3com S7906E Manuel De Montage

 

1-3 

The system ID of a device can be generated from the Router ID. For example, a router uses the IP 

address 168.10.1.1 of Loopback 0 as the Router ID, and the system ID in IS-IS can be obtained in the 

following way: 

z 

Extend each decimal number of the IP address to 3 digits by adding 0s from the left, like 

168.010.001.001; 

z 

Divide the extended IP address into 3 sections with 4 digits in each section to get the system ID 

1680.1000.1001. 

There are other methods to define a system ID. The principle is to make sure it can uniquely identify a 

host or router. 

4) SEL 

The NSAP Selector (SEL), or the N-SEL, is similar to the protocol identifier in IP. Different transport 

layer protocols correspond to different SELs. All SELs in IP are 00. 

5) Routing 

method 

Because the area information is identified in IS-IS addresses, a Level-1 router can easily identify 

packets destined to other areas.  

z 

A Level-1 router makes routing decisions based on the system ID. If the destination is not in the 

area, the packet is forwarded to the nearest Level-1-2 router. 

z 

A Level-2 router routes packets across areas according to the area address. 

A network entity title (NET) indicates the network layer information of an IS and does not include 

transport layer information. It is a special NSAP address with the SEL being 0. Therefore, the length of 

the NET is equal to the NSAP and is in the range 8 bytes to 20 bytes. 

Generally, a router only needs one NET, but it can have three NETs at most for smooth area merging 

and partitioning. When you configure multiple NETs, make sure their system IDs are the same. 

For example, a NET is ab.cdef.1234.5678.9abc.00, where, 

Area = ab.cdef, System ID = 1234.5678.9abc, and SEL = 00. 

IS-IS has a two-level hierarchy to support large scale networks. A large scale routing domain is divided 

into multiple Areas. Typically, a Level-1 router is deployed within an area, a Level-2 router is deployed 

between areas, and a Level-1-2 router is deployed between Level-1 and Level-2 routers.  

1) Level-1 

router 

A Level-1 router establishes neighbor relationships with Level-1 and Level-1-2 routers in the same area. 

The LSDB maintained by the Level-1 router contains the local area routing information. It directs the 

packets destined for an outside area to the nearest Level-1-2 router. 

2) Level-2 

router 

A Level-2 router establishes neighbor relationships with the Level-2 and Level-1-2 routers in the same 

or in different areas. It maintains a Level-2 LSDB which contains inter-area routing information. All the 

Level-2 and Level-1-2 routers must be contiguous to form the backbone of a routing domain.