OSPF vs ISIS - Which do you prefer & why?

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
configurations.

-Wayne

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
http://www.typo.org/~web/