Integrated IS-IS Configuration on Cisco IOS

Lessons Discussion
21

Hello, everyone.

Cisco documentation completely omits the IDP part of the NET.

The NET may be 8 to 20 octets in length and consists of three parts

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Apparently, the Area address an be 1 to 13 bytes in length. The Sys-ID Is 6 bytes and the NSEL is 1 byte. This together creates 20 bytes.

There doesn’t appear to be any space for IDP. Even if I configure a NET address that uses it, such as:

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It includes the AFI and IDI in the area address. Does Cisco ignore or not recognize the first two? Or are they used so little that they decided to completely omit it?

A Cisco Live presentation says this:

IS-IS Addressing - Sample NET Addresses
:one: 8-byte Area ID / System ID

Area Address        Station ID        SEL
┌──────────┐        ┌──────────────┐  ┌──┐
07          .1921.6800.1001          .00
└──────────┘        └──────────────┘  └──┘
     Area
  • Area Address: 07
  • Station ID (System ID): 1921.6800.1001
  • SEL (NSEL): 00

:two: 10-byte Private AFI / Area ID / System ID

Area Address
┌──┬──────┐        ┌──────────────┐  ┌──┐
49 .1234   .1921.6800.1001          .00
└──┴──────┘        └──────────────┘  └──┘
 AFI  Area         Station ID        SEL
  • AFI: 49 (Private)
  • Area ID: 1234
  • Station ID (System ID): 1921.6800.1001
  • SEL (NSEL): 00

:three: OSI NSAP Format

Area Address
┌──┬────┬──────┐   ┌──────────────┐  ┌──┐
49 .0456 .1234   .1921.6800.1001    .00
└──┴────┴──────┘   └──────────────┘  └──┘
 AFI Domain  Area       Station ID   SEL
  • AFI: 49
  • Domain (IDI): 0456
  • Area (HODSP): 1234
  • Station ID (System ID): 1921.6800.1001
  • SEL (NSEL): 00

Apparently, the Area Address consists of the AFI, IDI, and the Area number.. So the area address is not the same as the area or is it? I’ve tried changing the AFI or Domain while keeping the Area number the same and the routers formed only a L2 adjacency so I don’t think they considered the area to be the same :smiley:

Thank you
David

22

Hello David

There are many acronyms used here and this can get confusing!! Let’s first see what the official authoritative addressing scheme is and then we’ll see how Cisco interprets it:

According to the official ISO standard, the NET format is like so:

  • AFI at 8 bits
  • IDI variable length at 0 or more bits
  • The HODSP is variable length between 0 and 96 bits (based on the constraints of the rest of the fields)
  • The system ID at 48 bits
  • the NSEL at 8 bits (which is always 0 for IS-IS use)

Note that:

  • The AFI + IDI is defined as the Inter-Domain Part or the IDP
  • And overall, the Area address is the AFI + IDI + HODSP
  • And the max size of the Area address is 13 bytes
  • However, because virtually all IS-IS deployments use an AFI of 49 (private addressing), the IDI is almost always zero length. Thus, the AFI is essentially the IDP in this case.

So the overall structure is this:

|    Area Address    |   System ID   |  NSEL |
|    IDP    | HODSP  |   System ID   |  NSEL |
|AFI  | IDI | HODSP  |   System ID   |  NSEL |

Now keep in mind that IS-IS didn’t invent these fields, they are part of the OSI Layer 3 addressing standard, and IS-IS simply uses them.

Now going on to Cisco. Indeed, Cisco does not mention the IDP or IDI or the AFI for that matter (I didn’t find it in the documentation you sent). I see it in the Cisco documentation you shared, where it does mention the AFI, but the rest of it calls it simply the “area,” where officially this is the HODSP.

Cisco simply doesn’t go into the details of the structure of the area address simply because it is not necessary for the purposes of IS-IS. This is a practical simplification, not a technical omission.

I hope this has been helpful!

Laz

PS I replaced your image with text that says the same thing for copyright purposes.