Posted on October 6, 2016 1:22 pm
 |  Asked by Lukas Hubschmid
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Hello folks,

Following situation:
– 2 datacenters
– 2 spines per datacenter (short name: S1 & S2 in DC1, S3 & S4 in DC2)
– multiple leafs per datacenter connected to both spines via bowtie MLAG
– S1 & S2 forms MLAG pair, S3 & S4 forms MLAG pair
– CWDM: S1 connected to S3, S2 connected to S4 (ring)
– bowtie MLAG between MLAG pairs S1/S2 and S3/S4
– MSTP with 1 MST instance

What is the best practise for configuring MSTP root bridges?
Configuring S1 as primary and S2-4 as secondary? Or S1 AND S2 as primary and S3+S4 as secondary?
As far as I know, in an MLAG pair, the MSTP configuration of the primary MLAG peer (in thise case S1 for S1&S2, S3 for S3&S4) is being used. But what happens if S1 goes offline and I configure S2 as secondary?

Many thanks in advance!

Posted by Sergey Kolobov
Answered on October 14, 2016 3:33 pm

Hi Lukas,

MLAG peers running STP in a way when the active MLAG peer is a root for the STP. MLAG peers are visible to STP as a single switch. Their MLAG System ID (MSI) is the MLAG domain’s MAC-address. The MSI is automatically derived when the MLAG forms and does not match the bridge MAC address of either peer. Each peer uses the MSI in STP.

In case when you have two MLAG peers interconnected between each others, you would have two separate MSTP instances which will figure out between themselves the active topology. In this case if primary of one of the pair goes down, secondary picks up the duty. Despite this, there are multiple scenarios can happened with potential split brain situation. In split brain, either STP can fail to receive BPDUs, because each peer is primary and therefore will not tunnel BPDUs to the other peer. When STP does not receive BPDUs, interesting things can happen, but this is different story. 

The best way to proceed with configuration – is official configuration guide and contact sales engineer for detailed help.


Hope this helps.

Best regards,


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