From: Tatyana Shekhtman (tatyana@xxxxxxxxx)
Date: Tue Apr 27 1999 - 15:35:08 GMT-3
>From Cisco CCO OSPF Design document:
"Forcing the Router ID Choice with a Loopback Interface OSPF uses the largest I
P address configured on the interfaces as its router ID. If the interface assoc
iated with this IP address is ever unavailable, or if the address is removed, t
he OSPF process mu
st recalculate a new router ID and flood all its routing information out its in
terfaces. If a loopback interface is configured with an IP address, OSPF will
default to using this IP address as its router ID, even if other interfaces ha
ve larger IP addre
sses. Because loopback interfaces never go down, greater stability throughout
your OSPF network is achieved. "
One more pointer from personal experience. In relatively large networks, it he
lps to use Class C addresses for loopback interfaces, since they are all config
ured with /32 net mask. Why? Because IP cache gets populated with entries for
IPs according to
the tightest mask in a particular network. So, if you have /32 addresses in a
class A, and somebody uses an application that does autodiscovery of your netwo
rk, then your routers' cache tables might "explode". This is an unpleasant les
son I learned at m
y previous job working for a very large company... (..but hey! no lesson is a b
ad lesson, right?!)
Regards,
Tatyana.
At 09:01 AM 4/27/1999 +0100, Derek Fage wrote:
>Hi there,
>
>I was looking at the troubleshooting scenarios currently on www.ccieprep.com
>and have a question for the group.
>
>One of the scenarios was with OSPF and a virtual link that is no longer
>working after lunch. It turns out that what happens is that one of the
>routers has been rebooted which caused the router id to change. This was
>becuase the router id was set when OSPF was started, and then another
>interface was configured for IP with a higher address so that this became
>the router-id when the router was rebooted.
>
>My question is regarding how to assign IP addresses when initially
>configuring the lab. Is it better to configure loopback addresses to stop
>this sort of thing happening ?
>
>Derek...
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