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IS-IS Protocol Addresses



IS-IS (Intermediate System to Intermediate System) Protocol was developed as a OSI network protocol, not TCP/IP. So, normally IS-IS Protocol not uses IP address, but it uses NET (Network Entity Title) addresses. Let’s check this address issue a little.

In IS-IS Protocol, NSAP (Network Service Access Point) is used as network layer address. These addresses are assigned per node, not per interface.

And SNPA (Subnetwork Point of Attachment) is the Layer 2 addresses for IS-IS Protocol.

NSAP consist of three parts. These are:
– Area-address
– SystemID
– N-Selector

Area address also has some parts inside it. These parts are AFI,IDI and HODSP.

IS-IS Protocol, Address Format

ISIS Address Format

Above, we have checked an example ISIS address, 49.0115.125a.ab44.1234.11111.0123.abcd.aaaa.00. AS you can see, 49 is the AFI. 0115.125a.ab44.1234 is the IDI part. For Area Address, last part is HODSP and its value is 1111.

System ID part of this example address is 0123.abcd.aaaa and the last part SEL part is 00.

We can also divide the NSAP address into two parts as Inter Domain Part and Domain Specific Part.

Inter Domain Part consists of AFI (Authority and Format ID) and IDI (Inter Domain ID).

Domain Specific Part consists of High Order Domain Specific Part, System ID and NSEL part. Here, High Order DSP is the subnet, System ID is the identifier of the device in area and the NSEL is the service identifier of NSAP address. When NSEL is equal to “00”, it is called Network Entity Title (NET) and it shows the device itself.

NETs are 8 to 20 bytes long. But in general, they are 10 bytes long.

System ID can be thought like OSPF Router ID. NSEL is like port/socket in TCP / IP.

You need to request an official NSAP prefix if you will use CLNS routing. Even if you will need only IP routing when using IS-IS Protocol, you always needs to use NSAP addresses, but this time you can add the private address space AFI 49.

AFI 49 is used as local addresses. This addresses are not advertised outsite of CLNS network.

In IS-IS ProtocolL2 Multicast Addresses are also used. And LSPs are sent to these addresses. In a broadcast network, L1 updates are sent to 0180:C200:0014 and L2 updates are sent to 0180:C200:0015.

In Point-to-Point networks 0900:2b00:0005 address is used as L2 address.

The other articles on IS-IS Protocol are below:

Routing With IS-IS Protocol – Part 1
Routing With IS-IS Protocol – Part 2 (IS-IS Addresses)
Routing With IS-IS Protocol – Part 3 (IS-IS Packet Types)
Routing With IS-IS Protocol – Part 4 (IS-IS Adjacency)
Routing With IS-IS Protocol – Part 5 (IS-IS versus OSPF)

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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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