09-26-2008 10:04 AM - edited 03-06-2019 01:37 AM
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!
Solved! Go to Solution.
09-26-2008 10:15 AM
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
09-26-2008 11:03 AM
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
09-26-2008 10:15 AM
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
09-26-2008 11:02 AM
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 !
09-26-2008 11:03 AM
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
09-26-2008 02:43 PM
Thanks a lot Jon!
10-01-2008 06:06 AM
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
10-01-2008 06:20 AM
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
10-01-2008 06:45 AM
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
10-01-2008 08:33 AM
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
10-02-2008 11:49 AM
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
10-02-2008 01:15 PM
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
10-03-2008 01:46 AM
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
10-03-2008 08:52 AM
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
10-02-2008 01:21 PM
Milan
Just to add to Francois post, the link i sent describes this perfectly - see Case 1 and Case 2.
Jon
10-01-2008 10:21 AM
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
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