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| 7.6 |  | Integrated IS-IS Operation in a WAN Environment |
| 7.6.1 |  | Point-to-point and point-to-multipoint operation with IS-IS |

WANs are typically implemented as either point-to-point or point-to-multipoint. WANs do not support broadcasts, thus the term NBMA.

Point-to-point WANs can be leased circuits between two routers. A point-to-point WAN has two devices attached, one device at each end of the circuit. Such links commonly run Cisco HDLC or Point-to-Point Protocol (PPP). These WAN links correspond exactly to the Integrated IS-IS classification of a point-to-point network.

> Note: A point-to-point circuit is still regarded as an NBMA network, just as a back-to-back Ethernet connection is still a LAN. Both are examples of multiple access networks that have only two devices attached.

Dialup networks using dial-on-demand routing (DDR) can be configured as either point-to-point or point-to-multipoint WANs. Legacy DDR connections using dialer map statements are NBMA networks, despite the fact that they may use PPP as their line protocol. This is because a single dialer interface can support multiple destinations. Dialer profiles and dialer virtual profiles are point-to-point connections, where one dialer profile equates to one remote profile. These connections can suffer from the same loss-of-neighbor delays as other NBMA networks. Dialer virtual profiles are point-to-point connections where the interface drops immediately if the remote end disconnects, leading to faster neighbor loss detection and faster convergence. Dial interfaces and dialer profiles are not dealt with in this course. As a general rule, avoid using IS-IS over dialup, except to provide dial backup functionality.

IS-IS can work only over NBMA clouds, such as Frame Relay, configured with a full mesh. Anything less than a full mesh could create serious connectivity and routing issues. However, even if a full mesh is configured, this is no guarantee that a full mesh will exist at all times. A failure in the underlying switched WAN network, or a misconfiguration on one or more routers, could break the full mesh either temporarily or permanently.

Avoid NBMA multipoint configurations for IS-IS networks. Use point-to-point subinterfaces instead.

Point-to-point interfaces should usually be explicitly configured with an IP subnet. In this case, a 31-bit mask, with the IOS release 12.2 required, or a 30-bit mask is applied. In modern IP networks using private addressing and variable length subnetting, there are usually plenty of spare IP addresses to apply to point-to-point interfaces. Alternatively, in conformance with RFC 1195, IP unnumbered can be used with IS-IS on point-to-point interfaces.

Recall that, on a point-to-point link, a single IIH PDU type is used. These IIHs specify whether the adjacency is L1, L2, or both. When the adjacency is established, each neighbor sends a CSNP describing the contents of its link-state database. Each router then requests any missing LSPs from the neighbor using PSNPs and acknowledges the receipt of the LSPs with PSNPs. This activity reduces the amount of routing traffic across the point-to-point link. Each router exchanges only the information missing from its link-state database rather than the entire link-state database of its neighbor router.

| Web Links Configuring Integrated IS-IS http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56f.html