PVST+ and RSTP

[amit-singh]

Hi All,

I am carrying a project where I and migrating lan for one of my customer using 3COM to Cisco. They are using 3Com 7700 as L3 and 4400 as L2 switch which all gets uplink to 7700.I have enabled PVST+ on my cisco 4506 and CST on 4400 amd its working as expected ( in lab env). Today I have enabled RSTP on 3COM 4400 (which is default) and I could see that my root bridge 4506 is still communicating with all the 3COM 4400 switches and working fine.

Question I have: Is PVST+ backward compatible with RSTP.

regards,

-amit singh

[Francois Tallet]

Hi Amit,

The answer to your question implies a combinaison of the two following requirements:

-1-the interaction of PVST (whether it is rapid pvst or not) and a pure IEEE bridge, only running one instance of the spanning tree - I guess that's what the 3com is, I have no experience of this device.

-2-the interaction between RSTP and STP.

-1- can be discussed. Cisco has always been running PVST+, and it seems that it is working correctly with most third party devices. There are some configurations that are not possible though, because we are running an IEEE STP on vlan 1 only (the root bridge for the other vlans are necessarily on a Cisco device for example). Anyway, I guess that if PVST was working with STP, then there should be no issue (again, my ignorance of the 3com is a problem, I guess this CST is STP)

-2-If RSTP on this 3com is the Rapid Spanning Tree of the IEEE, then yes, it is able to fall back in STP mode and interact with Cisco's PVST (again, on vlan 1). RSTP is backward compatible with STP.

The only way to get rid of PVST in a Cisco box is to run MST (and MST is also backward compatible to RSTP and STP).

Hope this helps,

Francois

[amit-singh]

Francois,

Thanks for the reply.

Yes 3Com is running RSTP (IEEE) and I could see that it is working fine with PVST+. I found a DOC as well whihc says that PVST+ is backward compatible with PVST+. Could you please eleborate this " are running an IEEE STP on vlan 1 only " ?? what about the other Vlans.

When any third party switch sends a STP bpdu, it will include all the VLAN ID's in the same BPDU frame.They all will have a common root bridge and I cannot do STP load-balancing as with PVST/PVST+.I think with above statement you mean that Cisco switch will receive the CST BPDU only on Vlan 1 but it will still be a root bridge for all the other Vlans.

Waiting...

regards,

-amit singh

[Francois Tallet]

Hi Amit,

The IEEE STP only runs a single instance for all the vlans -> no vlan load balancing in IEEE STP/RSTP (MST is the only IEEE protocol that provides this option).

PVST+ interact with this unique IEEE STP instance in vlan 1. Meaning that for vlan 1, the root can be anywhere in the network.

For all other instances than vlan 1, the root can only be in a PVST+ area (because there is nobody running STP for this vlan in the IEEE area). For these vlans, the IEEE area is seen as a big shared segment through which PVST+ BPDUs are flooded. I think that's the simpler way of describing the model.

Regards,

Francois