How switches detect different mstp regions

[minh_hieu]

Hi community,

It is said that a switch detects if  its neighbor switches belong to the same mstp region by comparing its parameter with the neighbor ones.

"An MSTP switch can also detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (version 3) associated with a different region, or an RST BPDU (version 2)."

I haven't had chance to test but I wonder if 2 switches have the same mstp configuration (mstp revision, name, hello time, max hops, number of instances...), except VLAN mapping to instance (for example only different in one VLAN), do they consider themselves to belong to 2 different regions?

Thanks and regards,

Hieu

[Peter Paluch · Accepted solution]

Hello Hieu,

I haven't had chance to test but I wonder  if 2 switches have the same mstp configuration (mstp revision, name,  hello time, max hops, number of instances...), except VLAN mapping to  instance (for example only different in one VLAN), do they consider  themselves to belong to 2 different regions?

Yes, absolutely, they consider themselves to be in different regions. The VLAN-to-instance mapping is not carried in MST BPDUs, but instead, an MD5-hash is computed over this VLAN-to-instance mapping table, and this hash is carried in MST BPDUs. If these mapping tables differ just in a single mapping, the MD5 hash values will differ as well, and so the MST-speaking switches will understand they are running in different regions.

Best regards,

Peter

[minh_hieu]

Thank Peter.

It is very useful information. Highly appreciate your comment

Br,

Hieu

[Peter Paluch]

Hello Hieu,

It has been a pleasure.

I forgot to mention that if you actually want to see the MD5 digest that is computed from the VLAN-to-instance mapping table, use the following command:

show span mst config digest

On older IOSes, the digest keyword is hidden but nevertheless supported. This command will produce something like:

SW-Dist1#show span mst config digest
Name      [GVOZA]
Revision  1     Instances configured 2
Digest          0xFFD44ED23C057194BC88E858AD0BB32D
Pre-std Digest  0xBD3B76CBB8941D9B51D30DBA3CD5F445
SW-Dist1#

There are usually two digests displayed, one is computed according to an incomplete MSTP specification that Cisco implemented before the specification was finalized (the "Pre-std"), the other is computed according to the current MSTP standard. If only a single hash value is displayed then the switch implements only the older pre-standard MSTP. Using this hash value, you can quickly compare if the two switches are using the same VLAN-to-instance mapping.

Best regards,

Peter