[c-nsp] Spanning tree compatibility with RSTP

Phil Mayers p.mayers at imperial.ac.uk
Tue Nov 28 11:20:44 EST 2006

Collins, Richard (SNL US) wrote:
> Hello,
> I have some switches which are to be connected with some third-party
> equipment which runs only classical RSTP.  
> What is the recommended procedure - go to mst mode and set all vlans to
> one MST instance and match the timers with the third party RSTP?  Can
> one still run with multiple MST instances?

Frankly, this is a tedious procedure on ciscos. The STP support on 
Foundrys and Extremes is far better.

What you would do in an ideal world (on one of those boxes) is run the 
STP on the native/untagged vlan in 802.1d/w mode, run *no* STP on the 
tagged vlans, and bind the tagged vlans forward/blocked state to the 
untagged vlan.

Cisco are you hearing this - I want:

spanning-tree vlan 456 follow 123
! This feature already works for the vlan numbered "1"
spanning-tree vlan 123 ieee-pdus

In the absence of that, we went for the following situation:

  1. Run 802.1d/w on the downstream switches, but NOT on the ports 
facing the 6500, because it's not needed. This prevents front-panel 
ports looping vlan 123 -> 456 and PVST shutting both those vlans down on 
the link to that switch

  2. Run PVST on the vlans as *well* and have them pass transparently 
through the 1d/w cloud. This permits the switch to be dual-attached to 
that vlan.

This works very well, however we have pretty simple edge switches (3Com 
4400s - surprisingly good boxen) which don't "see" the PVST at all. As 
other posters have said, some boxes that *think* they can talk PVST 
can't and it can get messy.

MST is an option, however we couldn't use it because one can only be a 
member of *one* MST domain on the ciscos, which frankly sucks.

I would be interested to hear how other people have handled this, in 
particular the single MST process hassles.

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