802.1q trunk and bpdu

Answered Question
Sep 26th, 2008
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hi every body!A

A switch is connected to another switch by 802.1q trunk.

Does switch send only bpdu belonging to stp instance, native vlan mapped to?

thanks a lot!

Correct Answer by Jon Marshall about 8 years 10 months ago

Which makes a lot of sense because 3rd party switches do not support PVST so they have a CST (Common Spanning Tree).


Thanks for the info Sarah - rated.


Jon

Correct Answer by Istvan_Rabai about 8 years 10 months ago

Hi Sarah,


The switch sends bpdus for each vlan that is configured and not pruned from the trunk with the "switchport trunk allowed vlan" command.


Cheers:

Istvan

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Correct Answer
Istvan_Rabai Fri, 09/26/2008 - 10:15
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Hi Sarah,


The switch sends bpdus for each vlan that is configured and not pruned from the trunk with the "switchport trunk allowed vlan" command.


Cheers:

Istvan

sarahr202 Fri, 09/26/2008 - 11:02
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i find out the following:

if both switches i.e sw1 and sw2 are cisco switches, then switch will send bpdu for native vlan, bpdu for each vlans

If cisco switch is connected to third party switch, then only bpdu belonging to native vlan is sent.

Thanks a lot !

Correct Answer
Jon Marshall Fri, 09/26/2008 - 11:03
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Which makes a lot of sense because 3rd party switches do not support PVST so they have a CST (Common Spanning Tree).


Thanks for the info Sarah - rated.


Jon

milan.kulik Wed, 10/01/2008 - 06:06
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Hi,


I'm afraid this is incorrect.

Cisco switch is not able to detect if the neighbor switch is a Cisco one or a third party one.

Cisco switch will send BPDUs for all VLANs from a trunk port if PVSTP+ configured.


See BPDU Formats section in

http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml#stp


BR,

Milan

Jon Marshall Wed, 10/01/2008 - 06:20
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Milan


If a cisco switch is connected via an 802.1q trunk to a non-cisco switch it sends an IEEE BPDU on the native vlan, ordinarily vlan 1 and then sends SSTP (Shared Spanning-tree) BPDU's for each other vlan. The non-cisco switch cannot interpret these SSTP BPDU's and merely forwards them.


So strictly speaking you are correct (and rated :) ). What i was trying to get across was that what Sarah was seeing was logical in that she would only see the untagged IEEE BPDU for the native vlan.


Hope i've made sense.


Jon



milan.kulik Wed, 10/01/2008 - 06:45
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Hi Jon,


the CCO document says:

"PVST+ interoperates with 802.1Q mono Spanning Tree through the so-called Common Spanning Tree (CST) over an 802.1Q trunk. The CST is always on VLAN 1, so this VLAN needs to be enabled on the trunk to interoperate with other vendors. CST BPDUs are transmitted, always untagged, to the IEEE Standard Bridge-Group (MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42). For completeness of description, a parallel set of BPDUs are also transmitted to the Cisco shared Spanning Tree MAC address for VLAN 1."


This leads me to a consequent question:

"What happens, if there is following chain of switches connected by trunks:

Cisco1---ThirdParty---Cisco2

?"

Let's say Cisco1 sends BPDUs on the trunk to the ThirdParty.

ThirdParty receives the IEEE BPDU for VLAN1 (native by default). It understands it, increases the BPDU cost and sends a IEEE BPDU to Cisco2.

But ThirdParty also received a Cisco shared Spanning Tree BPDU for VLAN1.

It doesn't understand it, so simply forwards to Cisco2.

What happens at Cisco2? It received two BPDUs for VLAN1, each with different cost!!

So which one does it choose?


And if we use another VLAN as the native one (or even a different one on each trunk) the question becomes even more complicated :-((


Do you know an answer to my question?

I plan to build a lab "when I have some time" :-)


BR,

Milan




Francois Tallet Wed, 10/01/2008 - 08:33
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The Cisco bridge always ignore the BPDUs for vlan 1 sent to the SSTP address. Vlan 1 only uses BPDUs sent the the IEEE address.

