Re: How is a break/reconfiguration propagated in an RSTP network?

anoop wrote:
> Christopher Nelson wrote:
>
> > My biggest remaining question is what's "less time" or "much faster".
> > The paper says that STP took 30 seconds to a minute or more to recover
> > and that RSTP is faster. I'm seeing 2-3 second recovery when the
> > primary path is broken. Is that expected and acceptable? When I read
> > the description in the document refereneced above, it seems that the
> > network should be able to converge in 10s of milliseconds but I'm not
> > seeing that.
>
> The 10s of msec for recovery is not generally true. It happens only
> for certain failures that are "easy" to repair based on network
> topology -- cases where it is obvious to the bridge experiencing
> failure that the root has not failed and that the probability of causing
> a loop by opening one of its blocked ports is very small. Even for
> these cases the repair time is usually ~100 msec. Further, the
> actual repair time also depends on the quality of the implementation
> and the way a port failure is communicated to the software running
> STP on the switch.
>
> For the kind of failure you have 3-6s sounds reasonable. I suspect
> you would do a bit better if you had sw1 directly connected to sw4
> and repeated the test, but it still won't be on the order of 10s of
> msec.

I've dug further with Ethereal and what I see seems really wrong but
I'm far from having RSTP be intuitive so I'm hoping for some feedback.

At steady state with no connections being broken or established, Sw2 is
sending Sw4 BPDUs like this:

Flags: Agreement, Forwarding, Learning, Port Role: Designated,
Proposal
Root ID: Sw1
Root path cost: 200000
Bridge ID: Sw2
Message Age: 1

If I then break the connection between Sw1 and Sw2, Sw2 sends Sw4:

Flags: Agreement, Forwarding, Learning, Port Role: Designated
Root ID: Sw2
Root path cost: 0
Bridge ID: Sw2
Message Age: 0

What doesn't make sense to me is that once things have settled out in
steady state, I'd expect Proposal to be _cleared_ and when Sw2 tries to
assume the root role, I'd expect it to be _set_.

Is there something I don't understand or is the state of Proposal
backwards in these traces?

Chris

.

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