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Leaking BGP-LU Prefixes into Flexible Algorithm

You can leak BGP-LU prefixes into the IGP with flex algo prefix-SIDs. You can configure the prefix-segment (and metric) in the policy-statement to leak BGP-LU learned prefixes into flex algo.

Figure 1: BGP-LU – Flexible Algorithm Leaking BGP-LU – Flexible Algorithm Leaking

For example, in the topology shown in #id-understanding-isis-flexible-algorithm-for-segment-routing, the IGP domain includes flex algos 128 and 129. The device R9 resides outside the IGP domain. The device R9 is not reachable via flex algo in the IGP domain. Any traffic destined for device R9 follows a flex algo path till device R5 and then follows the device R5 to R9 link.

When flex algo prefix-SIDs are leaked from BGP-LU to an IGP instance, FAPM sub-TLV will be advertised with the metric derived from the export policy or the route’s own metric. The metric defined in the export policy has higher precedence over the route’s own metric. Additionally, IS-IS installs a stitched route in the mpls.0 tables to stitch incoming MPLS traffic from BGP-LU to IS-IS.