The IGP instance SPF routine performs the Dijkstra tree calculation on the topology with IP links only and saves the information in both the unicast routing table and in the multicast routing table. It then performs the IP reach calculation in the multicast routing table for each prefix family that disabled IGP shortcuts. Concurrently, it lays the tunnels on the tree and performs the IP reach calculation in the unicast routing table for each prefix family that enabled IGP shortcuts.
The following are the details of the resolution of prefix families in the unicast or multicast routing tables.
OSPF supports IPv4 prefixes by enabling family=ipv4. IPv4 prefix resolution in the unicast routing table can mix IP and tunnel next hops with the preference given to tunnel next hops. A maximum of 64 ECMP tunnel and IP next hops can be programmed for an IPv4 prefix.
OSPFv3 supports IPv6 prefixes by enabling family=ipv6. IPv6 prefix resolution in the unicast routing table can mix IP and tunnel next hops with the preference given to tunnel next hops. A maximum of 64 ECMP tunnel and IP next hops can be programmed for an IPv6 prefix.
IS-IS supports IPv4 prefixes in MT=0 by enabling family=ipv4 and ipv6 prefixes in both MT=0 and MT=2 by enabling family=ipv6. IPv4 and IPv6 prefix resolution in the unicast routing table can mix IP and tunnel next hops with the preference given to tunnel next hops. A maximum of 64 ECMP tunnel and IP next hops can be programmed for an IPv4 or IPv6 prefix.
family=ipv4 also enables the resolution in the unicast routing table of LDP IPv4 prefix FEC in OSPF or IS-IS. When prefer-tunnel-in-tunnel is enabled (disabled) in LDP, an LDP FEC selects tunnel next hops (IP next hops) only and does not mix these next hop types when both are eligible in the unicast routing table.
A maximum of 32 ECMP tunnels next hops can be programmed for a LDP FEC.
LDP IPv6 prefix FECs are not supported over IPv4 IGP shortcuts when enabling family=ipv6. A consequence of this is that if the corresponding IPv6 prefix resolves to tunnel next hops only, the LDP IPv6 prefix FEC remains unresolved.
In all cases, the IP reach calculation in the unicast routing table first follows the ECMP tunnel and IP next hop selection rules, described in ECMP Considerations, when resolving a prefix over IGP shortcuts. After the set of ECMP tunnel and IP next hops have been selected, the preference of tunnel type is then applied based on the user setting of the resolution of the family of the prefix. If the user enabled resolution of the prefix family to both RSVP-TE and SR-TE tunnel types, the TTM tunnel preference value is used to select one type for the prefix. In other words, RSVP-TE LSP type is preferred to a SR-TE LSP type on a per-prefix basis.
One or more SR-TE LSPs can be selected in the unicast routing table if resolution=filter and the resolution-filter=sr-te.
One or more SR-TE LSPs can also be selected in the unicast routing table if resolution=any and one or more SR-TE LSPs are available but no RSVP-TE LSPs are available for resolving the prefix by IGP.
An intra-area IP prefix of family=ipv4, or family=ipv6, or an LDP IPv4 prefix FEC always resolves to a single type of tunnel rsvp-te or sr-te. rsvp-te type is preferred if both types are allowed by the prefix family resolution and both types exist in the set of tunnel next hops of the prefix. The feature does not support mixing tunnel types per prefix.
An inter-area IP prefix of family=ipv4, or family=ipv6, or an LDP IPv4 prefix FECs always resolves to a single tunnel type and selects the tunnel next hops to the advertising ABR node from the most preferred tunnel type if the prefix family resolution allowed both types. If the prefix resolves to multiple ABR next hops, ABR nodes with the preferred tunnel type are selected. In other words, if RSVP-TE LSPs exist to at least one ABR node, ABR nodes that are the tail-end of only SR-TE LSPs are not used in the set of ECMP tunnel next hops for the inter-area prefix.
The feature does not support configuring a different tunnel type per prefix family in resolution-filter. The no shutdown command within the igp-shortcut context fails if the user configured family=ipv4 to resolve to sr-te and family=ipv6 to rsvp-te or the other way around. This is true for both inter-area and intra-area prefixes.
The feature does, however, support selecting the best tunnel-type per prefix within each family as described in (5). For instance, family=ipv4 and family=ipv6 can both configure resolution=any. On a per prefix-basis, the best tunnel type is selected, therefore allowing both tunnel types to be deployed in the network.
The user can set resolution=disabled for each family independently, which disables IGP shortcuts for the corresponding prefix family in this IGP instance. IP Prefixes and LDP FECs of this family resolve over IP links in the multicast routing table.