Label deduction with flexible-algorithm

A method comprising:

• receiving, by a first router, a route advertisement including a label stack of a plurality of base node labels that uniquely identifies a second router, wherein the plurality of base node labels includes a base node label for the second router that is shared with another router and a base node label for an area border router in an Interior Gateway Protocol area including the second router, wherein the plurality of base node labels is associated with a segment routing path computed without a flexible-algorithm, wherein the segment routing path computed without the flexible-algorithm is not a constraint-based path;
• receiving, by the first router, a packet destined for the second router;
• in response to receiving the packet destined for the second router, deducing, by the first router and from the plurality of base node labels, a node label for the second router by offsetting the base node label of the second router to fall within a node label range of the flexible-algorithm and a node label for the area border router by offsetting the base node label of the area border router to fall within the node label range of the flexible-algorithm, wherein the deduced node label for the second router and the deduced node label for the area border router are associated with a segment routing path computed with the flexible-algorithm, wherein the segment routing path computed with the flexible-algorithm is a constraint-based path;
• constructing, by the first router, a label stack including the deduced node label for the second router and the deduced node label for the area border router to steer the packet to the second router along the segment routing path computed with the flexible-algorithm;
• and
• forwarding, by the first router, the packet encapsulated with the label stack including the node label for the second router and the node label for the area border router to steer the packet to the second router and the area border router along the segment routing path computed with the flexible-algorithm.

A system comprising:

• a plurality of routers;
• and
• a plurality of area border routers, wherein a first router of the plurality of routers and a first area border router of the plurality of area border routers is logically grouped in a first Interior Gateway Protocol (IGP) area, wherein a second router of the plurality of routers and a second area border router of the plurality of area border routers is logically grouped into a second IGP area, wherein the first router, the second router, the first area border router, and the second area border router participate in a flexible-algorithm;
• wherein the first area border router is configured to:
• receive, from the first router, a base node label for the first router, the base node label associated with a segment routing path computed without the flexible-algorithm, wherein the segment routing path computed without the flexible-algorithm is not a constraint-based path;
• and
• send, to the second area border router, a route advertisement including the base node label for the first router;
• and
• wherein the second router is configured to:
• receive, from the second area border router, the route advertisement including the base node label for the first router;
• receive a packet destined for the first router;
• in response to receiving the packet destined for the first router, deduce, from the base node label, a node label for the first router by offsetting the base node label of the first router to fall within a node label range of the flexible-algorithm, the deduced node label associated with a segment routing path computed with the flexible-algorithm, wherein the segment routing path computed with the flexible-algorithm is a constraint-based path;
• construct a label stack including the deduced node label for the first router to steer the packet to the first router via the segment routing path computed with the flexible-algorithm;
• and
• forward the packet encapsulated with the label stack including the node label for the first router to steer the packet to the first router along the segment routing path computed with the flexible-algorithm.