Posted on October 5, 2015 11:47 am
 |  Asked by Victor
 |  920 views
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Hi,

I have a question regarding running mlag between two switches and having isolated Vlans per switch. If I am running mlag do I need to define the same vlan on each switch even if I am not going to use the vlan on the neighbor switch? I am asking from a mlag perspective.

Thank you,

Victor

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Posted by Mark Berly
Answered on October 5, 2015 11:54 am

Technically defining a VLAN on only one of the MLAG peers IF the hosts are only local to the switch, it is the MLAG primary and there are no other hosts using that VLAN on the peer switch will work.

Operationally I would discourage you from doing this at some point in the future a host or redundant link from a host will get plugged into the MLAG peer switch and you will spend the next 4 hours debugging.

I have found that in general consistency in configuration will save you many headaches.

NOTE: the STP agent is only active on the MLAG primary, which is why you need to define the VLAN on it for the VLAN to be active

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Posted by Victor
Answered on October 5, 2015 11:58 am

I ask because I currently have a setup where I have to switches in a mlag domain. And unless I define the same vlan on whichever switch is the primary switch from an mlag perspective the secondary switch will not put the ports into a forwarding state. I understand your point regarding keeping general consistency in configuration. However, sometimes you just don’t want to add extra configs to a device because it can also cause confusion. Thank you

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Posted by Mark Berly
Answered on October 5, 2015 12:04 pm

You hit on the exact issue with not having a consistent configuration between two devices. The exact operational model is based on which code version you are running as MLAG has evolved over time as well all of the coding and associated testing revolves around the idea that you will have the configurations in sync. By not defining the VLANs on both of the MLAG peers you are asking for problems, maybe now or maybe in the future…

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Posted by Victor
Answered on October 5, 2015 12:38 pm

Okay, got it. Thank you for your help.

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