CCNP BSCI Official Exam Certification Guide, Fourth Edition

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

SPF algorithm.