Not correct MSTP BPDU by Cisco 2950

[antonertelecom]

IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA8a, RELEASE SOFTWARE (fc1)

This switch send out not correct MSTP Bpdus.

While CIST information seems to be OK, MST Extension has another &

unknown format.

The MST extension length field in the Cisco BPDU is in the wrong location:

it occupies the same location as "MST config ID format selector" defined in the 802.1s.

It seems, that the 16 bytes configuration digest in the BPDU from Cisco

does not follow the recommendation of the 802.1s. They have different

values compared with those value shown in 802.1s

Mst extension of BPDU which sended by Cisco 2950:

MST Extension, Length: 0 (Malformed: number of MSTI messages!!! Must be 64)

MST Config ID format selector: 64

MST Config name:

MST Config revision: 0

MST Config digest: BB3B6C15EF8D089BB55ED10D24DF44DE

CIST Internal Root Path Cost: 2147483660

CIST Bridge Identifier: 34217 / 21:00:00:00:00:00

CIST Remaining hops: 20

The same problem was with cisco 3750. But it was solved after IOS change (C3750ME Software (C3750ME-I5-M), Version 12.2(25)SEG1).

Please solve this problem for all your products. If you know about problem, Why does users have to note you?

[Francois Tallet · Accepted solution]

Cisco shipped MST before it was an IEEE standard. The MD5 digest in the MST BPDU depend on a key that was not published before the standard was out. So we basically knew that we would not be able to create a BPDU that was compliant to the standard and this explains why we did not even bother cleaning up the format of the BPDU which is slightly different. This allowed Cisco customers to benefit from MST almost 2 years before it was available on most competitor's platforms.

This pre-standard version won't be able to form a region with a standard switch, but it can still interoperate using the CIST with standard MST or RSTP.

The latest versions of the IOS do support MST in its standard format (I think some platforms will only be able to run the pre-standard because they don't have enough RAM to run the latest IOS on which standard MST is implemented).

All this information is available on CCO.

Regards,

Francois

[antonertelecom]

Thank you for your exhaustive response. Please counsile me. My network consists of Cisco 3508 (only pvst) as root, Cisco 2950 (mst pre-standart,pvst,rapid-pvst) and Dlink DES-3526 (stp,rstp,mstp). Which spanning tree standart can I use in my network?

[Francois Tallet]

Hi!

I'm sorry to tell you that, but if you're not in the worst case scenario, that's probably close to it:-(

If you want to be as close to the standard as possible, run mst (even pre-standard) on the 2950 and RSTP or MSTP on the DLink (because the 2950 is prestandard, you will not be able to form a region with the DLink, which is why running RSTP or MST will not make any difference).

The interaction Cisco MSTP/PVST is relatively inconvenient, specially if the PVST switch is the root. Check http://www.cisco.com/warp/public/473/147.html for an overview.

If you want to do some load balancing, I would probably suggest you run rapid-PVST on the 2950 and again RSTP on the DLink.

Hope this helps a little bit, if it's not really comforting...

Regards,

Francois