule buy only disallowing 1 VLAN (Vlan 1) from instance 0. Either that or allow VLAN 1 so that all VLANs in instance 0 are allowed.

I hope you understand what I´m getting at.

Thanks again for your help.

Steve

[Kevin Dorrell]

That is correct. But AFAIK, if you find that a particular stack only has VLANs in instance 2, then you could clear all the VLANs of instance 1 from the trunk quite safely, but none of the VLANs of instance 2.

But when it comes to instance 0, I hesitate to answer. I wouldn't risk it, because that would imply that VLAN 1 gets cleared, and I couldn't say whether the behavior would be good. Can anyone else come in and clarify that?

Kevin Dorrell

Luxembourg