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slow MST handshake?

In an all-Cisco MST environment, will "spanning-tree mst forward-time XXX" affect the speed of the fast switch-to-switch handshake?  Or only affect edge ports (how many seconds to wait, when there is no fast handshake)?

I'd like to slow-down the handshake in an MST environment.  Ideally, it would be nice to have some Event Dampening-like controls on STP state transitions... without EEM scripts.

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Peter Paluch
Hall of Fame Cisco Employee

Hi Bradley,

MST is predominantly event-driven, and the timers are there only to provide backward compatibility with the legacy STP that does not support the Proposal/Agreement mechanism and where the port has to transition through the Discarding/Learning/Forwarding sequence by means of timers. Increasing the forward time is not going to "slow down" the handshaking between switches - the Proposal/Agreement handshake is independent.

Why would you want to slow down the handshake? Can you provide us with a bigger picture of your situation and the reason you believe that slowing down MST would solve your issue?

Best regards,
Peter

Hi, Peter.  This regards a customer's MST network, operating in a ring topology.  Each ring segment iscarried over a metroE point-to-point segment, but without link propagation. 

One segment of this network was mostly down, but working intermittently.  Each time it would start working, MST converged quickly (fast handshake) and traffic was routed over the bad link.  But when the link would quit passing frames again (several seconds later), it seems that reconvergence was slow (max_age timer).  In a babbling condition (e.g. 3 seconds up, 27 seconds down, repeat), you would spend more time blocking, than forwarding.

Ideally, the underlying network should propagate link failure, allowing for sub-second reconvergence on link-down; and also allowing interface event dampening, to prevent east/west flapping.

I can also imagine using EEM to watch for MST instance root transition from east to west, which would could trigger a high port cost on the east-facing port.