IPv6: IS-IS or OSPFv3

Justin Shore justin at justinshore.com
Mon Jan 5 17:43:25 UTC 2009

Kevin Oberman wrote:
> I would hope you have a backbone well enough secured that you don't need
> to rely on this, but it does make me a bit more relaxed and makes me
> wish we were using ISIS for IPv4, as well. The time and disruption
> involved in converting is something that will keep us running OSPF for
> IPv4 for a long time, though. I remember the 'fun' of converting from
> IGRP to OSPF about 13 years ago and I'd prefer to retire before a
> repeat.

I did the OSPF --> IS-IS migration some time back and here's some of the 
info I found at the time.


Vijay did a nice presentation on AOL's migration to IS-IS.  IIRC AOL 
migrated everything in 2 days.  Day 1 was to migrate their test POP and 
hone their script.  All remaining POPs were migrated on Day 2.  I 
believe he said it went well.  There have been several other documented 
migrations too:


I migrated my SP from a flat OSPF network (end to end area 0) to IS-IS. 
  The OSPF setup was seriously screwed up.  Someone got the bright idea 
to changes admin distances on some OSPF speakers, introduce a default in 
some places with static defaults in others, redistributing like it was 
going out of style, redisting a static for a large customer subnet on P2 
instead of P1 which is what PE1 actually connected to (and not 
advertising the route from PE1 for some unknown reason), etc.  The old 
setup was a nightmare.

The IS-IS migration went fairly well after I got some major bugs worked 
out on our 7600s.  I implemented IS-IS overtop of OSPF.  Some OSPF 
speakers had admin distances of 80 and some were default.  IS-IS slipped 
in over top with no problems.  I raised IS-IS to 254 for the initial 
phase anyway just to be safe.  Once I had IS-IS up I verified it learned 
all the expected routes via IS-IS.  Then I lowered its admin distance 
back to the default and bumped OSPF up to 254.  Shortly thereafter I 
nuked OSPF from each device.  It was hitless.  I never could get IS-IS 
to work with multiple areas.  The 7600s made a smelly mess on the CO 
floor every time I tried.  In the end I went with a L2-only IS-IS 
network.  Still it works well for the most part.  I've had about as much 
trouble with IS-IS as I have had with OSPF.  Occasionally some random 
router will get a burr under it's saddle and jack up the MTU on the CLNS 
packets beyond the interface's max.  The receiving router will drop the 
padded frame as too big.  Fixing this can sometimes happen with a 
shut/no shut.  Sometimes I can nuke the entire IS-IS config and re-add 
the config.  Other times I simply have to reboot.  This doesn't happen 
too often; it's usually several hours after I rock the IS-IS boat so to 
speak.  Still, I wouldn't go back to OSPF for this SP.


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