IS-IS

IS-IS (Intermediate System-Intermediate System)

IS-IS is an Interior Gateway Protocol (IGP) meaning that it is intended for use within an administrative domain or network. It is not intended for routing between Autonomous Systems a job that is the purpose of an Exterior Gateway Protocol, such as Border Gateway Protocol (BGP).

IS-IS is a link-state routing protocol, meaning that it operates by reliably flooding topology information throughout a network of routers. Each router then independently builds a picture of the network's topology. Packets or datagrams are forwarded based on the best topological path through the network to the destination.

The main advantage of a link state routing protocol is that the complete knowledge of topology allows routers to calculate routes that satisfy particular characteristics and features.

A disadvantage of a link state routing protocol is that it does not scale well as more routers are added to the routing domain. Increasing the number of routers increases the size and frequency of the topology updates, and also the length of time it takes to calculate end-to-end routes. This lack of scalability makes a link state routing protocol unsuitable for routing across the Internet, which is the reason why IGPs only route traffic within a single AS.

Similarities between OSPF and IS-IS

They are both link state protocols and uses same algorithm (Dijkstra) for computing the best path to a network or destination.

They both support variable length subnet masks and can use multicast to discorver neighbouring router using hello packets,

They both can authenticate routing updates.

Diffrences

IS-IS can support more routers in an area than OSPF. This makes IS-IS more favoable among ISP’s environments.

IS-IS is neutral regarding the type of network addresses for which it can route. OSPF, on the other hand, was designed for IPv4. Thus IS-IS was easily adapted to support IPv6, while the OSPF protocol needed a major overhaul (OSPF v3).

IS-IS differs from OSPF in the way that "areas" are defined and routed between. In OSPF, areas are delineated on the interface such that an area border router (ABR) is actually in two or more areas at once, effectively creating the borders between areas inside the ABR, whereas in IS-IS area borders are in between routers.