[c-nsp] OSPF to ISIS migartion

Justin Shore justin at justinshore.com
Wed Sep 23 09:02:59 EDT 2009

jack daniels wrote:
> Hi all ,
> I have got a project for an ISP ( also LDP configured ) runnning OSPF to
> migrate to IS-IS.
> I was planning to runnn dual IGP , as ospf with AD 110 and ISIS with AD 115
> , OSPF will always be preffered.
> I was planning the challenges for migration, below are the ones which I
> could think of , please give your inputs on WHAT CONSIDERATIONS TO KEEP IN
> Also request you to share some docs for migration of OSPF to ISIS<<<<<
> 1)  MEMORY/CPU utilisation
> 2)  no. of routes in routing table and database
> 3)  harware of the cisco devices ( 7206/12K)

Vijay from AOL did such a migration.  If memory serves me correctly he 
migrated all of AOL in 2 days.  Day 1 was the migration of their test 
POP.  Day 2 was everything in production.


Here's another NANOG IS-IS presentation.


If you search NANOG's resources page you'll find several IS-IS 


How many routes does your IGP carry today?  Perhaps you should first 
move your customer routers into iBGP before you proceed with the IGP 
change.  That will ensure that your IGP is small.  Unless it's extremely 
large or your edge routers are on the verge of collapse, I wouldn't 
worry about the size too much.

You need to make sure that IS-IS is supported on all your current 
platforms.  There are some that you may currently utilize that don't 
support IS-IS at all.  For example most fixed configuration Cat switches 
don't support it, though a few do (ME3750 for example).  You may need to 
bump featuresets to get full IS-IS support in some cases as well so 
definitely make use of Cisco's Feature Navigator.

Also, and I'm speaking from my own experience with a thoroughly messed 
up OSPF deployment to a flat L2 IS-IS deployment, when you start the 
migration don't stop until the migration is complete.  Do not leave bits 
and pieces of the network unmigrated, assuming that you'll come back 
later and fix it.  I made that mistake.  I guarantee you that dualing 
IGPs and/or redistribution will bite you in the ass.  There is nothing 
quite like the look on one's face when you discover that the routing 
between your core routers happens to get routed through a stack of EMI 
3750s that is connected to both core routers.  Oh yeah.  Not pretty. 
Don't make the same mistake.  Do it Vijay's way; all at once.

Best of luck

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