Rapid-PVST+ and RSTP

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Jun 3rd, 2009
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Hi,


We have a core that runs Cisco switches but the access-layer switches use Linksys. Although the high-end Linksys switches. Currently, Cisco is running PVST and the Linksys runs STP. We would like to switch it to RSTP. The Linksys switches support RSTP but Cisco only supports Rapid-PVST+. How is this going to work?


I am thinking that the STP for the VLAN 1 will be the one used by the Linksys switches. If coreA and coreB is connected by a trunk then the Linksys is connected to both coreA and B, if coreA is the RB for VLAN1, then the connection of Linksys to coreA will be the RP for all VLANs since it doesn't run Rapid-PVST. Correct me if I'm wrong.


Now, I'm thinking that my other option is MST since the Linksys switch supports MST. This way, I can support RSTP. Is this a good solution?


Thanks in advance.


John

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Giuseppe Larosa Wed, 06/03/2009 - 09:06
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Hello John,


>> Now, I'm thinking that my other option is MST since the Linksys switch supports MST. This way, I can support RSTP. Is this a good solution?


This is the best solution.

Notice that to be in same MST region the following must match:

MST region name (string)

MST revision number (number)

vlans to MST instances mapping (an hash of this table is sent in the BPDU if it doesn't match the switches see each other as in different regions)


MST requires careful planning: the above parameters have to be manually configured in all switches (no automatic propagation).


A useful advice is the following:

decide a Vlan plan that includes all the possible 4094 vlans.

You can map subsets of them to each MST instance.

That is you map already Vlans that are not in use.

If you do so you do the job once.


When a new Vlan is needed you decide what topology should have = you pull a vlan X from the subset associated to MST instance y.


In this way your network is ready to grow and you avoid all the management overhead of MST.


see


http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.2SXF/native/configuration/guide/mst.html


Be aware that some cisco switches actually support a pre-standard version sometimes called MSTP so you need to check the effective support of MST on all cisco switches.


Edit:

about RPVST and 802.1W some years ago a collegue of mine was not able to make it working. I'm afraid they can both fall back to legacy 802.1D.

It is better to use MST in your case.


Hope to help

Giuseppe


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