spanning tree between cisco and nortel switches

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Feb 22nd, 2009
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Hi all, with cisco I know they use pvst, however we need to connect my cisco switches with some nortel 470 and 5510 switches, the only thing is, the nortel use the same spanning tree instance for all vlans, if I trunk between the switches what will happen? and what should I do ?

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naveen_b81 Mon, 02/23/2009 - 01:05
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If you ensure that you have a single uplink between the cluster of cisco to cluster of nortel siwthces, then u should not face any problem. However if you want to be dead sure, you can run MST with instance on the cisco switches to work with the Nortel switches.

carl_townshend Mon, 02/23/2009 - 03:31
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hi there, thanks for the replies, would it not be easier to create an instance of spanning tree per vlan on the Nortel switches? they all run dual uplinks to the distribution layer and we need the stp.

I also wanted to run rpvst on my cisco devices, Am I right in saying that when the cisco switch sees an old stp bpdu it will switch back to normal stp just for that link ?

milan.kulik Mon, 02/23/2009 - 05:16
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AFAIK, low-end Nortel switches support only 8 STP instances. So in theory, you could configure an STP instance per VLAN, if you use only 8 VLANs.

But you would still be in trouble, as Cisco is using a proprietary MAC destination address and BPDU format extensions.

I've got a feeling Nortel enables configuring the destination MAC, but I'm not sure if BPDUs would be accepted on the Cisco trunk side.

In any case, this would be a challenge and I'd recommend studying Nortel config guides carefully first.

If I remember correctly, high-end Nortel switches provide "Cisco like" PVSTP mode, but I'm afraid it's not your case.

You could also use mono-STP (mapping all VLANs STP to VLAN1 STP instance) in Nortel LAN part, but it's still a little dangerous as Cisco STP might get confused by Cisco BPDUs flooded by Nortel switches (as not recognizing the special Cisco MAC adrress, they would treat them as any other multicast).

See "BPDU Formats" in

for more details.



carl_townshend Tue, 02/24/2009 - 07:01
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Hi there, After speaking with some consultants today, they say we can use MST between the cisco and nortel devices.

I have never used MST before, I have read about it however. Can you please clarify how this will work for me. Can I still use a mixture of rstp for my cisco devices and use MST to my nortel devices, what will the config look like?

many thanks


PETROBRASEISA Thu, 02/26/2009 - 10:39
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Hi Carl

I had the same problem, but with Nortel 450. In my case, Cisco's Switches only accepted Nortel Cluster as the root, when I configured MST on they. To know more about MST read the Paper from Cisco "Understanding Multiple Spanning tree Protocol (802.1s)" ID:24248. It helped me a lot.


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