MSTP traffic interruptions.

[Ridma Ransara Jayaweera]

Hi All

we have 3750G stack connecting to H3C switch cluster connecting through two trunk portchannels.

and we have MSTP configured since H3C support only MST.

MST is configured as below.

instance 1 1,168,178,198

instance 2 148,158,188

instance 1 LANS are extended to H3C,instance 2 VLANS are local to 3750 stack.

As a new requirement we need to add a another VLAN to this topology.and we are planing to add this VALN to instance 1

can there be any traffic interruptions.

pls comment.

Thanks

[Peter Paluch · Accepted solution]

Hello Ridma,

You should declare a maintenance window for this change because there will be transient outages in the connectivity. The reason is that as soon as you add a new VLAN to the instance 1 on a switch, the MSTP region configuration on that switch will change and will be different from the region configuration on other switches until you configure all switches similarly. As long as there are differing MSTP region configurations, there are multiple regions in the network, and the convergence on a common spanning tree interconnecting these regions will cause the transient outages I have been talking before. You would avoid the outages only if you were able to activate the updated MSTP region configuration on all switches at the same time instant which is clearly not possible.

It is recommended to pre-deploy MSTP by mapping VLANs into individual instances even before these VLANs exist. For example, you could map all VLANs in the range 1-500 into instance 1 and VLANs in the range 501-1000 into instance 2 (just an example). This does not mean you are creating them, you are just saying which instance they belong into if they are ever created. Afterwards, even if you create a VLAN that did not exist before, the MSTP configuration does not change and there will be no outage in the network.

Best regards,

Peter

[Ridma Ransara Jayaweera]

Thank you very much for the explanation sir.