SR-TE LSP path computation using hop-to-label translation

MPLS passes the path information to the TE-DB, which converts the list of hops into a label stack as follows:

• A loose hop with an address matching any interface (loopback or not) of a router (identified by router ID) is always translated to a node SID. If the prefix matching the hop address has a node SID in the TE database, it is selected by preference. If not, the node SID of any loopback interface of the same router that owns the hop address is selected. In the latter case, the lowest IP-address of that router that has a /32 Prefix-SID is selected.

• A strict hop with an address matching any interface (loopback or not) of a router (identified by router ID) is always translated to an adjacency SID. If the hop address matches the host address reachable in a local subnet from the previous hop, then the adjacency SID of that adjacency is selected. If the hop address matches a loopback interface, it is translated to the adjacency SID of any link from the previous hop which terminates on the router owning the loopback. The adjacency SID label of the selected link is used.

  In both cases, it is possible to have multiple matching previous hops in the case of a LAN interface. In this case, the adjacency-SID with the lowest interface address is selected.

• In addition to the IGP instance that resolved the prefix of the destination address of the LSP in the RTM, all IGP instances are scanned from the lowest to the highest instance ID, beginning with IS-IS instances and then OSPF instances. For the first instance via which all specified path hop addresses can be translated, the label stack is selected. The hop-to-label translation tool does not support paths that cross area boundaries. All SID/labels of a path are therefore taken from the same IGP area and instance.

• Unnumbered network IP interfaces, which are supported in the router’s TE database, can be selected when converting the hops into an adjacency SID label when the user has entered the address of a loopback interface as a strict hop; however, the user cannot configure an unnumbered interface as a hop in the path definition.

  Note: For the hop-to-label translation to operate, the user must enable TE on the network links, meaning to add the network interfaces to MPLS and RSVP. In addition, the user must enable the traffic-engineering option on all participating router IGP instances. Note that if any router has the database-export option enabled in the participating IGP instances to populate the learned IGP link state information into the TE-DB, then enabling of the traffic-engineering option is not required. For consistency purposes, it is recommended to have the traffic-engineering option always enabled.