IGP choice

Jameson, Daniel Daniel.Jameson at tdstelecom.com
Fri Oct 23 18:56:43 UTC 2015

A lot of  carriers use ISIS in the core so they can make use of the' overload bit' with a  'set-overload-bit on-startup wait-for-bgp".  Keeps them from black holing Traffic while BGP reconverges.,  when you have millions of routes to converge it can take forever.  It's also a really handy tool when you're troubleshooting or repairing a link,  set the OL bit,  and traffic gracefully moves,  then when you're done it gracefully moves back.  You can do the same thing with the Metric,  and Cost in OSPF,  just not quite  as elegant.

Largely I think it's preference,  ISIS and OSPF tackle most of the same stuff just in different ways.


-----Original Message-----
From: NANOG [mailto:nanog-bounces at nanog.org] On Behalf Of Matthew Petach
Sent: Friday, October 23, 2015 11:31 AM
To: marcel.duregards at yahoo.fr
Cc: nanog at nanog.org
Subject: Re: IGP choice

On Fri, Oct 23, 2015 at 1:41 AM, marcel.duregards at yahoo.fr <marcel.duregards at yahoo.fr> wrote:
> sorry for that, but the only one I've heard about switching his core 
> IGP is Yahoo. I've no precision, and it's really interest me.
> I know that there had OSPF in the DC area, and ISIS in the core, and 
> decide to switch the core from ISIS to OSPF.

Wait, what?
*checks memory*
*checks routers*

Nope.  Definitely went the other way; OSPF -> IS-IS in the core.

> Why spend so much time/risk to switch from ISIS to OSPF, _in the core_ 
> a not so minor impact/task ?
> So I could guess it's for maintain only one IGP and have standardized 
> config. But why OSPF against ISIS ? What could be the drivers? People 
> skills (more people know OSPF than ISIS) --> operational reason ?

I'm sorry you received the wrong information, the migration was from OSPF to IS-IS, not the other way around.



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