[c-nsp] TCN's - Causing brief outages on ASR1K

[Pete Templin]

You can run RSTP or MST all day long on a switch to get rapid STP 
convergence, but you'll only gain the rapidness of RSTP/MST on ports 
where they neighbor is actually participating in the correct STP 
variant. Routers don't participate in STP, so the 4948 has to treat 
those ports as legacy STP. Whenever there's a root placement event, the 
4948 has to block the port until the STP process/timers can confirm that 
there's no superior root bridge hiding inside or behind the router.

Now, if there's a small enough event going on that SHOULDN'T be causing 
a root placement event but IS, that could be a bug in the 4948 code.

However, I'd say very strongly that you SHOULD have portfast [trunk] 
towards any devices that aren't participating in the STP process, unless 
those devices are capable of creating an L2 loop.

On 12/15/2014 1:18 AM, Antoine Monnier wrote:
> A TCN will cause all the learned MAC addresses to be flushed by the
> switiches, but it will not "block" traffic. So the TCN on its own should
> not be the cause of OSPF and LDP flaps.
>
> Is your switch running out of space for all the learned MAC addresses?
>
>   I dont see how enabling "portfast trunk" would help in that scenario (it
> should only change the behavior if an interface flaps).
> Has the source of TCN being identified? Configuring ports as "portfast"
> will lower the probability of generating TCN, that may be why they advised
> you to do this. However applying to a port that is stable (no interface
> flap) is not really going to help for this specific problem.
>
>
>
> On Mon, Dec 15, 2014 at 9:06 AM, Lukas Tribus <luky-37 at hotmail.com> wrote:
>>
>>> Is it expected behaviour for a TCN to cause a flap on an ASR...We have
>> many other POP's with switches
>>> 4948's/4500's etc(trunk)->ASR+7200's and do not have spanning-tree
>> portfast trunk enabled, and
>>> they do not experience any flaps?
>> This has nothing todo with the ASR1k at all. Its expected behavior that
>> STP on the switch will block traffic
>> when there's a reconvergence, especially when malconfigured (like not
>> using portfast on
>> router or host connected links).
>>
>> Why this doesn't happen on your 7k2 we can't tell, there are a lot of
>> moving parts that only you know
>> (for example whether you are using pvst, rapid-pvst or mst and where
>> exactly the root of those particular
>> vlans is).
>>
>>
>>
>> Regards,
>>
>> Lukas
>>
>>
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