Is cisco's MST compatible with the standard 802.1s?

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Mar 31st, 2008
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Hi, all

I tried to connect cisco 2950 and 4006 with other vender's device, say hp procure switch. but MST seems not compatible to each other. 2950 works fine with 4006, but treated hp's box as rstp neighbor.

i did google this forum and found some conversation about it. i just wonder it is a bug or not. if is, has it been fixed already?


#sh ver

Cisco Internetwork Operating System Software

IOS (tm) C2950 Software (C2950-I6Q4L2-M), Version 12.1(22)EA2, RELEASE SOFTWARE (fc1)

Copyright (c) 1986-2004 by cisco Systems, Inc.

Compiled Sun 07-Nov-04 23:14 by antonino

Image text-base: 0x80010000, data-base: 0x8055E000

ROM: Bootstrap program is C2950 boot loader

j2950 uptime is 3 weeks, 1 day, 9 hours, 58 minutes

System returned to ROM by power-on

System image file is "flash:c2950-i6q4l2-mz.121-22.EA2.bin"

cisco WS-C2950-24 (RC32300) processor (revision J0) with 21055K bytes of memory.

Processor board ID FOC0711Z13P

Last reset from system-reset

Running Standard Image

24 FastEthernet/IEEE 802.3 interface(s)

32K bytes of flash-simulated non-volatile configuration memory.

Base ethernet MAC Address: 00:0D:BD:01:AC:00

Motherboard assembly number: 73-5781-11

Power supply part number: 34-0965-01

Motherboard serial number: FOC07291R9C

Power supply serial number: PHI072206LY

Model revision number: J0

Motherboard revision number: A0

Model number: WS-C2950-24

System serial number: FOC0711Z13P

Configuration register is 0xF

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nambi_gct Tue, 04/01/2008 - 07:40
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In my experince it is fully complaiant with the standard.Have you configured the name, revision and instance consistently.Only if the MST region of the neighbor is different this will happen.

kangzhen Tue, 04/01/2008 - 09:39
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I configured the same region name on all boxes, create 2 instance and assign several vlan on each of them.

i can verify the configure by sniffer the STP packet. only difference between them is MST config digest and config id format selector. i can find the command to change that "format selector" and just wondered if it will affect MST behavior?


Francois Tallet Tue, 04/01/2008 - 12:29
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Cisco shipped MST before the standard was released. Your device is probably running this "pre-standard" version. I think 12.1 IOS only has the pre-standard version and some switches cannot upgrade to 12.2 and will be limited to this pre-standard version.

"Standard" Cisco bridges (12.2 and beyond afaik) are able to form regions with pre-standard ones. Third party bridges (or more broadly, switches that only implement the pure IEEE standard) won't be able to form a region with pre-standard Cisco boxes. You will be limited to RSTP use between those (you lose the ability to form a region, which means that you cannot do load balancing based on instances).



kangzhen Fri, 04/04/2008 - 17:01
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thanks for your information.

since i don't have ver 12.2, I just tried to compare the "config digest" between 2 boxes.

even remove all vlan from instance and let all VID map to CIST, cisco's config digest does not match IEEE's standard (refer to IEEE 802.1s, table 13-2), but hp's box does.



Francois Tallet Fri, 04/04/2008 - 17:09
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Yes, and that's pretty much why you won't be able to form a region between a third party standard and a Cisco pre-standard box. You can only be part of the same region if the region name, revision number and configuration digest match. The key used to generate the configuration digest was released by the IEEE just when the standard was published (precisely for the purpose of preventing pre-standard implementation). Cisco provided its pre-standard MST several months before this event... Note that there are few other subtle differences between our pre-standard BPDU and the official one.

Anyway, as a result, pre-standard Cisco and third party standard switches can only share the CIST information. That's what's happening when the regions are different. So you can still run MST between the two types of device, but it's going to fall back to RSTP capability in the way that you can only use a single instance between them.



kangzhen Fri, 04/04/2008 - 18:00
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thanks again, Francois.

one more question is since cisco claimed 802.1s is supported long time ago, if we wanna purchase some box, how can i tell if pre-standard or real -standard 802.1s is supported? Because we need them to talk with other vendor's box.



