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OSPF & IS-IS Router ID

When you think about Router ID in any link state protocol  two requirements come to mind:

  1. Each router must be identified by a Router ID and an Area to exist in.
  2. This Router ID must be unique inside a single IGP domain.

IS-IS refers to the this ID as a System ID (SysID) and OSPF refers to this ID as a Router ID (RID). There are some best practices that can be used to ensure the uniqueness of the router ID within the IGP domain.

In a previous post Mohammed discussed the OSPF RID, so I will focus on the SysID used by IS-IS in the following points:

  • Both SysID and Area ID is configured with the same command.
  • Unlike OSPF RID the SysID in IS-IS must be configured manually.
  • The NET address consists of the following fields:

  1. The AFI, the Authority and Format Identifier (1 byte). This usually set to 49 in IP networks which means locally assigned.
  2. The Area ID is a variable length field from 0 t0 12 bytes.
  3. The SysID is  a 6 bytes length field, this must be unique for every router.
  4. The N-selector always set to 00 for any intermediate system (1byte).

Now to make sure that every Router in the network has its own SysID and that this ID is unique, one of the following methods are used which you can choose from:

  1. Use the MAC address as a SysID. The MAC address is 6 bytes long as the SysID and it is globally unique so it satisfies the requirements. However, MAC addresses are not so reader friendly and routers sometimes does not have broadcast interfaces to use its MAC.
  2. Use the loopback address to form the SysID. Example Lo:192.168.1.1 –> 1921.1681.0001.  I prefer this method myself because it keeps things unified. You can also encode the dotted decimal values into hex.
  3. Sequentially assign the SysID to routers. Example: 0000.000.0001, 0000.0000.0002 …. This method is simple and reliable if you have good documentation.

You are free to choose the method that best works for you and suits your network.

You don’t have to cook fancy or complicated masterpieces – just good food from fresh ingredients.     Julia Child

5 comments

  1. I have not seen any network running isis

  2. nice tutorial, can u plz explain AFI more?

  3. @Rady,

    ISIS is widely used in service provider networks and carriers, the protocol can scale very well and in the same time simple in implementation

  4. Peter,

    Originally with ISO addressing the AFI identifies the assigning authority and the format of the address, however actually with IP only routers the AFI has no actual meaning, it is just a part of the AreaID, moreover most commonly an AFI of 49 is used as per the NSAP format standards.

    I hope that I’ve been informative.

    BR,
    Mohammed Mahmoud.

  5. Good explanation. Wouldn’t the IP Address encoded as the System ID be: 1921.6800.1001 instead of 1921.1681.0001, or am I missing something?

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