Forwarding Adjacency

The forwarding adjacency feature can be enabled independently from the IGP shortcut feature in CLI. To enable forwarding adjacency, the user enters the following command in IS-IS or OSPF:

• config>router>isis>advertise-tunnel-link

• config>router>ospf>advertise-tunnel-link

If both igp-shortcut and advertise-tunnel-link options are enabled for a specific IGP instance, then the advertise-tunnel-link wins. With this feature, ISIS or OSPF advertises an RSVP LSP as a link so that other routers in the network can include it in their SPF computations. An SR-TE LSP is not supported with forwarding adjacency. The RSVP LSP is advertised as an unnumbered point-to-point link and the link LSP/LSA has no TE opaque sub-TLVs as per RFC 3906 Calculating Interior Gateway Protocol (IGP) Routes Over Traffic Engineering Tunnels.

When the forwarding adjacency feature is enabled, each node advertises a p2p unnumbered link for each best metric tunnel to the router-id of any endpoint node. The node does not include the tunnels as IGP shortcuts in SPF computation directly. Instead, when the LSA/LSP advertising the corresponding P2P unnumbered link is installed in the local routing database, then the node performs an SPF using it like any other link LSA/LSP. The link bidirectional check requires that a link, regular link or tunnel link, exists in the reverse direction for the tunnel to be used in SPF.

The forwarding adjacency feature supports forwarding of both IPv4 and IPv6 prefixes. Specifically, it supports family IPv4 in OSPFv2, family IPv6 in OSPFv3, families IPv4 and IPv6 in ISIS MT=0, and family IPv6 in ISIS MT=2. Note that the igp-shortcut option under the LSP name governs the use of the LSP with both the igp-shortcut and the advertise-tunnel-link options in IGP. Table 1 describes the interactions of the actions of the forwarding adjacency feature.