OSPF vs ISIS - Which do you prefer & why?

Mark Tinka mark.tinka at seacom.mu
Thu Nov 10 06:12:05 UTC 2016

On 10/Nov/16 04:45, RT Parrish wrote:

> 1) Network Topology support - The differences between a single OSPF
> backbone area and a contiguous set of Level-2 adjacencies will occasionally
> be a deciding factor.

L2 IS-IS can be as chatty as single-area OSPF. That said, IS-IS has
native tools to reduce that chatter (like PRC, and iSPF), but to be
honest, I'm not sure it makes much of a difference given today's faster
router CPU's.

> 2) Feature Support on a per vendor basis - Some vendors will roll new
> features out in one or the other protocols prior to the other.  Segment
> Routing and some of its enhancements come to mind as being in ISIS first.

I've noticed that the delay between when IS-IS and/or OSPF pick up a
feature the other already has is reasonable. By the time an OSPF has
completed evaluating whether they need LFA, it would have been
implemented in the IGP.

I suppose back then, there was a much bigger between when features made
it between both protocols, but things seem to be on par nowadays.

> 3) Layer 2 adjacencies - I think someone already mentioned that you form
> adjacencies at layer 2 which also means that with a single adj you can
> support multiple protocols (v4/v6). OSPF would require two different
> instances to support both. Maybe good, maybe not. Depends on your desired
> level of isolation between the two.

OSPFv3 can support the advertisement of IPv4 prefixes. But you'd still
need an IPv6 link layer.

> 4) CPU performance is academic at this point - The SPF calculations in most
> networks would require next to no difference between the two protocols even
> if running both IPv4 and v6.


> End of the day, use the right tool/vendor/technology for the right job.



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