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How Does a Switch Knows About MST Boundary Port

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                  

  • LAN Switching and Routing
1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

How Does a Switch Knows About MST Boundary Port

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

1 REPLY
Cisco Employee

How Does a Switch Knows About MST Boundary Port

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

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