The IGP most commonly used in large service provider networks

Like OSPF, it uses the Dijkstra algorithm to calculate the best path

Originally designed to use CLNS – Connectionless Network Service

ISIS protocol data units (PDUs) are used to transmit the routing information

Just like OSPF, a singe AS can be divided into smaller groups called areas

Level 1 & 2 Routers

Level 1 routers route within an area or towards a Level 2 system

Level 2 routers route between areas and toward other Autonomic Systems

Level 1/2 router sets the attached bit in the Level 1 PDU’s to indicate that it’s attached to a Level 2 router

Level 1 routers create a default route for interarea prefixes, which points to the closest Level 1/2 router

NET – Network Entity Title Address

To enable ISIS, you have to set a NET address on an interface, preferably lo0, this is essentially setting the system ID for each router.

The standard for these addresses can vary depending on your personal design, but a good reference design is below:

AFI – Authority format identifier, the first two numbers, which are commonly just set to 49.

The following four numbers after the AFI can be used to designate the area.

System ID – Following this, you could use the router ID to set this number, this is where it starts to define the individual router.

N Selector – the last two numbers, which can be left at 00