How Does a Switch Knows About MST Boundary Port

[daudparvez]

Hi All,

I have read in ine MST tutorial that a switch does not exchange M-Records out the Boundary Port, i.e, a pot connected to either another MST Region or an STP/RSTP switch.

My question is that how does a switch knows that it's certain port is Boundary Port ? As far as i guess, initially a switch will emit MST BPDU with M-Records, but when it recieves MST BPDU from another Region with a different Region Name, it will mark it's port Boundary and will cease sending M-Records out this port.

Can someone please explain it a bit more ????

Regards,

Daud Parvez                  

[Peter Paluch · Accepted solution]

Hello Daud,

You are very correct in your understanding. The switch will indeed start by sending MST BPDUs on all its Designated ports. However, if it receives a MST BPDU that has either a different region name, revision or the MD5 hash of the VLAN-to-instance mapping table, or that is not a MST BPDU at all, the switch will mark the receiving port as a boundary port, knowing that the neighboring switch is configured differently.

Now, if the neighboring switch runs MSTP but in a different region, the switches will continue to send MST BPDUs including the M-records. However, knowing that the neighbor is in a different MST region, both switches will process only the CIST parts of the MST BPDU, ignoring other M-records.

If the neighboring switch runs a different STP version, the MST switch will fall back to using that version on the appropriate port.

Please feel welcome to ask further!

Best regards,

Peter