Information Technology Reference
In-Depth Information
IS-IS also defines three optional metrics (costs): delay, expense, and error. The delay cost metric
reflects the amount of delay on the link. The expense cost metric reflects the communications
cost associated with using the link. The error cost metric reflects the error rate of the link. IS-IS
maintains a mapping of these four metrics to the quality - of - service (QoS) option in the CLNP
packet header. IS-IS uses these mappings to compute routes through the internetwork. Cisco
routers do not support the three optional metrics.
IS-IS Operation
This section discusses IS-IS areas, designated ISs, authentication, and the network entity title.
Network Entity Title (NET)
To configure the IS-IS routing protocol, a NET must be configured on every IS. Although IS-IS
can be configured to route IP, the communication between ISs uses OSI protocol data units
(PDUs), not IP packets, and runs directly over the data-link layer. The NET is the OSI address
that each IS uses to communicate, by using OSI PDUs. NET addresses range from 8 to 20 bytes.
The NET is made up of two major fields, the initial domain part (IDP) and the domain specific
part (DSP). The IDP consists of a 1-byte authority and format identifier (AFI) and an initial
domain identifier (IDI). The value of the AFI determines the length of the IDI and the specific
encoding format of the DSP. The DSP is composed of a High-Order DSP (HO-DSP), an Area
ID, a System ID and a selector (SEL). All ISs within the routing domain have the same IDP and
HO-DSP value.
IS-IS divides the NET into an area address and a system address, as shown in Figure 8-8.
Network Entity Title
Figure 8-8
IDP
DSP
A
F
I
HO-
DSP
IDI
Area
System ID
SEL
IS-IS Area Address
System Address
L2 ISs use the Area ID. The area address is composed of all bits from the AFI to the HO-DSP.
The System ID must be the same length for all ISs in an area, and for Cisco routers, they must
be 6 bytes in length. Usually, an IS MAC address identifies each unique IS. The SEL is configured
 
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