MSTP and Redundant crossover ports with different priority

Unanswered Question
Feb 27th, 2010

Hello,

Currently I have rpvst+ spanning tree setup when I have devices connected to the switch that bridge 2 vlans.

For example i have vlan 10 on wan interface and vlan 20 on other lan interface of the devices.

When that devices dies both vlans are bridged because devices has a buildin passthrough in case of power or device failure.

This is ok with rpvst+.  In addition to above i have 2 ports:

one assigned to vlan 10

and one to vlan 20

and there is phisical cable that connects them.

Those ports are used to bridge vlan 10 and 20 in case the above device is completely dead and the passthrough does not work.

To accomplish this i set different priority for those ports to allow forwarding incase of above failure.

set spantree portpri    1/-2  48

visually it looks like this

vlan10---------|   lan  |

cross      --- |     vlan10

through   --- |     vlan20

vlan20---------| wan  |

Above works great with rpvst+ but now i want to switch to mstp and cross connect cables like above would create the loop for mstp so they would go to blocking state because there is only one set of bpdu exchagned so when it sees vlan 10 and 20 on the same instnce it goes to blocking.

I could create separate instance for those vlans but it would only work with no extra cross over connection.

Can something like above be accomplished using mstp?

I have this problem too.
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Ganesh Hariharan Sat, 02/27/2010 - 23:30

Hi,

It will be helpful if you can share your requirement in diagramtic view to us so that we can guide or suggests as per your reuirement.

Ganesh.H

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