IS-IS

Intermediate System to Intermediate System:
A protocol used to determine the best way to forward datagrams through a packet switched network.

• A IGP protocol
• Not intended for routing between ASes
• Floods topology information throughout the network
• Packets are forwarded based on best topological path through network
• Uses Dijkstra's Algorithm
• Preferable to service providers while OSPF is preferable to enterprise networks
  

Whats the difference from OSPF:
Because both are link state, both support Classless networks, use multicast to discover neighboring routers using hello packets, and support authentication of routing updates, these protocols are very similar.

• OSPF routes IP and is a layer 3 protocol (on IP), IS-IS is an OSI protocol (same as CLNS), and does not use IP to carry routing information.
• A topological map is built of the network, which indicate which IP subnets each IS-IS router can reach, using the lowest cost to an IP subnet to forward traffic.
• Differs from OSPF in how topology is flooded. It is less chatty, and can support larger networks, therefore favorable in ISP (Internet Service Provider) networks. It easily adaptive to support IPV6 due to the fact its neutral regarding network address routing.
• The "Area" concept is different - IS-IS routers use Levels, with Level 1 being and intra (within) area, Level 2 being an inter (between) area, or level 1 and 2 routers. Level 2 routers can only form relationships with other level 2 routers, and level 1 with level 1. 1-2 routers exchange with both kinds and are used for the communication between the two. IS-IS borders are in between routers, designated as level 2 or 1-2. This results in a IS-IS router being a part of a single area, and does not require area 0. OSPF creates a web topology and IS-IS creates a logical topology of a backbone or level 2 routers with branches of level 1-2 and level 1 routers forming individual routers.