So in this session we're going to take a look at two interior routing protocols, IS-IS and OSPF. Both of them are Link State Routing Protocols using the Dijkstra SPF algorithm. So what's the difference between them then? And why do so many ISP engineers spend so much time arguing about which is superior? Once we saw in the OSPF session OSPF stands for Open Shortest Path First Open means it's an open standard, and developed by the IETF OSPF working group in RFC 1247. The current version is OSPFv2 RFC 2328. The Shortest Path First refers to Edsger Dijkstra's algorithm for producing a shortest path tree through a graph. IS-IS stands for intermediate system to intermediate system. ISO 10589 specifies the OSI IS-IS routing protocol for CLNS. A Link State protocol with a two-level hierarchical architecture. And it has Type/Length/Value options to enhance the protocol. RFC 1195 added IP support and we call this integrated IS-IS. Integrated IS-IS runs on top of the data link layer. So let's look at the two protocols, both are IGPs, Interior Gateway Protocols. They distribute routing information between routers belonging to a single autonomous system. And both use Edsger Dijkstra's algorithm. They support Classless Inter-Domain Routing. They support Variable Subnet Length Masking. They support authentication. They support Multi-path, and they support unnumbered links. So the slide shows the different terminologies used in OSPF and IS-IS for the same devices. In OSPF we talk about a Host. In IS-IS it's an End System. In OSPF we talk about a Router. In IS-IS it's an Intermediate System. A Link in OSPF is a Circuit in IS-IS. A Packet in OSPF is a Protocol Data Unit in IS-IS. Designated Router in OSPF is a Designated IS in IS-IS. OSPF has a Backup Designated Router and there's no such thing in IS-IS. OSPF's Link-State Advertisement is a Link-State PDU in IS-IS. A Hello packet in OSPF is the IIH PDU in IS-IS. And the Database Description in OSPF is the Complete sequence number PDU in IS-IS. OSPF talks about an Area, IS-IS has a Sub domain. OSPF has a non-backbone area, IS-IS refers to this as a Level-1 or station. OSPF's backbone area is Level-2 in IS-IS. The Area Border Router in OSPF is an L1L2 device in IS-IS. Autonomous System Boundary Router in OSPF is any Intermediate System in IS-IS. What about the transport? OSPF uses IP as a transport, Protocol number 89. IS-IS is directly encapsulated in Layer 2. It has no dependency on IP.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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