Integrated IS-IS Design Considerations 271
By default, six-bit metrics are conﬁgured on the outgoing interface. A 10-bit ﬁeld describes the
total path cost. These default metrics are referred to as narrow.
Because it considered these inadequate, Cisco increased the metric size to 24 bits. This larger
metric ﬁeld provides more granularity to distinguish between paths and is referred to as wide.
To determine shortest path, the lowest metric is chosen, internal paths are chosen over external
paths, and Level 1 routes have precedence over Level 2 routes.
The default metric is the only metric supported by Cisco, because each metric used in Integrated
IS-IS requires a different link-state database calculation for both the Level 1 and Level 2 routes.
Integrated IS-IS Design Considerations
Optimizing the network relies on careful planning and design. Although each network is
constrained by physical and technical limitations, you must strive to design your network to meet
the needs of its users and accommodate the demands of various applications.
In Integrated IS-IS, the fundamental design considerations are areas and addressing. The next
sections describe the Integrated IS-IS design considerations of area design, route summarization,
and nonbroadcast multiaccess (NBMA) modeling solutions.
Area Design of Integrated IS-IS Routers
When designing a network, you have to compromise. Typically, the trade-off is between
reliability and speed. What is most efﬁcient for the network is determined by the requirements
of the network and the resources available.
In designing Integrated IS-IS networks and the hierarchical design, you need to consider the data
ﬂow in addition to the resources required by the routing protocol. Tuning the update process might
be sufﬁcient, although this results in compromising resources and reliability. If you reduce the
update timers, the databases converge more quickly, but the network could be depleted of
resources necessary to route data.
In area design, two SPF databases need to be maintained; this requires the use of additional
resources for those routers that straddle more than one level.
Some typical designs include the following:
■ A totally ﬂat network that uses only Level 1 routing. This design will not scale because any
change in the network requires a ﬂood of LSPs to all routers, which consequently run the