Recently, I have been involved in a friendly dispute whether the IS-IS routing protocol is only a remnant of the past, being gradually phased out and replaced by OSPF, or whether the IS-IS is still being intensively used and "flourishing", so to say. My personal opinion is that the IS-IS is a very fine routing protocol with no reasons to be considered obsolete but I do not have any real-world experiences about its deployment.
Can you perhaps share your knowledge and experiences about the IS-IS deployment in real world? Is it growing or diminishing? How do you perceive its future? Thank you very much for all your ideas!
IS-IS is not on its way out just yet. It is just not popular with enterprise companies I find.
It is very very popular still within Service Providers and especially as the core IGP within an MPLS network. I work in a Service Provider role and the majority of my customers are using IS-IS within their MPLS core and I encourage it. The main reason it is chosen over OSPF is:
1) It scales better within a single area compared to OSPF (single area required for true RSVP-TE)
2) It is EXCELLENT for routing IPv6 because it does not run over IP so to make it route IPv6 you just have to turn it on. Whereas with OSPF you have to run a whole new routing protocol to route IPv6 as well.
3) IS-IS is a little more secure as it doesn't use IP for transport so if someone accidently changes IP addresses or deletes them, routing is not effected.
So in my view, IS-IS is certainly here to stay for quite a while longer, especially with the advent of IPv6 coming up as you only need one routing protocol to route IPv4 and IPv6 which might even appeal to the enterprise market as well.
my current customer is a big enterprise and in this network IS-IS is used as the IGP.
In Europe OSPF is most used, just to make an example it is used in several backbone networks of ISPs.
In my opinion one of the possible drawback of IS-IS is the fact that is not an IP based protocol.
This is not only a problem for its diffusion, but it can also create some little issues in the networks.
I mean that modern modular QoS is IP oriented and not all cisco platforms have the hidden system queue for routing protocols.
This means for example that IS-IS has an higher probability to see a neighborship to fall down in comparison to OSPF.
to be more practical I can see alarms on NMS about an IS-IS neighborship failing even if the link is healthy and it had no line state transitions.
The adjacency is rebuilt immediately but I have the impression that with OSPF using IP CS6 we can know in what class this traffic is placed.
I can say that some years ago when I was working for a service provider I could see Cisco representives trying to push IS-IS usage instead of OSPF (with little to no success).
IS-IS has two advantages: it is very fast in convergence;
Its TLV structure makes it easy to update for example to support IPv6.
We can compare this with BGP address-families where everytime a new need can be addressed by adding a new address-family.
OSPFv3 supports IPv6 but it doesn't support IPv4, OSPFv2 supports IPv4 but not IPv6.
So if IPv6 should finally become a success deploying a single multi-family process instead of two separate processes can be seen as an advantage of IS-IS.
In US IS-IS is widely deployed by big ISPs because they started to use it before OSPFv2 was defined in all its details.
There are also some bugs affecting IS-IS for example one in 12.2(18)SXFy (with y roughly 9 or 11) that causes IS-IS to freeze after a big topology change instead of recalculating and creating black holes for traffic.
The workaround is a clear of the IS-IS process. I've seen this in our network during one migration and in another couple of cases.
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