Help with RSTP - more useful BPDUs?

Hi All

I'm new to forums, so not sure if starting this is OK, or if should have looked harder here for existing threads, however..
I get STP, but while trying to nail RSTP port roles, I ran into this explanation:

Understanding Rapid Spanning Tree Protocol (802.1w) [Spanning Tree Protocol] - Cisco Systems

I don't understand what "more useful" means within the context of "An alternative port receives more useful BPDUs from another bridge...." And "A backup port receives more useful BPDUs from the same bridge"
I don't quite get what types of value stored in the BPDU are "more useful". An example of more useful and less useful would be helpful. I can't work out why the BPDUs received in the examples above, should be "more useful" in the given situations.

Also so far in my studies, I've never seen switch ports connected to what looks like a common 10BASE5 bus (could be a hub I guess). If anyone could enlighten me I would be forever indebted.

Thanks guys,

zuul

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Comments

Roguetadhg

Welcome!

10base5? common thicknet switch?

The most useful, in the way the site is describing it: The method to compare BPDUs from itself, and other switches.

Basicly, you'd need to follow the election process of normal STP (and hope Big Bird stays employed :P)

The alternate port is the "best alternative" port on a different switch. If Switch B goes down, the alternate port on switch A would be able to pickup the slack for a faster convergence.

zuul

Roguetadhg wrote: »

Welcome!

10base5? common thicknet switch?

The most useful, in the way the site is describing it: The method to compare BPDUs from itself, and other switches.

Basicly, you'd need to follow the election process of normal STP (and hope Big Bird stays employed :P)

The alternate port is the "best alternative" port on a different switch. If Switch B goes down, the alternate port on switch A would be able to pickup the slack for a faster convergence.

Thanks a lot Roguetadhg,
- OK I know it's nothing to do with 10base5 (clutching at straws), just unfamiliar with this type of drawing, hoped some light could be thrown..
- Also, If I'm interpreting what you said correctly - like it could be that bridge B has the lower cost to the root (not in this case going by the drawing) or ,lower BID, port priority etc, than bridge A? And it's these values within the BPDU that regarded as more useful?

- The other point, "A backup port receives more useful BPDUs from the same bridge"
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- Do you think this is showing two ports on the same bridge directly connected to each other?
- Big bird? was that the PBS Romney quote? English elections aren't quite as showbiz yet but we're getting there - need to catch up with the hoopla

Cheers
Z

zuul

Hi Roguetadhg,
Thanks for the welcome and thanks for the reply. I replied to you on Friday but this may not have been successful or is still awaiting moderation. If I don't see anything soon I'll try again.
Cheers,
Zuul
____________________________________________________
08/10/12
OK, like it could be that bridge B has the lower cost to the root (not in this case going by the drawing above) or ,lower BID, port priority etc, than bridge A? And it's these values within the BPDU that are regarded as more useful? And it's also this that's responsible for:
1) The fact that the designated port is on B and the alternate port is on A in the first place.
2) The alternate port (on A) receives more useful BPDUs from the designated port (on

because the designated port "can send the best BPDU on the segment to which it is connected" (which would be more useful BPDUs than the alternate port on A could deliver).

The other point, "A backup port receives more useful BPDUs from the same bridge"
Can't get my head round this. Not sure under what circumstances a bridge would be receiving BPDUs from another one of its ports?

Cheers all.
Z

zuul

For anyone interested, I found some good stuff on this. It explains what my study material didn't.
It's on the The Cisco Learning Network, here:
https://learningnetwork.cisco.com/docs/DOC-2549
CCNA TV- 17-Dec-07 - Rapid Spanning-Tree Protocol
And it's at about 16:00 minutes into the video.
You will need to be logged into the CLN to see it.
Thanks to those who have contributed.