Spanning tree calculation

[dharminderrana]

Hi Friends,

My query is " if i have 4 switches A, B, C and D and no hardcore spanning tree priority is configured for Vlan X and Y.

But for vlan X and Y switch A is root bridge by default spanning tree calculation. Now i want to configure switch A as root bridge and switch B and backup root bridge by configuring spanning tree priority. So in this case, i am making switch A a root bridge for vlan X and Y by spanning tree priority command so will spanning tree recalculate again and traffic will impact on vlan X & Y or not.

Thanks

[Kevin Dorrell]

Yes, I believe it will recalculate.  The bridge ID is a concatenation of the priority and the MAC address of the switch.  So as far STP it is concerned, this is a new root bridge.

Kevin Dorrell

Luxembourg

[dharminderrana]

Hi Kevin,

Thanks for your reply. So if i change the priority on switch B, C and D higher then default priority on Switch A in that will it impact the whole spanning tree or only on switch B, C and D.

Thanks

[Kevin Dorrell]

If you raise the priority on B, C and D so as to ensure that A stays root, then there should be no re-calculation because the root bridge stays the same.

Kevin

[Peter Paluch]

Hello Kevin,

Raising the priorities on B, C and D may actually be accompanied by a localized STP recalc because, depending on the topology, the choice of root ports or designated ports on the neighbors of these switches is dependent on the sending bridge priorities as well. Whether this will be the case really depends on the exact topology and perhaps even on the sequence of modifying the priorities on individual switches.

Best regards,

Peter

[Kevin Dorrell]

Hello Peter,

You are absolutely right ... sorry, I took my eye off the ball there.Of course, the downstream switch does use the upstream forwarding switch's ID as a tie breaker between equal cost root port candidates.  In many cases that will not make a difference because there will already be a port that wins outright because of the root cost.  But yes, it could go to the tie breakers: forwarding switch id, port priority on neighbor, and internal port number.

Kevin