I've come across a wierd OSPF issue in a lab scenario I thought might be fun
for everyone to comment on (instead of a bunch of gossip on salaries and
brain dumps)
The scenario has 5 routers (NLI-lab 7) and the last thing I want to do is
upset NLI by divulging their lab scenario so I'll try to briefly go over
what I thought was interesting without divulging the whole thing.
There are 3 routers running OSPF on the top. 2 of the 3 have OSPF on one
serial interface and RIP on the other serial interface -fully redistributing
both ways. On the bottom there are 2 routers running RIP on their serial
interfaces (which are connected to the RIP serials on the redistributing
routers) then those 2 bottom RIP only routers are connected together via an
ethernet segment which are both running RIP on the ethernets as well.
r1 --- r2 --- r3
| |
| |
r4 --------- r5
I believe the two redistributing routers would be considered ASBR's in OSPF.
I had originally thought that both of the ASBR's would choose the OSPF route
to the ethernet segment between the 2 routers running only RIP, since the
routes are identical in mask length, the next thing is to look at the
administrative distance. Since OSPF has is lower it should choose the OSPF
route, but if they both use the OSPF route you would obviosly create a
routing loop, which cannot happen in OSPF since it creates a topological
database.
What ends up happening is one of the ASBR's uses the OSPF route and the
other one uses the RIP route, dependancy upon which one comes to full first
will use the RIP route, and the other one will use the OSPF route.
It seems OSPF will only allow one external exit point for each outside
subnet. But why don't both of the ASBR's choose the RIP routes and equal
cost load balance out of the external links as it does internally?
I think it's a neat scenario. I really liked it, the overall issue was to
learn how to control the routing process to most efficiently route to the
destination, but I learned alot about the way OSPF works too.
>>>Brian
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