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MPLS Guide
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.
Refer to the 7705 SAR Routing Protocols Guide, "IS-IS Command Reference", for
more information on 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.
Refer to 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
For the IGP context, the config>router>isis>interface>bfd-enable command
string is used; refer to the 7705 SAR Routing Protocols Guide, "IS-IS Command
Reference".
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Label Distribution Protocol
LDP
Commands.
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