Configuring IS-IS on NBMA Media

This section reviews configuration examples for popular nonbroadcast multiaccess media (NBMA), such as Asynchronous Transfer Mode (ATM), Frame Relay (FR), and Integrated Data Services Network (ISDN). IS-IS treats this media as broadcast media when they are configured as multipoint. This assumes that devices connected to such media are fully meshed with each other, which might not be always the case. When routers connecting to NBMA media are not fully meshed, routing might not work well because synchronization of the LS database might be flawed. In such cases, it is necessary to ensure that the DIS is connected to every node on the NBMA cloud, so that the periodic Complete Sequence Number Packets (CSNP) sent out by the DIS reaches every router.

The Cisco IOS Software offers an alternative for multipoint NBMA configuration. The option is known as point-to-point and allows the logical connections underlying such media (permanent virtual circuits, PVCs) to be modeled as point-to-point links. The point-to-point NBMA links simplify the NBMA topology for operation of routing protocols, by reflecting actual connectivity between network devices.

For highly meshed NMBA clouds with point-to-point setups, the IS-IS mesh group feature provides a means to reduce redundant flooding. IS-IS mesh groups are discussed in Chapter 5, "The IS-IS Link-State Database," in the section, "Flooding over NBMA Transport Media."

The following subsections feature configuration examples for ATM, FR, and ISDN. Both point-to-point and multipoint configuration examples are provided for ATM and FR.

ATM Configuration

The ATM point-to-point configuration procedure for IS-IS is similar to the serial point-to-point example discussed earlier. Figure 9-4 shows an ATM point-to-point example.

Figure 9-4. ATM point-to-point example.

If the ATM cloud is highly meshed with lots of PVC interconnections, an IS-IS mesh group configuration needs to be used to reduce the incidence of undesirable redundant flooding. Redundant flooding wastes network resources and can impact performance. The IS-IS mesh group feature is covered in Chatper 5 and also in Chapter 8, "Network Design Scenarios." The multipoint example shown in Figure 9-5 requires IP and CLNS map statements in the ATM map-list configuration section.

Figure 9-5. ATM multipoint example.

Frame Relay Configuration

Just as in the case of ATM, FR setups can be configured in point-to-point or multipoint modes. The IS-IS point-to-point configuration (see Figure 9-6) is similar to the serial point-to-point example with the obvious difference being the subinterface configuration and the FR map statement.

Figure 9-6. Frame Relay point-to-point example.

Figure 9-7 shows an FR multipoint configuration example. The point-to-point configuration is recommended over the multipoint configuration because it is maps the Layer 3 layout of the network to the underlying Layer 2 infrastructure with a clear indication of which routers have PVCs between them. This helps avoid problems with routing over a nonfully meshed multipoint cloud.

Figure 9-7. Frame Relay multipoint example.

Similar to the case of ATM, the IS-IS mesh group feature must always be considered for highly meshed FR environments with point-to-point configurations. This helps limit unnecessary flooding of LSPs.

ISDN Configuration

Figure 9-8 shows a typical multipoint ISDN configuration, which follows the generic multipoint approach discussed with regard to ATM and FR. The key differences in all three cases lie in the Layer 2 configuration specifics for each technology.

Figure 9-8. ISDN example.