OSPF and IS-IS Differences and Similarities

In the previous posts, we have discussed a lot about the OSPF and IS-IS routing protocols. If you missed them, we recommend you to have a quick look to review the basic concept and features of the IS-IS routing protocol. In this post, we will explain some of the major similarities and differences between the OSPF and IS-IS routing protocols. It will help you to distinguish between both the routing protocols. You can use the following link for know more about these protocols.

Similarities between OSPF and IS-IS Routing Protocols

There are a lot of similarities between the OSPF and IS-IS routing protocols. The following are some of the major similarities that one should keep in mind while implementing the OSPF or IS-IS routing protocol.

  1. Both the routing protocols are Interior Gateway Protocols (IGP).
  2. Both the routing protocols can only be used to distribute routing information between routers belonging to the same Autonomous System (AS).
  3. Both the routing protocols are link-state protocols.
  4. Both the routing protocols maintain a link-state database and run the Dijkstra SPF algorithm to compute the shortest path.
  5. Both the routing protocols use Hello packets to create and maintain adjacencies between the neighboring routers.
  6. Both the routing protocols use areas to create a hierarchical structure.
  7. Both the routing protocols are classless routing protocols and support CIDR, VLSM, and discontinuous network.
  8. Both the routing protocols elect a designated router in the broadcast networks.
  9. Both the routing protocols support authentication mechanisms.
  10. Both the routing protocols support the unlimited number of hops count.

Differences between OSPF and IS-IS Routing Protocols

Apart from these similarities, both the routing protocols differ from each other with many features. Both the routing protocols have some key differences that one should keep in mind because many people think (better to say confused) that both are the same protocols and the implementation of any protocol will not affect network performance a lot. But the following list will help you to distinguish both the routing protocols.

  1. OSPF supports NBMA and point-to-multipoint links, whereas IS-IS does not support.
  2. IS-IS runs on the data link layer, whereas OSPF runs on the network layer.
  3. OSPF supports virtual link, whereas IS-IS does not support.
  4. OSPF elects a DR and BDR, whereas IS-IS elects only a single DR called DIS.
  5. OSPF defines a backbone area called area 0 for inter-area advertisements, whereas IS-IS categorizes the domain into two layers.
  6. An OSPF router can belong to multiple areas whereas an IS-IS router can belong to only one area.
  7. OSPF uses Router ID, whereas IS-IS uses System ID to identify each router on the network.

OSPF and IS-IS Terminology

Apart from these similarities and differences between the OSPF and IS-IS routing protocols, one should also be familiar with the terminology used to refer the basic terms for both the routing protocols.

The following table list some of the basic terms used refer the components of the OSPF an IS-IS routing protocols.

OSPF and IS-IS Protocol Terminology

In this post, we have learned the similarities and differences between the OSPF and IS-IS routing protocols. Let us know, if you have something that can help us to improve the article. You are always welcome to provide your valuable suggestions.

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