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Spanning Tree History Lesson Request

For purely academic purposes, I'm trying to get an idea of how the 802.1D, 802.1Q, and Cisco spanning tree protocols have evolved over time.

I've been digging through web sites and looking at the 802.1 committee's pages.

The question I'm stuck on is when did the standard (.1q) allow for per-vlan spanning tree? I have read where 802.1s was integrated into 802.1q, which provided an MST capability in the standard...is that the same point when Per-Vlan Spanning Tree became available?

By Per-Vlan Spanning Tree, I'm referring specifically to the ability to define a different tree (or at least a different root bridge) for each VLAN. As I understand MST (based on Cisco MISTP), this is the ability to group multiple VLANs into the same tree, which is not what I'm after.

Can anyone help me out, or am I barking up the wrong tree, so to speak, with how the standard developed?

thanks

Jim

1 ACCEPTED SOLUTION

Accepted Solutions

Re: Spanning Tree History Lesson Request

That's correct. There has never been any per vlan STP in the standard. Cisco solution only makes sense because it's more flexible to use than the IEEE standard. However, PVST has some limitation in term of scalability, and at a point, it's better to move to MST.

Regards,

Francois

7 REPLIES

Re: Spanning Tree History Lesson Request

Hi Jim,

I wrote a rather high level "history" of the stp modes in the following paper: http://www.cisco.com/en/US/docs/solutions/Enterprise/Data_Center/DC_3_0/DC-3_0_IPInfra.html#wp1037394

Search for the "STP Mode Selection" section.

Let me know if you need more details...

Regards,

Francois

New Member

Re: Spanning Tree History Lesson Request

Francois,

Thank you for the informative link.

From what I'm reading here, 802.1s (which was integrated into the IEEE 802.1q-2003 standard) provided support for spanning tree with either a tree-per-vlan or tree-per-vlan-group. Is that about right?

Thanks

Jim

Re: Spanning Tree History Lesson Request

Hi Jim,

MST creates one spanning tree per instance. The instances are entirely independent from the vlans, so it's not PVST.

You can have up to 65 instances (64 MSTIs and one CIST) which allows you to play the PVST game up to 65 vlans (by mapping each vlan to a different instance;-) After that, you will necessarily have instances handling more than one vlan.

MST is the first time Cisco actually follows the standard.

Regards,

Francois

New Member

Re: Spanning Tree History Lesson Request

Francois,

So from this I understand that the IEEE standard, which is basically RSTP, does NOT support a tree-per-vlan? Only Cisco's PVST+/R-PVST+ will do that?

Thank You

Jim

Re: Spanning Tree History Lesson Request

That's correct. There has never been any per vlan STP in the standard. Cisco solution only makes sense because it's more flexible to use than the IEEE standard. However, PVST has some limitation in term of scalability, and at a point, it's better to move to MST.

Regards,

Francois

New Member

Re: Spanning Tree History Lesson Request

Outstanding! Thank You Francois. That clears up a lot

Bronze

Re: Spanning Tree History Lesson Request

Unfortunately, you will not find much about Per VLAN Spanning Tree (PVST+) when looking at IEEE documentation. The reason is that PVST+ is Cisco proprietary and there is not a standards document for it. Cisco does interoperate with the standards based Spanning Tree and you will find plenty of documents on Cisco.com about how they manage to do that.

Anyhow - here is a link with some history on the matter.

http://en.wikipedia.org/wiki/Spanning_tree_protocol

HTH,

Jim

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