Design Question: MST vs. RSTP

Unanswered Question
Sep 29th, 2009

I'm building out a new pre-production environment, and had planned on using Rapid-PVST on all switches, since all will be Cisco and that's what I've used in the past. However, the Load Balancers will be F5. In the event that the F5 loses a port, I would like to them run STP and switch to the blocking link rather than trigger a failover between the BigIP units.

I recently ran in to an issue where the number of VLAN instances exceeded the capacity for the line card. This environment will have about 50 VLANs trunked to 40 access switches. While I could work around the issue with filtering, just using MST might make more sense.

So what are the pros / cons of MST? I would think:

Pros

- Fewer STP instances (and lower CPU)

- Standard rather than Cisco Proprietary

Cons

- More complex to configure

- More complex to troubleshoot

Have I missed anything?

I have this problem too.
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Giuseppe Larosa Tue, 09/29/2009 - 21:12

Hello John,

MST requires a change at the planning level.

Most of threads about MST report the problem of making changes: adding a Vlan to an instance has an impact that has to be considered: for a while devices with mismatching parameters divide the MST region in two regions.

Cisco expert Francois Tallet has made a useful suggestion to build an MST network without this issue:

take the 4096 Vlan space and divide it in 64 subsets.

Associate each subset to an MST instance.

The point is that this can be done before creating L2 vlans so it can be once.

When a new vlan is needed pick a vlan-id from the subset that is associated to the MST instance you want to use.

It is enouogh to create the Vlan (for example on a VTP server) and to have it propagated on trunks.

No real change is needed at the MST level and this eliminates the impact of modifying it.

So we can say MST requires a different approach.

Hope to help

Giuseppe

xcz504d1114 Wed, 09/30/2009 - 06:07

Giuseppe did an excellent job in explaining MST and possible design approaches.

The biggest problem working with MST is going to come from user error. The MST name, revision number, instance mappings, VLAN names, everything has to match exactly. Unfortunately this is all done manually on every single device.

VTPv3 is available on some higher end devices that actually allows the MST information to be propogated through VTP.

You can also run RPVST on your cisco devices, and MST on your F5's, they should detect the boundary mismatch and default the boundary to a CST instance (Cisco will, I'm making an assumption on the F5).

I run MST on my network because of the number of spanning-tree instances, I break my VLAN's into several instances, but there is no reason why you can't have everything default into instance 0 and have a single instance, UNLESS you have a need to manually split traffic directions which is rare, or if you want "Core A" to be the SPT root for VLAN X, and "Core B" to be SPT root for VLAN Y, but again, that is rare to see.

HTH,

Craig

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