IS-IS System and Area IDs

IS-IS System and Area IDs:

In contrast to the OSPF AID and RID, which are expressed separately, the IS-IS AID and SysID are specified together in the Network Entity Title (NET). The NET is a special version of an ISO network service access point (NSAP) address, familiar to anyone who has worked with ISO protocols or with ATM. Figure 4.8 shows the basic format of a NET.

Figure 4.8. The format of a Network Entity Title.

Figure 4.8

As Figure 4.8 indicates, there are a few rules for configuring a NET:

  • The AFI must be 1 byte.
  • The remaining Area ID can be from 0 to 12 bytes.
  • The SysID must be 6 bytes.
  • The SEL must be 1 byte.

The NET is always specified in hexadecimal.

The Authority and Format Identifier (AFI) is actually a part of the Area ID, but is identified separately because of its special configuration rule. In ISO addresses, the AFI identifies the assigning authority of the address and the format of most of the rest of the address. But when the NET is assigned to a router in an IP-only network, the AFI has no real meaning separate from the rest of the AID.

The last byte, the NSAP Selector (SEL), is used in ISO protocols to identify an upper-layer function to which the address pointssomething like a port number in IP protocols. The SEL value 0x00 specifies the router itself. In an IP-only network, where there are no upper ISO protocol layers, a router never examines the SEL, which therefore can be set to any 1-byte value. Nonetheless, common practice is to always set the SEL to 0x00.

NET Configuration Tips

Unlike some OSPF implementations, there are no automatic selection mechanisms for the NETit is always manually configured. Although the NSAP format provides the possibility of some complex NET configurations, it is best in IP-only networks to keep it as simple as possible.

The simplest approach to configuring an area ID is to use only the AFI field for the AID. For example, the following NET specifies an AID of 05, a SysID of 00d0.b775.ff31 and a SEL of 00:


Alternatively, many network operators prefer to stay in compliance with NSAP format standards by specifying an AFI of 49, which is the NSAP AFI indicating a locally assigned address, and then specifying the AID after the AFI. The following NET uses an AFI of 49, and an AID of 0005, with the SysID and SEL the same as the last example:


There are several frequently used approaches to specifying a SysID. The first is to select the MAC address of an interface on the router the NET is to be configured on. Because both the MAC address and the SysID are 48 bits, the MAC address is perfectly adaptable as the SysID. And because the interface MAC is globally unique, you are guaranteed that the SysID based on it meets the requirements of being unique within the IS-IS domain. A possible liability of this approach is if the interface that the SysID is taken from is removed from the router and reused on another router. In this case, the MAC address of the same interface could be used inadvertently as the SysID on more than one router. Another problem with this approach is that some routers might not have any broadcast interfaces and so no MAC addresses to use.

Another common approach to configuring the SysID is to encode the 32-bit, IP-based loopback address or RID into a 48-bit SysID. For example, the loopback address might be used as the SysID:


The IP address could also be expressed in hexadecimal:


or perhaps:


However, changing the dotted-decimal address to hex adds an unnecessary complexity to the NET configuration, and can make record keeping and troubleshooting more difficult.

The simplest of all techniques is to assign SysID values sequentially, starting with 1:




Assuming that you document and assign the SysIDs in your network as carefully as you do IP addresses and 32-bit RIDs, the potential of assigning duplicate SysIDs is minimized. The consequences of duplicate SysIDs is the same as with the OSPF.

Some would argue that during troubleshooting the utility of having the IP-based RID encoded in the SysID outweighs the simplicity of sequential numbering. Dynamic Hostname Exchange, described later in this chapter, also eases troubleshooting. Only you can determine the right scheme for your network.


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