Foundation Summary

     

The "Foundation Summary" section of each chapter lists the most important facts from the chapter. Although this section does not list every fact from the chapter that will be on your exam, a well-prepared candidate should, at a minimum, know all the details in each "Foundation Summary" before going to take the exam.

Figure 11-2 shows how IS-IS selects the best path to be entered into the forwarding table.

Figure 11-2. Selecting the Best Path to Be Entered into the Forwarding Table

The routing process is divided into four stages:

• The update process

• The decision process

• The forwarding process

• The receive process

Some typical area designs include the following:

• A totally flat network that uses only Level 1 routing.

• A totally flat network using only Level 2 routing. As the network expands, Level 1 areas may be added.

• A totally flat network using the Cisco default configuration of every router running Level 1-2 routing.

• A hierarchical network where the core is running Level 2 routing with Level 1 areas connecting into the core . Level 1-2 routers are used to connect the areas.

By default, Cisco routers running Integrated IS-IS operate as Level 1-2 routers, which overcomes many potential pitfalls, including the following:

• Area partitions

• Loss of data

• Suboptimal routing decisions

However, this configuration consumes far more resources than a topology that uses a hierarchical design with Level 1, Level 2, and Level 1-2 routers.

Integrated IS-IS does not have a point-to-multipoint configuration option. The alternatives are as follows :

• Configure the link as multipoint, creating a broadcast environment that requires the election of a DIS.

• Configure the interfaces with subinterfaces that are point-to-point.