Cisco Catalyst RSTP support

Unanswered Question
Feb 13th, 2009

From my reading, it appears that the catalysts do not support pure RSTP, can anyone confirm this?

By pure RSTP, I mean plain old 802.1D-2004 compliant behavior. I am looking for a single tree structure so Rapid-PVST is out of the question.

The Catalysts do have RSTP over MST, that works by using instance 0 to maintain a single tree running RSTP. This uses MST BPDU's which wastes bandwidth because they contain extraneous information that the RSTP-only switch does not look at.

Responses appreciated, thanks.

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Giuseppe Larosa Fri, 02/13/2009 - 13:12

Hello Erlend,

the scenario is the one you have described.

Rapid STP is defined as 802.1W

802.1D is the old classic STP.

This makes difficult to perform multivendor tests with switches that support 802.1W as a single instance and not MST 802.1s.

The waste of bandwidth is a relative problem because the records for the other MST instances are not very big.

Actually if the switch thinks to be at an MST region border it can revert back to 802.1D.

The MST BPDU contains the MST region name, the revision number and an hash of the vlans to instances mappings.

So when you look for interoperability with non Cisco switches check that they can support MST 802.1s.

Hope to help


kraptor123 Fri, 02/13/2009 - 14:11

Let's say that we had a network of switches - Cisco and non-cisco's. The non-cisco's are running in 802.1w mode, and the Cisco is running in MST mode.

Assume that one of the Cisco switches is the root bridge. It will continue to send MST BPDU's throughout the lifetime of the network. The MST extension might be only a few bytes long, but isn't that inefficient since it corresponds to wasted bytes every few seconds.

Is there a different approach or any way to send out 802.1w BPDUs from the Cisco?

Giuseppe Larosa Sat, 02/14/2009 - 01:26

Hello Erlend,

>> Is there a different approach or any way to send out 802.1w BPDUs from the Cisco?

As far as I know the only options are proprietary rapid per vlan STP and MST 802.1s (and a pre-standard proprietary called MSTP).

Probably this comes from the fact that all STP modes are multi-instance in Cisco switches.

Hope to help



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