Spanning Tree modes

Unanswered Question

We have several 3500XL switches in our network along with newer models capable of running PVRST+.  Of course the 3500XL switches can only run PVST+ mode, which is what all the other switches are curently running.  We would like to switch our environment to use PVRST+.  I realize that we will have to continue running PVST for the 3500XL switches.  Can we run both modes on the newer switches and what problems will we run into as a result?

Thanks,

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Reza Sharifi Thu, 01/07/2010 - 19:22

Hi,

It looks like you still have the same 3 options which are MST, PVST and Rapid-pvst and by enabling rapid-pvst you also enable PVRST.

So Rapid-pvst and PVRST are the same.

Management-Switch(config)#spanning-tree mo ?
  mst         Multiple spanning tree mode
  pvst        Per-Vlan spanning tree mode
  rapid-pvst  Per-Vlan rapid spanning tree mode

Management-Switch(config)#

HTH

Reza

Thanks for the reply, but that is not it.  I realize that pvrst and Rapid pvst are the same.  However, the 3500xl switches will not support pvrst, but a 3750 and a 6509 does.  If the 6509 is connected to some 3750 switches and 3500xl switches, can I enable pvrst and still be compatible with the 3500 xl switches?

Thanks,

Jon Marshall Fri, 01/08/2010 - 00:00

[email protected]

Thanks for the reply, but that is not it.  I realize that pvrst and Rapid pvst are the same.  However, the 3500xl switches will not support pvrst, but a 3750 and a 6509 does.  If the 6509 is connected to some 3750 switches and 3500xl switches, can I enable pvrst and still be compatible with the 3500 xl switches?

Thanks,

Yes, you can run them both together. As Jerry says, Rapid PVST+ is backwards compatible.

The main issue to be aware of is that any vlan that exists on your 3500xl switches will have to use the standard 802.1d timers ie. it gets no benefit from rapid pvst+. So lets says you had 5 vlans in your setup.

1) vlan 5, 6 are on 6500 and 3750

2) vlan 7, 8 are on 6500, 3750 and 3500xl

3) vlan 9 is on 3500xl

1) will use rapid pvst+

3) will obviously not use rapid pvst+ ie. it will use standard 802.1d timers

2) is the key one. for these vlans also it will use standard 802.1d timers ie. it cannot use the much shorter rapid pvst+ timers.

So the key thing to do is to make sure that you only allow the vlans that have to be on the 3500xl switches ie. clear all vlans that are not needed on the 3500xl switches off the trunk links. If you don't and just allow all vlans then you will see no benefit for any of your vlans.

Jon

Giuseppe Larosa Fri, 01/08/2010 - 00:13

Hello Ancarr,

Rapid PVST is backward compatible on a per port basis with PVST+.

The newer switches will be using RPVST. Rapid PVST uses a newer version of BPDU and they can detect a legacy neighbor when receiving the BPDU from the neighbor.

Formally a switch can run in a single STP mode so we cannot say new switches can run both PVST+ and RPVST, but there is per port backward compatibility.

It is very important that trunk ports towards C3500XL are configured to allow only the necessary vlans, the vlans used on them

This can be done on both sides of the trunk using

switchport trunk allowed vlan x,y,z

In this way, vlans that are not extended to C3500XL can take benefit from RPVST as explained by Jon.

Hope to help

Giuseppe

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