The SSTP BPDU sent on vlan 1 is mainly used to help detecting native vlan inconsistency as far as I remember.

Regards,

Francois

milan.kulik Thu, 10/02/2008 - 11:49
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Hi Francois,


you are right.

100% precise would be saying "native VLAN" instead of "VLAN 1".

I found a document describing in details:

http://www.cisco.com/en/US/docs/ios/12_1t/12_1t3/feature/guide/dtbridge.html#wp1020686


Also this one is helpful:

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00801d11a0.shtml


Thanks and best regards,

Milan


Francois Tallet Thu, 10/02/2008 - 13:15
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Nope, I stand for what I said. The SSTP BPDU for vlan 1 is sent on vlan 1. It will be tagged if vlan 1 is not the native vlan. The IEEE BPDU for vlan 1 is always sent untagged (I don't like the term native vlan, which is a Cisco invention).

Regards,

Francois

milan.kulik Fri, 10/03/2008 - 01:46
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Hi Francois,


you are correct.

I think I understand it completely now :-)


What makes me still a little confused is the sentence "The CST BPDU (of VLAN 1, by default) is sent to the IEEE address." I found in

http://www.cisco.com/en/US/docs/ios/12_1t/12_1t3/feature/guide/dtbridge.html#wp1020686


Does it mean it's possible to use a different VLAN than VLAN1 for CST?


Thanks,

Milan




Francois Tallet Fri, 10/03/2008 - 08:52
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Hi Milan,

No, it's a default and cannot be configured. Initially, in Cisco's implementation, vlan 1 could not be disabled or even removed from trunks. That's why it was given this special role of being the CST.

Regards,

Francois

Jon Marshall Thu, 10/02/2008 - 13:21
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Milan


Just to add to Francois post, the link i sent describes this perfectly - see Case 1 and Case 2.


Jon

Jon Marshall Wed, 10/01/2008 - 10:21
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Milan


Have a look at the PVST+ explained document - about 2/3 of the way down. It covers exactly what happens when cisco switches are connected to non-cisco switches.


http://blog.internetworkexpert.com/category/ccie-routing-switching/bridging-switching/


From the perspective of the non-cisco switch it only really sees the BDPU for the CST, the other BPDU's it doesn't interpret. I think this is what Sarah was getting at :)


Jon

milan.kulik Thu, 10/02/2008 - 11:57
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Hi Jon,


thanks for the link!

I was more interested in a Cisco switch perspective - more complicated.

I found some explanations on CCO in the meantime, noticed in my reply to Francois.


Thanks anyway,

Best regards,

Milan

Jon Marshall Thu, 10/02/2008 - 12:07
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Milan


No problem altho to be honest i would have preferred you hadn't rated as 2 is generally accepted as not being helpful regardless of the official explanations.


In addtion my original response to Sarah was not "incorrect" as such because viewed from the perspective of the non-cisco switch you would actually only see the BPDU sent on vlan 1. I never stated that the Cisco switch would not send BPDU's for the other vlans.


Jon

milan.kulik Fri, 10/03/2008 - 01:03
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Hi Jon,


IMHO, incorrect was Sarah's sentence (please notice I was replying to her message):

"If cisco switch is connected to third party switch, then only bpdu belonging to native vlan is sent. "

I might be wrong, English is not my native language, but doesn't it sound like the Cisco switch would not send BPDU's for the other vlans?


Regarding rating:

It's everybody's right to evaluate the message according to he/she feels it helpful from his/her subjective point of view.

The link you provided was not exactly what I was looking for (showing what happens but not how in deep regarding BPDUs sent/received/accepted/ignored), that's why "somewhat helpful" which IMHO means "a little heplful".


Anyway, I'm glad having someone to discuss this question which was not clear to me for 2 years.


BR,

Milan

Jon Marshall Fri, 10/03/2008 - 01:12
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Milan


No problem at all and yes always good to discuss these things.


Jon

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