IGP choice

marcel.duregards at yahoo.fr marcel.duregards at yahoo.fr
Fri Oct 23 08:41:49 UTC 2015

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.
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 ?

In my understanding of both protocols, from 3 year old documentation (2012):

OSPF is more or less limited to hundred routers in the backbone area. 
Yeah, ok, but back in 2005 I know some ISP which run 200 routers in the 
backbone area (only one area) w/o problem. What about today ? protocol 
design limitation or resources (memory+cpu) limitation ? If ressources 
only, as of today we can put also 1000 ospf routers in one area...
Cisco recommend no more than 50 routers per area with OSPF. Is it a 
conservative value ?
It also depend on the number of networks/router, of course.

ISIS is not. ISIS scale up to thousand routers in the same area.
Some docs say that ISIS converge faster due to fewer LSP traffic 
(compare to OSPF which generate more LSA traffic, therefore use more 
CPU) and better timers. Timers can also be tuned with OSPF, so I do not 
sea a real argument with better timers for ISIS (same story between HSRP 
versus VRRP with better timers for VRRP).

As your doc say (reason to choose ISIS):
better convergence, better security, simplicity.


On 22.10.2015 19:25, Niels Bakker wrote:
> * marcel.duregards at yahoo.fr (marcel.duregards at yahoo.fr) [Thu 22 Oct
> 2015, 18:57 CEST]:
>> Anybody from Yahoo to share experience on IGP choice ?
> What a weird way to limit your audience.  This is NANOG, not Yahoo.
> Otherwise, http://userpages.umbc.edu/~vijay/work/ppt/oi.pdf
>      -- Niels.

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