At the early days of networking, ISO and IP protocols battled to be the adopted standard of the Internet. Sure IP won as we all see IP Internet but there is no ISO ones.

IS-IS is the protocol deployed by ISO that is used for routing NSAP packets between the routers. later ISO updated this routing protocol to route IP packets only or NSAP packets only or both of them and the new name is Integrated IS-IS.

IS-IS is for “Intermediate System -to- Intermediate System” which is the equivalent to “Router -to- Router” in the IP world. like the OSPF, ISIS is a link state protocol which depends on the areas and all areas must be connected to a backbone, but the backbone in IS-IS is slightly different from OSPF as in OSPF the backbone area is the Zero area which is connected to the all nonzero areas and in IS-IS the backbone is a contiguous chain of level 2 routers.

Although IS-IS is used for routing IP packets but it is using the NSAP addressing to identify the area, system ID of each router. NSAP addressing format is like the following:

Area [1 – 13 bytes]

System ID [6 bytes]

NSEL [1 byte]

Area ID: could be anything from 1 to 13 byte and should be identical within the same area.
example: Area ID: 49.005

System ID: Fixed 6 bytes the same like MAC address, could be the MAC or the IP or anything filling the 6 bytes.
example: 0000.FF15.0100

NSEL: it is equivalent to the port number in TCP/IP so it will be always “00” as there is no use for it in IP routing.

Full NSAP address examples:
01.1720.1600.0001.00
from this address we know that the NSEL is always 00 and the System ID will be 1720.1600.0001 and it fall in area 01

Routers types:

Level-2 routers —> Backbone routers should be contiguous connected to L2 or L1/2 routers in order to get an optimal routing between all areas.
It has only L2 routes in its routing table.

Level-1-2 routers —> Acts like the ABR/ASBR in OSPF, it is the default for every IS and can be connected to any type of routers.
It has both L2 and L1 routes in its routing table as it has two separate LSDB “Link State Data Bases” for L2 and L1 routers it is connected to.
This type of routers are usually used to connect L1 area with another area by a link to L2 or L1/2 router.

Level-1 routers —> These type of routers will have only L1 routes in its routing table and only one database for all L1 routers lying in its area, beside a default route(1) for the external areas pointing to the nearest L1/L2 router.

(1) default route to the Level-1-2 router is the same behavior of the totally stubby area in OSPF as the ABR injects a default route in the area’s routing table.

**Note**: Every router should lie in one area only and in order to configure another area, new ISIS process should be configured “not recommended”.

In IS-IS the default metric for every interface is 10 and we should change the metric manually for every interface in order to prevent IS-IS from acting as a hop count protocol and its maximum hops are 6 as the maximum metric value is 63 for L1 routing and 1023 for L1/2 & L2 routing. In order to solve this issue of the narrow metric style we should change the operation of the IS-IS to use a wide metric style “range from 1 to 16,777,214″ by using the command” metric-style wide” under the “router isis“.

an example for configuring a L1/2 router lying in area 15 and with system ID of AAAA.BBBB.CCCC

router isis netscouts net 15.AAAA.BBBB.CCCC.00 is-type level-1-2 metric-style wideinterface FastEthernet0/0 ip address 10.10.10.1 255.255.255.0 ip router isis netscouts isis metric 3000