Mixing PVRST and RSTP

Unanswered Question
Oct 16th, 2009

Hi all, we have 2 Cisco 6509 Sup720 core switches configured with PVRST and 2 3Com edge switches that support only one instance of RSTP for all vlans. What will happen if I connect them together? Does the 3Com switches will send a BPDU on each VLAN and our Cisco switches update their tree or could we expect to have convergence problem?

Thanks!

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Giuseppe Larosa Fri, 10/16/2009 - 10:48

Hello,

only one STP instance can be used, the suspect is that both may fall back to 802.1D on the common link

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

they should be able to detect they are both using 802.1W because version field=2 on these BPDUs.

So I guess a single rapid instance can exist.

Then as it happens with PVST+ Rapid PVST can tunnel its BPDUs in proprietary multicast frames for all non native vlans on the trunk in order to try to reach another Cisco switch downstream the 3COm

PVST+, RPVST+4

0x010b

01-00-0c-cc-cc-cd

see

http://www.cisco.com/en/US/products/hw/switches/ps700/products_white_paper09186a00801b49a4.shtml#cg5

your best move should be to leave the 3Com at the access layer.

MST cannot help because those devices miss the region concept and cannot be accepted as peers in an MST region.

Hope to help

Giuseppe

Francois Tallet Fri, 10/16/2009 - 17:14

The interaction here is between cisco proprietary PVST style RSTP and IEEE standard RSTP. What's going to happen is that vlan 1 in Cisco RPVST switches is going to interact with 3com devices (running full RSTP). Then, as Giuseppe said, all the other RPVST intances are going to treat the 3com devices as a big hub and flood their BPDUs over them. That's going to lead to sub-optimal topology and convergence.

However, moving the Cisco devices to MST really makes sense in that case. RSTP is designed to interact with MST's instance 0. So basically, by moving the Cisco devices to MST, you're going to run RSTP (real IEEE RSTP this time) across the whole network. The only drawback is that RSTP only runs a single instance and there will be no load balancing possible -> however convergence should be fast.

So you have the choice:

-1- some form of load balancing be slow convergence

-2- no load balancing but full RSTP benefit.

I'd definitely pick -2-.

Regards,

Francois

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