OSPF vs ISIS - Which do you prefer & why?

Mark Tinka mark.tinka at seacom.mu
Thu Nov 10 05:59:12 UTC 2016

On 9/Nov/16 19:12, Michael Bullut wrote:

> Greetings Team,
> ​While I haven't worked with IS-IS before but the only disadvantage I've
> encountered with OSPF is that it is resource intensive on the router it is
> running on which is why only one instance runs on any PE & P device on an
> ISP network. OSPF is pretty good in handling the core network routing while
> BGP & EGP handle the last-mile routing between PE & CE devices. BGP & EGP
> can run on top of OSPF. I came across this *article*
> <https://routingfreak.wordpress.com/2011/03/05/why-providers-still-prefer-is-is-over-ospf-when-designing-large-flat-topologies/>
> when
> scrolling the web a while back and I still want to find out if am the only
> one who thinks its a matter of choice between the two. Although there isn't
> distinct 1:1 argument, it's good we discuss it here and figure out why one
> prefer one over the other *(consider a huge flat network)**.* What say you
> ladies and gentlemen?

I've given a talk about this a couple of times since 2008. But our
reasons are to choosing IS-IS are:

  * No requirement to home everything back to Area 0 (Virtual Links are

  * Integrated IPv4/IPv6 protocol support in a single IGP implementation.

  * Single level (L2) deployment at scale.

  * Scalable TLV structure vs. Options structure for OSPFv2. OSPFv3
    employs a TLV structure, however.

  * Inherent scaling features, e.g., iSPF, PRC, e.t.c. Some of these may
    not be available on all vendor implementations.

If you're interested in reviewing the talk I gave on this, a lot more
details is in there at:


Ultimately, router CPU's are way faster now, and I could see a case for
running a single-area OSPFv2. So I'd likely not be religious about
forcing you down the IS-IS path.


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