PVST working with RSTP (not per VLAN) in the same LAN

[gerardoceballos]

Hi everyone,

I have to resolve a loop problem in a very bad configured LAN network.

-As an introduction to the LAN:

They have a Cisco 3750 switch as a core and then some Cisco SG-300 and Dlink DGS-1210 as distribution and access switches.

They have different STP "zones" (each one with it´s own root bridge) because they have some trunk ports with STP disabled. I know that in order to make a good configuration of STP I have to carefully reconfigure the entire LAN to have a single STP "zone" with only 1 root switch (I think the 3750 core switch is the best option here).

There are no loops at the moment, but they want to make some  connections that creates loops.

-My specific problem:

They have configured the Cisco 3750 core switch to use PVST (per VLAN STP, with different root priorities per each lan.. etc), but the Cisco SG-300 and DLink switchs doesn´t support PVST and are running RSTP and STP.

The problem is that there are some cases in wich I dont know why there is two different STP Zones. I have checked and in all the VLANS the core is sending BPDUs but not receiving any.  I don´t know why is this happening, I have checked that those trunk ports doesn´t have STP disable.

In one case a Cisco SG-300 has PortFast enabled in the trunk port to the core. Can this prevent the exchange of BPDUs and split the STP into two different zones?

In one case a Dlink does not have STP disable neither running PortFast, but it is not working neither. Could it be that PVST (in the core) and the RSTP(in the Dlink) are incompatible??  (I have checked and in other cases a DLINK and a Cisco SG-300 are negotiating well using RSTP in both of them).

Thanks everyone for the help,

Gerardo

[Peter Paluch]

Hi Gerardo,

The problem is that there are some cases in wich I dont know why there is two different STP Zones.

What exactly do you mean by this? Can you explain in detail how did you come to the conclusion there are two different STP zones in your network? Perhaps some exact configuration information or outputs from show spanning-tree would help.

However, in any case, it is strongly possible that the SG-300 and the D-Link are running only a version of STP or RSTP that does not recognize VLANs and simply derives a loop-free topology for all VLANs at once. In that case, to the Catalyst 3750, the SG-300 and D-Link switches would appear as STP partners in VLAN1 and they would interoperate. For other VLANs, these switches would neither originate nor process any received per-VLAN BPDUs. As a result, the Catalyst 3750 would think it is the only switch in the entire VLAN 2, VLAN 3, etc. This is because the per-VLAN BPDUs are sent to a different destination MAC address than ordinary BPDUs, and for non-PVST switches, such BPDUs appear to be simply some multicasted frames that will be flooded and not processed.

It is very important for you to attempt to make all switches run the same version of STP. Ideally, if the D-Link and SG-300 support the MST, you should migrate your entire network to MST as it would save you lots of troubles you would otherwise have with running different STP versions. So without even going further, please try to find out if all your switches support MST. Configuring it is fairly simple although it requires more pre-planning than PVST.

Best regards,
Peter