So let's look at the decisions that the service providers have to make. Which IGP should an ISP choose? Both OSPF and IS-IS use the Dijkstra SPF algorithm. Both exhibit the same convergence properties. IS-IS is less widely implemented on router platforms. IS-IS runs on the data link layer, OSPF runs on the IP layer. So why do we keep discussing the merits of each IGP so frequently? Well the biggest ISPs tend to use IS-IS. In the early 1990s Cisco's implementation of IS-IS was much more stable, and reliable than their OSPF implementation. So ISPs naturally would prefer IS-IS. The main IS-IS implementations were much more tunable than the equivalent OSPF implementations, because the biggest ISPs using IS-IS put more pressure on Cisco to implement these knobs. Moving forward a decade, the early Cisco OSPF implementation was substantially rewritten, and is competitive with IS-IS in features and performance throughout this century. Router vendors wishing a slice of the core market, need an IS-IS implementation as solid and as flexible as that from Cisco. Those with IS-IS and OSPF support tend to ensure they exhibit performance and feature parity for both interior protocols. So how do we go about choosing an IGP? OSPF has a rigid area design. All networks must have an area 0 core, with sub-areas distributed roundabout. And it suits ISPs with a central high-speed core network linking regional PoPs. As for IS-IS, it has a relaxed two level design. L2 routers must be linked through the backbone. And it suits ISPs with so called stringy networks, diverse infrastructure and so on. Not really fitting the central core model used by OSPF. It's much more flexible than OSPF as well, but can be easier to make mistakes also. There are some other considerations as well. Some people claim IS-IS is more secure. IS-IS runs on the data link layer, so therefore it's not possible to "attack" the IGP using IP as it is with OSPF. That's hardly an improvement in security, but at least having IGP running on the data link layer gives a perception of an improvement over OSPF. IS-IS is not dependent on IP addressing. IS-IS's NSAP addressing scheme avoids dependencies on IP as with OSPF. Reliability is often quoted as a reason as well. IS-IS has long been used by the majority of the world's biggest Internet Service Providers. And so there is a belief that equipment vendors pay more attention to IS-IS reliability, scalability, and features than they would do with OSPF. And here's another consideration, the migration to IPv6. Adding IPv6 means we need OSPFv2 and OSPFv3 in the network. OSPFv2 only supports IPv4, OSPFv3 only supports IPv6. This means we need to run two routing protocols with two sets of identical configuration. For IS-IS it's simply the addition of an IPv6 address-family. IS-IS is multi-protocol. And most networks using IS-IS today will deploy IS-IS single topology handling both v4 and v6. New deployments of IS-IS use multi-topology. Where they can run separate topologies for v4 and v6 if they require. And this allows an incremental deployment of IPv6 across the networks. Interesting enough, there is now RFC5838, which describes support of multiple address families for OSPFv3. However vendor support, at least at the time of this recording, was still fairly limited.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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