Forwarding adjacency

The forwarding adjacency feature can be enabled independently from the IGP shortcut feature in CLI. Use the following commands to enable forwarding adjacency 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, 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 described in 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 or LSP advertising the corresponding P2P unnumbered link is installed in the local routing database, the node performs an SPF and uses it like any other link LSA or LSP. The link bidirectional check requires that a link, regular 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: Impact of LSP level configuration on IGP shortcut and forwarding adjacency features describes the interactions of the forwarding adjacency feature.