Network Entity Titles

Network Entity Titles
So far, we’ve discussed the commonalities and differences between IS-IS and OSPF. The main focus
of Integrated IS-IS is how it can route IP packets. What we need to remember is that IS-IS is a CLNP
protocol, not a TCP/IP protocol. This means that even though routing IP is supported, IS-IS still
communicates with CLNS PDUs. An ISO addressing scheme must be implemented for IS-IS to function.
The ISO address is known as a network entity title (NET). The NET is used just like an IP
address to uniquely identify a router on the internetwork. The NET on Cisco devices may take the
form of a variety of standard Network Service Access Point (NSAP) formats. Each of these formats
has three common values:
Area ID
The area ID is typically a one- or two-octet field that precedes the system ID. The area
ID is used to signify the area the router belongs to. The area ID can span up to two octets if need
be. For the BSCI, we will concentrate on a one-octet area ID. For the NSAP addressing scheme
used with IS-IS, the area ID is everything to the left of the system ID.
System ID
The system ID is used to identify the router. It is similar to the router ID in OSPF.
The system ID can be up to eight octets in length. Cisco supports only a six-octet field for the
system ID. Whether or not the vendor you are using supports more than six octets, the number
of octets used must be the same throughout the IS-IS routing domain. Normally, the system ID
will be set to one of the MAC addresses on the router. This is not a rule, and the system ID can
be represented in a number of ways. The system ID that is used for a router must be unique
throughout the IS-IS routing domain. There must be only one system ID set per router, regardless
of the number of routing processes. The system ID must be common across all routing processes
on the same router.
SEL
The NSAP selector byte (SEL) is a one-octet field that represents the service being
offered at the network level of the device. For our implementation of IS-IS, the SEL will
always be set to 00. 00 represents the router. A good comparison to the SEL is the TCP or
UDP port number being associated with an IP address, forming a socket that identifies a
unique application instance on the internetwork. Basically, it tells you what service is being
referenced for the particular address.
As previously mentioned, the NET can come in and be entered in a variety of NSAP formats,
the last eight bytes of each serving the same function. Let’s go ahead and take a look at the three
most common formats, as shown in Figure 7.3.
As you can see, there are three common NSAP formats that the NET may be in:

Standard eight-octet format

OSI NSAP format

GOSIP format
It is important that you know these three formats exist. The BSCI exam concentrates mainly on
the standard eight-octet format. So that will be the one we will focus on in this chapter.