At http://www.cisco.com/cpress/cc/td/cpress/design/ospf/on0407.htm#xtocid1636523
(which is an extracted chapter from 'OSPF Network Design Solutions', by Tom
Thomas), there is a bit that states...
"Cisco's OSPF implementation enables you to alter certain interface-specific
OSPF parameters, as needed. You are not required to alter any of these
parameters, but some interface parameters must be consistent across all routers
in an attached network. Those are the parameters set by the following commands:
ip ospf hello-interval
ip ospf dead-interval
ip ospf authentication-key
timers spf spf-delay spf-holdtime
Therefore, be sure that if you do configure any of these parameters, the
configurations for all routers on your network have compatible values. "
The first three I can understand, and I don't have a problem with these
parameters having to match on all routers on the network. But I can't see why
the spf timers should have to match. And in any case, that one's not an
interface-specific parameter.
For those who haven't used this command before, 'spf-delay' is the delay time,
in seconds, between when OSPF receives a topology change and when it starts a
SPF calculation. 'spf-holdtime' is the minimum time, in seconds, between two
consecutive SPF calculations. The command reference on CCO doesn't mention that
the spf timers have to match on all routers. I can see that if they are
mismatched by too much, it will take longer for routers to converge to a
consistent view of the network, but would it cause any other problems?
I've had a look at RFC 2328, and am no wiser, although I will happily admit I
did not read all 240 pages. Why would one router care how long another router
has waited between SPF calculations? Or is this an error in the book/website -
can they in fact be different everywhere (obviously it's simpler if they're the
same, but does it do nasty things to OSPF if they're not)?
JMcL
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