Stitching in the LDP-to-SR Direction

Stitching in the data plane in the LDP-to-SR direction is based on the LDP module monitoring the TTM for an SR tunnel of a prefix matching an entry in the LDP TTM export policy.

Figure: Stitching in the LDP-to-SR Direction

In Figure: Stitching in the LDP-to-SR Direction, router R1 is at the boundary between an SR domain and an LDP 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:

  1. Router R1 receives a prefix SID sub-TLV in an IS-IS IP reachability TLV originated by router Ry for prefix Y.

  2. R1 resolves the prefix SID and programs an NHLFE on the link toward the next hop in the SR domain. R1 programs an SR ILM and points it to the NHLFE.

  3. Because R1 is programmed to stitch LDP to SR, LDP in R1 checks the TTM and finds the SR tunnel to prefix Y. LDP programs an LDP ILM and points it to the SR tunnel. As a result, both the SR ILM and LDP ILM are now pointing to the SR tunnel, one via the SR NHLFE and the other via the SR tunnel endpoint.

  4. R1 advertises the LDP FEC for prefix Y to all its LDP peers. R2 is now able to install an LDP tunnel toward Ry.

  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.

    Note:

    If R1 has already resolved an LDP FEC for prefix Y, it has an ILM assigned to it. However, this ILM is not 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.

  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.

Parent topic: LDP-to-Segment Routing Stitching for IPv4 /32 Prefixes (IS-IS)