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MSTP - RSTP interop on Ring

I have RSTP rings consisting of Alcatel 7450's and Foundry FES switches.  I want to insert Cisco ASR901's into these rings and have been testing in the lab.  It appears that the ASR901's only support MSTP in the EVC configuration.  So I would prefer to keep RSTP on the Alcatel and Foundry's and enable MSTP on only the ASR901's. 

Topology is:

7450-2 <=> Foundry-2 <=> ASR901-2 <=> Foundry-1 <=> ASR901-1<=> 7450-1 with a VPLS between the 7450's.

If I enable MSTP in the Foundry's, Alcatel and ASR901 everything works fine.  If I keep RSTP on the Foundries and 7450's, then STP also works, with the exception of a Foundry-2 switch running RSTP, that is located between 2 ASR901's running MSTP.  This switch does not seem to interact with the other switches (i.e. thinks it is the root bridge).

Since there are mulitple MSTP boundary's, do the ASR901's need to be in different IST's/instances?


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MSTP - RSTP interop on Ring


based on the concept that MST is based on RSTP and all instances of MST only work inside their region, the only instance that interacts with outside is the instance 0 , IST is responsible for all the bdpu transimission and interactions with outsiders and it works automatically. it is advised NOT to map any vlans to IST (instance 0) in an MSTP environment.

you set the instance - vlan mappings throughout the network and it should be fine, with backward compatibility.

Hope it Helps,


Hope it Helps!

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MSTP - RSTP interop on Ring

Thanks for the reply.  I did try changing both ASR901's to MST instance 1 and mapping vlans 2-4094, then I turned on RSTP in the Foundry's and Alcatel nodes.  With this config, RSTP didn't seem to work in any node.  They all thought they were the root bridge.

When I previously used MST instance 0 in the ASR's (all other nodes running RSTP), only the Foundry that was located between the 2 ASR's incorrectly thought it was the root bridge and all other nodes recognized the correct root bridge.

I haven't tried it yet, but I am wondering if each ASR needs to be in a different MST instance since there are multiple boundary's between MSTP and RSTP.

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