OSPF vs ISIS - Which do you prefer & why?

Wayne Bouchard web at typo.org
Thu Nov 10 06:41:50 UTC 2016

This generally supports my own view that it depends on the topology
and the real or potential scale/scope. In my experience, IS-IS is just
all around better in a flat, highly interconnected environment such as
an ISP or other broadly scaled network. If you have a very (almost
exclusively) heirarchical structure and pretty good control over IP
addressing and can use summarization effectively, then OSPF can make
your core networking much simpler. On a small network that doesn't
look to grow at leaps and bounds, I'd favor OSPF. On a large, complex
network or a network that has the potential to grow without any sort
of predefined structure (ie, more demand based), then IS-IS is
probably your win. Note that this doesn't factor in multiple IS-IS
levels, something I don't have a great deal of experience with.
Mostly, networks I've been associated with just run one great big,
gigantic level 0, though they did also experiment with other


On Thu, Nov 10, 2016 at 07:59:12AM +0200, Mark Tinka wrote:
> 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
>     evil).
>   * 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:
> http://www.apricot.net/apricot2009/images/lecture_files/isis_deployment.pdf
> 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.
> Mark.

Wayne Bouchard
web at typo.org
Network Dude

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