Spanning-tree mode with 2 links

[sebastien3]

Hello,

I have two identical links with Q-in-Q support on 3750X. I want to create path redundancy if one of the links breaks.

Link 1 should always have priority for VLAN50 and should only go through link 1, never link 2.

The two links should pass VLANs 10,20 and 30 and avoid making a network loop.

What is the best solution for speed of convergence + redundancy ?

spanning-tree mode rapid-pvst + spanning-tree backbonefast

Thanks !

[balaji.bandi]

If you make the respected switches for the VLAN to be Root bridge for the STP to be Priority it will take that path as best, if that path not available it will be take other path. Rapid STP wil have faster convergency for sure.

You only see the Loop for 10,20, 30 (since 50 have only one path and allowed in that Link1)

So for 10,20,30, Link2 will be STP Priority and Root bridge

For VLAN 50 Link 1 will be Root bridge / Priority

.

[sebastien3]

@balaji.bandiOK !

So I leave my configuration with only rpvst and do not add spanning-tree backbonefast.

Thanks !

[Georg Pauwen]

Hello,

in order to make Vlan 50 always go through link 1, on the interface that belongs to Link 1, configure:

spanning-tree vlan 50 priority 0

For the other Vlans, if you leave everything at the default spanning tree settings, either link will be selected, without creating a loop.

Backbone Fast needs to be enabled on all switches in order to be effective, it is (obviously) designed to detect indirect link failures. It has nothing to do with link preference.

[sebastien3]

Hello @Georg Pauwen 

Thank for spanning-tree vlan 50 priority 0

Backbone Fast needs to be enabled on all switches in order to be effective, it is (obviously) designed to detect indirect link failures. It has nothing to do with link preference.

Yes ... In my case the two Q-in-Q links go through the network of an ISP which I do not have access...

What is your opinion on this configuration ? I have to activate backbone fast ?

[Giuseppe Larosa]

Hello @sebastien3 ,

if you are using Rapid PVST you don't need backbone fast as this was a Cisco feature introduced for Cisco PVST.

Actually Rapid PVST implements the equivalent of backbone fast by default it is part of the standard and as noted by @Georg Pauwen  backbone fast was and is just a way to detect indirect failures.

The SP on the path will not interact with your STP it will likely use p2p L2 transport services or VPLS.

Hope to help

Giuseppe

[sebastien3]

Hello,

I have a question about the convergence time.

If I cut link 1, it takes about 40 seconds for traffic to flow to the second link.

Why RSTP is not faster in seconds (2 to 5 seconds?)

Thanks for your help