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