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RSTP and MSTP Diameter

With 802.1d, max number of bridges from host-host was seven. This seemed mainly due to the fact of the amount of time it took to propogate BPDU from root to edge bridge to ensure current BPDU information on edge switch did not age out.

Given that 802.1s handshakes BPDU information every 2 seconds (default) between switches and no longer uses the max-age information, does this mean that the diameter can be safely increased? The capability to now configure max hop count for all instances including IST would seem to suggest this is the case.



Community Member

Re: RSTP and MSTP Diameter

You should be able to increase the diameter without any problems.

Community Member

Re: RSTP and MSTP Diameter

With 802.1s the default number off Hops from the root is set to 20.

The number off hop is reduced by 1 at each bridge.

The max number of hops can be 40 !!

See this URL :

MST Configuration

Message Age and Hop Count

IST and MST instances do not use the Message Age and Maximum Age timer settings in the BPDU. IST and MST use a separate hop count mechanism that is very similar to the IP TTL mechanism.

You can configure each MST bridge with a maximum hop count.

The root bridge of the instance sends a BPDU (or M-record) with the remaining hop count that is equal to the maximum hop count.

When a bridge receives a BPDU (or M-record), it decrements the received remaining hop count by one.

The bridge discards the BPDU (M-record) and ages out the information held for the port if the count reaches zero after decrementing.

The nonroot bridges propagate the decremented count as the remaining hop count in the BPDUs (M-records) they generate.

The Message Age and Maximum Age timer settings in the RST portion of the BPDU remain the same throughout the region, and the same values are propagated by the region's designated ports at the boundary.

Best regards

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