Francois Tallet Fri, 04/04/2008 - 18:11
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Hi Zhen,

It's not really the platform but the IOS version that you need to check. The problem is that some old switches cannot be upgraded to a release that supports MST. As long as you can run at least 12.2, you are fine.



kangzhen Mon, 04/21/2008 - 13:42
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Finally I got the box running 12.2. pls refer to the below info for details.


Cisco Internetwork Operating System Software

IOS (tm) s72033_rp Software (s72033_rp-PSV-M), Version 12.2(18)SXD7b, RELEASE SO

FTWARE (fc1)

Technical Support:

Copyright (c) 1986-2006 by cisco Systems, Inc.

Compiled Fri 08-Dec-06 12:38 by ccai

Image text-base: 0x4002100C, data-base: 0x42040000

ROM: System Bootstrap, Version 12.2(14r)S9, RELEASE SOFTWARE (fc1)

BOOTLDR: s72033_rp Software (s72033_rp-PSV-M), Version 12.2(18)SXD7b, RELEASE SO

FTWARE (fc1)

sw6509-bb2 uptime is 6 weeks, 4 hours, 7 minutes

Time since sw6509-bb2 switched to active is 6 weeks, 4 hours, 6 minutes

System returned to ROM by power-on (SP by power-on)

System image file is "sup-bootflash:s72033-psv-mz.122-18.SXD7b.bin"

cisco WS-C6509 (R7000) processor (revision 2.0) with 458720K/65536K bytes of mem


Processor board ID SCA042802D2

SR71000 CPU at 600Mhz, Implementation 0x504, Rev 1.2, 512KB L2 Cache

Last reset from power-on

X.25 software, Version 3.0.0.

Bridging software.

24 Virtual Ethernet/IEEE 802.3 interface(s)

144 FastEthernet/IEEE 802.3 interface(s)

66 Gigabit Ethernet/IEEE 802.3 interface(s)

1917K bytes of non-volatile configuration memory.

8192K bytes of packet buffer memory.

65536K bytes of Flash internal SIMM (Sector size 512K).

Configuration register is 0x2102


after enable the MST, i still got the wrong message digest which is same as 12.1.

so is that build not NEW enough?

Thanks again


Francois Tallet Mon, 04/21/2008 - 13:53
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I thought you had some 29xx that were in 12.1?

Do a "show span summary". It should be clearly stated that this release is in an "IEEE standard" mode. I'm confident you'll get your sup720 to run MST standard!



kangzhen Mon, 04/21/2008 - 14:35
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I had c2950 but can't find 12.2 for that platform.

below is the output of the command "sh span summary"


sw6509-bb2#sh spanning-tree summary

Switch is in mst mode

Root bridge for: MST03, MST05

EtherChannel misconfig guard is enabled

Extended system ID is enabled

Portfast Default is disabled

PortFast BPDU Guard Default is disabled

Portfast BPDU Filter Default is disabled

Loopguard Default is disabled

UplinkFast is disabled

BackboneFast is disabled

Pathcost method used is long

Name Blocking Listening Learning Forwarding STP Active

---------------------- -------- --------- -------- ---------- ----------

MST00 0 0 0 105 105

MST03 0 0 0 4 4

MST05 0 0 0 4 4

---------------------- -------- --------- -------- ---------- ----------

3 msts 0 0 0 113 113


thanks for ur info.


kangzhen Mon, 04/21/2008 - 15:26
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12.2(18)SXD7b is the latest one i can find so far.

i will let you know as soon as i got the chance to test the latest image you mentioned.

thanks anyway.


Francois Tallet Mon, 04/21/2008 - 16:28
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Sure. But don't take the one specified in the release notes, take the latest!




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