Stitching In The Sr-To-Ldp Direction - Nokia 7705 SAR-W Series Manual

MPLS Guide
Note: If R1 has already resolved an LDP FEC for prefix Y, it has an ILM assigned to it.
However, this ILM will not be updated to point toward the SR tunnel because LDP attempts
a resolution in the RTM before attempting a resolution in the TTM. Therefore, an LDP tunnel
is selected before an SR tunnel. Similarly, if an LDP FEC is received after the stitching is
programmed, the LDP ILM is updated to point to the LDP NHLFE because LDP is able to
resolve the LDP FEC in the RTM.
5.1.14.2
Stitching in the data plane in the SR-to-LDP direction is based on the IGP monitoring
the TTM for an LDP tunnel of a prefix matching an entry in the SR TTM export policy.
In
domain and is configured to stitch between SR and LDP. Link R1-R2 is LDP-enabled
but router R2 does not support SR or SR is disabled.
The following steps are performed by the boundary router R1 to configure stitching:
3HE 18686 AAAB TQZZA
5. If R1 finds multiple SR tunnels to destination prefix Y, R1 uses the lowest
instance ID in the IS-IS protocol to select the tunnel.
6. If the user configured multiple from statements or did not include the from
statement but added a default action of accept for the IS-IS protocol, R1 selects
the tunnel to destination prefix Y by using the lowest instance ID in the IS-IS
protocol.
7. The user enables SR in R2. R2 resolves the prefix SID for prefix Y and installs
the SR ILM and the SR NHLFE. R2 is now able to forward packets over the SR
tunnel to router Ry. There is no activity in R1 because the SR ILM is already
programmed.
8. The user disables LDP over the R1-R2 interface in both directions. This causes
the LDP FEC ILM and NHLFE to be removed in R1 and in R2, which can then
only do forwarding using the SR tunnel toward Ry.

Stitching in the SR-to-LDP Direction

Figure 23, router R1 is at the boundary between an SR domain and an LDP
1. R1 receives an LDP FEC for prefix X from router Rx in the LDP domain. The
RTM in R1 indicates that the interface to R2 is the next hop for prefix X.
2. LDP in R1 resolves the received FEC in the RTM and creates an LDP ILM for
the FEC with an ingress label (for example, label L1), and points it to an LDP
NHLFE toward R2 with egress label L2.
3. R1 receives a prefix SID sub-TLV from the R5 mapping server for prefix X.
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