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 In this article, we will discuss the similarities and differences between OSPF and ISIS Protocols.

OSPF and ISIS Similarities

In this article we will compare OSPF and ISIS and discuss their similarities and some key differences between them. 
There are many similarities between the OSPF and ISIS. Below are few main similarities between both protocols.
  • Both OSPF and ISIS are Interior Gateway Protocols and used for routing in internal domains or autonomous systems.
  • Both the routing protocols are link-state protocols.
  • Both OSPF and ISIS maintain a link-state database (LSDB) and run the Dijkstra SPF algorithm to compute the shortest path.
  • Both the routing protocols use Hello packets to create and maintain adjacency between the neighboring nodes.
  • Both OSPF and ISIS use areas to maintain a hierarchical structure.
  • Both the routing protocols are classless routing protocols and support CIDR, VLSM, and discontinuous network.
  • Both the routing protocols elect a designated router in the broadcast networks. However, ISIS only elects a DIS (Designated IS). There is no backup DIS.
  • Both the IGPs support authentication mechanisms.
  • Both the IGPs support the unlimited number of hops count.
  • Both the IGPs support Classless Inter Domain Routing, Variable Length Subnet Mask, IP unnumbered links, Authentication, and Multi-paths.

OSPF and ISIS Dissimilarities

Apart from the similarities explained above, both the routing protocols also have some dissimilarities as explained below:

  • The default administrative distance of OSPF is 110 and ISIS is 115.
  • OSPF behaves differently with different kind of networks for example NBMA and point-to-multipoint links, whereas IS-IS only supports two type of networks (Broadcast and Point-to-Point)
  • ISIS runs on top of data link layer(encapsulated in Data link layer), whereas OSPF runs on top of IP (Protocol no. 89), i.e. at the Transport layer. To further elaborate, OSPF requires IP connectivity to form adjacency between two nodes, however, ISIS does not require any IP connectivity to for adjacency because it uses NSAP addresses (example 0049.0001.1111.1111.1111.00) which is a unique value per node configured under ISIS routing process. As ISIS on top of Data Link layer, it is not possible to attack ISIS at IP. NSAP fields are explained as below:
          49 -- Private Domain
          0001 -- Area
         1111.1111.1111 -- System ID (like mac address)
          00 -- NSEL -- Signifies that the node is a router. i.e if a node is router then nsel is 00

  • OSPF supports virtual link to connect an remote area to backbone area 0, whereas IS-IS does not support virtual link concept.
  • OSPF elects a DR and BDR in the shared network, whereas IS-IS elects only a single DR called DIS.
  • OSPF refreshes its complete database table every 30 mins (or at max after 60 mins) however, ISIS doesn't refresh its entire database periodically like OSPF.
  • Unlike OSPF, in the ISIS protocol,  SPF is only used to calculate the reachability to routers based on their NSAP addresses. Once best paths are calculated, SPF in not run again when any network prefix goes down or fluctuates. SPF will only run again if a router/node goes down. ISIS uses another algorithm called partial route calculation (PRC) for IP routing table calculation.
  • OSPF defines a backbone area called area 0 for inter-area advertisements, whereas ISIS categorizes the domain uses various ISIS levels, for example L1, L1/L2, L2. The L2 routers (L2 routing information) form the backbone area in ISIS.
  • An OSPF router can belong to multiple areas, for example, an ABR can be part of two or more areas at a time whereas an ISIS router can belong to only one area.
  • OSPF uses Router ID, whereas ISIS uses System ID to identify each router on the network.
  • Compared to OSPF, ISIS is more easy to configure and flexible and scalable in operation and therefore is widely configured as an IGP in Service Provider core backbones.
  • While migrating from IPv4 to IPv6, we have to jump from OSPFv2 to OSPFv3 at config level, however, ISIS simply requires addition of IPv6 address family and that's it.