Re: Migrating from EIGRP to OSPF

[Cthulu, CCIE Candidate It's Not Dagon]

Hey, Brian,

Thanks for the info.  I have some comments/clarification.


>EIGRP carries a lower administrative distance vs. EIGRP.
CR:  You mean EIGRP (90) has a lower AD than OSPF (110)?


So you can
>basically turn up OSPF on your routers, and then when it all looks kosher,
>what I would do is raise the administrative distance of EIGRP above
>OSPF..........this will cause the EIGRP routes to phase out and OSPF to
>be used.  If it all goes crazy........you can revert by simply moving
>EIGRP back to the default administrative distance.  If you were to have
>cleared out your EIGRP config......it would be difficult to put things
>back..........

CR:  That is an excellent suggestion!  It would be easy to write a script 
that lower/raise as needed.  Thank you!


>Ok, why would a more specific route be inserted in OSPF vs. EIGRP?  If you 
>deploy your OSPF to match that of your EIGRP, this should not
>happen.......its definitly avoidable.

CR: I was not clear on this.  What I meant to say was that if EIGRP has a 
more specific/longest match route than OSPF (or vice versa), that route will 
be inserted in the routing table rather than the OSPF route.  Recalling past 
groupstudy discussions, a learned route gets inserted in the routing table 
in order of preference of:

1. Most specific/longest match
2. administrative distance
3. cost (metrics)


I had a situation where a more specific EIGRP route stayed in the routing 
table even though adjustments has been made to prefer OSPF. For example, if 
you enter a network 1.1.1.1 0.0.0.0 area 0 command under OSPF, the mask that 
gets advertised with this route by OSPF is whatever mask you have on the 
interface.  Another thing is that if you advertise loopback interfaces, OSPF 
treats them as stub hosts (with a /32 mask);  therfore, the route from OSPF 
will be inserted in the routing table as it is more specific/longest match, 
rather than the same route learned from EIGRP with its lower administrative 
distance.  These are the types of problems I want to minimize.


Many thanks, Brian, for the suggestions and feedback!

Charles

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