This feature provides stitching between an LDP FEC and an SR node SID route for the same IPv4 /32 IS-IS prefix by allowing the export of SR tunnels from the Tunnel Table Manager (TTM) to LDP (IGP). In the LDP-to-SR data path direction, the LDP tunnel table route export policy supports the exporting of SR tunnels from the TTM to LDP.
A route policy option is configured to support LDP-to-SR stitching using the config>router>policy-options context. See the 7705 SAR Router Configuration Guide, ‟Configuring LDP-to-Segment Routing Stitching Policies”, for a configuration example and to ‟Route Policy Command Reference” for information about the commands that are used.
After the route policy option is configured, the SR tunnels are exported from the TTM into LDP (IGP) using the config>router>ldp>export-tunnel-table command. See LDP Command Reference for more information about this command.
When configuring a route policy option, the user can restrict the exporting of SR tunnels from the TTM to LDP from a specific prefix list by excluding the prefix from the list.
The user can also restrict the exporting of SR tunnels from the TTM to a specific IS-IS IGP instance by specifying the instance ID in the from protocol statement. The from protocol statement is valid only when the protocol value is isis. Policy entries with any other protocol value are ignored when the route policy is applied. If the user configures multiple from protocol statements in the same policy or does not include the from protocol statement but adds a default-action of accept, then LDP routing uses the lowest instance ID in the IS-IS protocol to select the SR tunnel.
When the routing policy is enabled, LDP checks the SR tunnel entries in the TTM. Whenever an LDP FEC primary next hop cannot be resolved using an RTM route and an SR tunnel of type isis to the same destination IPv4 /32 prefix matches an entry in the export policy, LDP programs an LDP ILM and stitches it to the SR node SID tunnel endpoint. LDP then originates a FEC for the prefix and redistributes it to its LDP peer. When a LDP FEC is stitched to an SR tunnel, forwarded packets benefit from the protection of the LFA/remote LFA or TI-LFA backup next hop of the SR tunnel.
When resolving a FEC, LDP attempts a resolution in the RTM before attempting a resolution in the TTM, when both are available. That is, a swapping operation from the LDP ILM to an LDP NHLFE is attempted before stitching the LDP ILM to an SR tunnel endpoint.
In the SR-to-LDP data path direction, the SR mapping server provides a global policy for the prefixes corresponding to the LDP FECs the SR needs to stitch to. Therefore, a tunnel table export policy is not used. The user enables the exporting of the LDP tunnels for FEC prefixes advertised by the mapping server to an IGP instance using the command config>router>isis>segment-routing>export-tunnel-table ldp. See the 7705 SAR Routing Protocols Guide, ‟IS-IS Command Reference”, for more information about this command.
When the export-tunnel-table ldp command is enabled, the IGP monitors the LDP tunnel entries in the TTM. Whenever an IPv4 /32 LDP tunnel destination matches a prefix for which the IGP received a prefix SID sub-TLV from the mapping server, the IGP instructs the SR module to program the SR ILM and to stitch it to the LDP tunnel endpoint. The SR ILM can stitch to an LDP FEC resolved over the LDP link. When an SR tunnel is stitched to an LDP FEC, forwarded packets benefit from the protection of the LFA backup next hop of the LDP FEC.
When resolving a node SID, the IGP attempts a resolution of the prefix SID received in an IP reachability TLV before attempting a resolution of a prefix SID received via the mapping server, when both are available. That is, a swapping operation of the SR ILM to an SR NHLFE is attempted before stitching it to an LDP tunnel endpoint. See the 7705 SAR Routing Protocols Guide, ‟Prefix SID Resolution for a Segment Routing Mapping Server”, for more information about prefix SID resolution.
It is recommended that the bfd-enable option be enabled on the interfaces for both LDP and IGP contexts to speed up the failure detection and the activation of the LFA/remote LFA backup next hop in either direction. This applies particularly for remote failures. For the LDP context, the config>router>ldp>interface-parameters>interface>bfd-enable command string is used; see LDP Commands. For the IGP context, the config>router>isis>interface>bfd-enable command string is used; see the 7705 SAR Routing Protocols Guide, ‟IS-IS Command Reference”.
The sections that follow describe how stitching is performed in the LDP-to-SR and SR-to-LDP data path directions.