MST/PVST Boundary

Answered Question
Aug 3rd, 2007
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I have an MST region with istance 1 mapped to vlans 1-500. The MST boundary switch has an interface which connects to another switch running PVST.


My question is, when a BPDU is sent from within the MST instance 1 for VLAN 45 (for example), will the PVST region recieve BPDUs for just VLAN 45 or for all VLANs mapped to the MST instance (1-500)?


I am confused by this line in the 6500 series command reference guide;


"The ports that belong to the MST switch at the boundary simulate PVST+ and send PVST+ BPDUs for all the VLANs."


Any help would be much appreciated!


Thanks,

Tom.

Correct Answer by Francois Tallet about 9 years 11 months ago

Hi Tom,

The Cisco MST switch detects neighbor running PVST as soon as it received a PVST BPDU. Then, it replicates the BPDU for instance 0 (the CIST) on each an every vlan configured on the port. This is a boundary port, meaning that only the CIST information is relevant (your MST instance 1 information will not be used). So on the PVST switch, each vlan receives a BPDU coming from the MST bridge, and each vlan instance receives information from MST instance 0.

HTH,

Francois

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Chris Jones Fri, 08/03/2007 - 04:37
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An mst region appears to the rest of the network as ONE bridge, so the switch that connects mst to pvst will send a bpdu representing the entire region.

reglend-xhango Mon, 08/06/2007 - 05:24
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It is better for you to Switch only to PVST+

because if there is a loop on MST Region it will block all VLAN's for PVST+.

Correct Answer
Francois Tallet Mon, 08/06/2007 - 10:17
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Hi Tom,

The Cisco MST switch detects neighbor running PVST as soon as it received a PVST BPDU. Then, it replicates the BPDU for instance 0 (the CIST) on each an every vlan configured on the port. This is a boundary port, meaning that only the CIST information is relevant (your MST instance 1 information will not be used). So on the PVST switch, each vlan receives a BPDU coming from the MST bridge, and each vlan instance receives information from MST instance 0.

HTH,

Francois

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