[c-nsp] OSPF: inconsistent SPF/LSA throttle timers

[Lukas Tribus]

Hey guys,

we are currently running a small OSPF network with about 50+ boxes using
default IOS timers. We would like to tune LSA/SPF throttle timers.

Now, because some of the boxes have a decent CPU (ASR1000, 6500, etc) running
more important traffic, and others have a small cpu, like ME3400 layer 3
switches, we wanted to tune the OSPF timers on those important boxes
specifically. SPF + RIB update on the ME3400 amounts for up to 250 ms,
which is why we prefer to leave them as-is, or at least, roll those
changes out gradually.


Here comes the question: a concern was raised, that if we are running an
OSPF area with different LSA/SPF timers on the boxes, we may hit a race
condition where a box discards an incoming LSA, and the OSPF database only
recovers after the LSA is refreshed (after the 30 or 50 minutes) or it
doesn't recover at all when using "flood-reduction" [1].

Personally, I don't think thats the case. Also, none of the documentation
I read suggests that. It would also mean that we cannot change the throttle
timers in a live network gradually, but need to shutdown the entire OSPF
network to adjust the timers. I can't believe thats the case.

I do agree that it would be better to have a fully consistent configuration,
and we will try to achieve that at one point, but until then, does anyone
run a production network with inconsistent LSA/SPF throttle timers?


Of course we will have temporary routing loops while reconvergence takes
place, but that is not the concern here.


I have my doubts that big heterogeneous networks really use consistent
SPF/LSA timers across all boxes.



I would appreciate any input from you guys.



thanks,

Lukas


[1] http://www.cisco.com/en/US/docs/ios/12_1t/12_1t2/feature/guide/dt_ospff.html