In OSPF protocol any of the router’s interfaces can be assigned to a particular area, however the concept of area in IS-IS is different. Here in general, every single router belongs to an Area. The idea of this comes from the fact that IS-IS was initially created to route Connectionless Network Protocol (CLNP) where the address belongs to a device (Router), whereas in Internet Protocol (IP) the address belongs to the particular interface.
IS-IS protocol has two levels or hierarchy, Level 1 and Level 2. Level 1 corresponds OSPF intra-area routing whereas Level 2 corresponds with the OSPF backbone Area 0 routing. Level 2 areas join all the areas with the backbone area. Every Cisco router comes by default as Level 1-2 (L1/L2) router to allow for easy configuration and deployment.
A Level 1 router can become adjacent with the Level 1 and Level 1-2 (L1/L2) router. A Level 2 router can become adjacent with Level 2 or Level 1-2 (L1/L2) router. There is no adjacency between L1 only and L2 only router.
IS-IS Level 1 (L1) Router
An IS-IS Level 1 router has the link state information of its own area for all the intra-area topology. In order to route packets to other areas it uses the closest Level 2 capable (L1/L2) router. Level 1 Area behaves pretty much as OSPF totally stubby area. L1 only router send L1 Hellos.
IS-IS Level 1-2 (L1/L2) Router
An IS-IS L1/L2 router maintains two link state database information. One is for Level 1 and the other for Level 2.Hence two distinct Shortest Path First (SPF) calculations are run, one on Level 1 link state database and other on the Level 2 link state database. IS-IS Level 1-2 router behaves very close to OSPF Area Border Router (ABR). L1/L2 router sends both L1 and L2 hellos.
As default behaviour L1/L2 router will only allow one way passage of prefixes from L1 Area to L2 Area, but not in reverse.
However if it is required to move prefixes from L2 Area to L1 Area then redistribute command under IS-IS configuration is required.
IS-IS Level 2 (L2) Router An IS-IS Level 2 router has the link state information for the intra-area as well as inter-area routing. L2 router sends only L2 hellos. IS-IS Level 2 area can be compared with OSPF backbone area 0.
If one IS-IS router receives an ISIS hello packet with higher MTU than it can support (on the interface) it discards the hello hence the adjacency doesn’t come up. In best practice MTU must be same on both the ends. There are only two network types in IS-IS. Broadcast and Point-to-Point. Broadcast is default network type. If one end is configured with “isis network point-to-point” and other end is default network type. The hellos will be discarded and adjacency will not come up. Hence network type must match on both the ends.
How to define area and isis id
Router 1: interface Loopback0 ip address 172.16.1.1 255.255.255.255 ! router isis 1 net 49.0001.1720.1600.1001.00 1) pad loopback with zeros (we pad each octet to 3 digits): 172.16.1.1 ---> 172.016.001.001 2) rearrange the bits to 3 groups of 4 digits: 172.016.001.001---> 1720.1600.1001 3) add area prefix (49.0001) and NSEL suffix (.00) 49.0001.1720.1600.1001.00