OSPF vs ISIS - Which do you prefer & why?

James Bensley jwbensley at gmail.com
Thu Nov 10 10:17:47 UTC 2016

On 10 November 2016 at 05:59, Mark Tinka <mark.tinka at seacom.mu> 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:

I don't think there is much of a debate to be had any more, the gap
between them is so small now (OSPFv3 and ISIS that is, no one would
deploy OSPFv2 now in greenfield right?):

>   * No requirement to home everything back to Area 0 (Virtual Links are
>     evil).

This is a good point I think.

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

This is in OSPv3.

>   * Single level (L2) deployment at scale.

Single area 0 deployment at scale? Bit of a moot point unless you
compare a specific device model and specific code version in two
identical deployments, its not much to do with the protocol but the
vendor implementation and the brute force.

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

OSPv3 has this.

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

OSPF has these too.

> 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.

Yeah this ^ I don't think there is a stronge case for either protocol.

Somenoe mentioned the AOL NANOG talk about migrating from OSPF to
ISIS. There was a NANOG talk recently-ish about someone migrating from
OSPF to BGP. There wasn't even a need for an IGP, BGP scalled better
for them (in the DC).

BGP these days supports PIC and BFD etc, how much longer to IGPs have? :)


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