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MST interaction with RSTP

Hi in the attached diagram where the core network runs MST with no vlans in MST0. It has 2 indipendant client networks connecting to the MST network with dual links one client network using vlan100 which in the MST network is mapped to MSTI1 and client 2 using vlan 2 mapped to MST2 in the MST network. My question is if client 2 experiences say a link flap on the active link will this have any effect on client 1s network.

Kind Regards

Kev

4 REPLIES

Re: MST interaction with RSTP

Yes, the topology change indication that is likely generated by the link flap will go through the MST region. If the RST region is a third party IEEE bridge btw, this will have an impact on all the vlans (as it is running a single instance of RSTP for all the vlans). If it is a region running Cisco's Rapid-PVST, the topology change will be limited to the instance in the MST region to which the vlan is mapped.

Note that a topology change indication is not an STP reconvergence: just an indication to flush the cam entries of certain ports.

Regards,

Francois

New Member

Re: MST interaction with RSTP

Thankyou for the below, does this meen that nothing will be effected in the MST region as its using MST Intance 0 but if client 1s link flaps then the cam table will be flushed in vlan 200 for client 2 but vlan 200 within the MST region will be un-affected as only MST 0 would be effected. All the connected clients are running rapid spanning tree/PVST.

Kind Regards

Kev

Re: MST interaction with RSTP

Hi Kev,

No actually, I meant the contrary;-)

In the best case, only the instance to which vlan 200 is mapped will be affected (which means that several vlans mapped to this instance will be flushed). Else, all the instances (thus all the vlans) will be flushed.

This is necessary to prevent black holing. If for instance an uplink of your RSTP region was going down, you would need to update the cam entries in the MST region so that the mac addresses of the stations connected in the RSTP region now point to the backup uplink, not the one that went down.

Regards,

Francois

Re: MST interaction with RSTP

I realized I did not read your question accurately. What I say about the topology change in the MST region also applies to the remote RSTP region.

Regards,

Francois

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