IS-IS(ISO 10589) is Integrated System to Integrated System which was initially designed by DEC for OSI then tuned to work for TCP/IP protocol by the IETF in RFC 1195. In other words we can say a robust protocol which now supports the both ip as well as CLNP applications. It is a link state routing protocol and uses the same DIJKSTRA algorithm as being used by OSPF (Open Shortest Path First) to calculate the path cost.
IS-IS maintains three types of tables:-
1. Neighbor Table
2. Routing Table
3. Topology Table
Address Format Used By IS-IS
OSI uses connectionless Network Service aka CLNS which is equivalent to IP in TCP/IP. CLNP assigning addresses to a router called Network Service Access Point(NSAP) addresses or Network Entity Title(NET) addresses. IS-IS uses a one address per node. Net addresses can be up to 20 bytes in length. CISCO implementation of NET address tells about area, system id and NSAP selector.
Types Of Router
1. Level 1 (L1) – It maintains the topology database of its own area.
2. Level 2 router consider as backbone router.
3. Level 1-2 routers are interfaces between the Level 1 and Level 2.
The main advantage of using IS-IS because of it’s TLV (Type/Length/Value) which makes it unique from legacy OSPF. It is a hearsay that support of IPv6 in OSPF requires lot of modification from it’s root but the presence of TLV in IS-IS helped it to support IPv6 with no more changes.