STP port Roles in 802.1d aka PVST

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Jul 2nd, 2008
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My understanding (and please correct me if I'm wrong) is that if I am running PVST than I am running the standard 802.1d STP but on a per vlan basis, and if there is a trunk that ISL would be used across the trunk, (PVST+ is the same but uses 802.1q across the trunk). Anyway I am running PVST so I should be running 802.1d, but when I do a show spanning-tree active on one of my down stream switches, I see some of the ports roles listed as "Altn" which to me means Alternate, but this confuses me because Alternate is a role in RSTP, not STP. Are PVST and PVST+ not really running 802.1d? but instead also versions of rapid STP? Why does role alternate show up on a PVST configuration?


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Francois Tallet Wed, 07/02/2008 - 09:15
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Hi Jason,

It is true that the port roles were introduced with RSTP. However, they definitely make sense for STP (for instance, the difference between an alternate and a backup port is useful to uplinkfast, and it's also nice to see clearly what the root port is) so we decided to display the role in STP mode also (something we did not do for CatOS afaik).

Now, just for the sake of nitpicking, STP has been in fact deprecated when 802.1w amended 802.1D. An 802.1D switch is an RSTP switch! Running STP nowadays should be in fact running RSTP in STP compatibility mode. And in that case, the switch would definitely display the RSTP roles.

I think that PVST is a sufficient deviation from 802.1D that it makes irrelevant arguing about whether the display of the RSTP role impact the compliance to the standard, so I definitely don't want to insist on that;-)



javickas1 Wed, 07/02/2008 - 09:29
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Greetings Francois

Thanks for the response. Just for the record I wasn't trying to nitpick. I just wanted to make sure I understood what I saw. I wish cisco would have documents which not only show what the output of a command looks like, but also what they mean. I'm putting together some training material so I wanted to make sure I understood what I saw. I have a couple of 2950's which are somewhat limited as switches go, but I discovered while looking thru some broader documents that cisco also has a CST spanning tree mode (not available on the 2950). Would CST mode be the pure historical STP? And as such would the port roles not be listed by RSTP names?

Francois Tallet Wed, 07/02/2008 - 09:40
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Hi again Jason,

I know you were not nitpicking, I was;-) Sometimes I just want to type things into this forum that I'm the only one to be interested in.

The only modes we have are: PVST, rapid-PVST and MST. I don't know exactly what the document you are referring to means by CST mode, but the documentation is not always very clear. MST is the only mode that is providing real standard behavior, with no per-vlan instantiation of the protocol. That's the only mode where we can claim we run a Common Spanning Tree (CST). In PVST mode, the CST is vlan 1, but only controls the state of vlan 1. So it is arguably not a real CST.

The RSTP roles have been introduced when we coded MST, around 2001/2002. Since then, all the STP ports have been showing an RSTP role, regardless of the mode.

Feel free to nitpick;-)



javickas1 Wed, 07/02/2008 - 11:00
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I'm don't recall where I found the CST but here's something I cut and pasted from that source.

Common Spanning-Tree (CST) assumes one spanning-tree instance for the entire bridged network, regardless of the number of VLANs. This implementation reduces CPU load since only one Spanning Tree instance is maintained for the entire network. This implementation can be used when only one Layer 2 topology is needed in the network.

I have another question though, regarding the BPDUs. They would appear to be sent to a multicast MAC address. It would appear the traditional STP address is 01-80-C2-00-00-00

It would also appear that RSTP uses 01-00-0C-CC-CC-CD

Is that an RSTP standard? or is this address cisco specific?

Also are there any other multicast MAC addresses associated with SPanning Tree. IANA only lists the original one.


Francois Tallet Wed, 07/02/2008 - 11:44
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The CST is the only instance that STP and RSTP bridge are running (I'm talking about "standard" STP/RSTP bridges, not Cisco bridges running PVST). The CST is also run network wide in MST, both within and between MST region (it's also called CIST for this reason: common and internal spanning tree).

PVST bridges instantiate the whole STP (or RSTP for rapid-PVST) protocol on a per-vlan basis. This is a Cisco proprietary technique. Basically, vlan 1 runs the CST (i.e. sends BPDU to the standard mac address 0180.c200.00) and interacts with third party bridges. Vlans other than 1 use the cisco specific mac address that you mentioned (0100.0ccc...). This mac address is flooded by third party bridges, that only run the CST. So third party bridges are thus "transparent" to PVST instances.

Again, this is a proprietary mechanism, MST is the only purely standard version of STP we support.



grichardson661 Fri, 01/16/2009 - 06:25
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Interesting post! I've been playing with STP for the BCMSN and also noticed port states / and port roles from the rapid ST when working with PVST. I have a question regarding the BCMSN exam. Does anybody know if the port roles/port states in the exam are going to be treated as the Cisco Press book states? For example, in the exam environment you would think a port role - backup state you would think RSTP is running not PVST. But in the real world this may not be the case. I know you can confirm this easily with certain show commands, but in the exam those commands may not be available. The question could be multiple choice.

Any info would be great.



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