Reverse SPF Algorithm for Node-Protecting Remote LFA
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Solution Overview
Problem
The computation of node-protecting remote Loop-Free Alternate (LFA) routes in network routing is inefficient, particularly due to the costly and non-scalable nature of performing forward SPF from every PQ node, which hinders effective network resilience and traffic management during failures.
Innovation Solution
A method is implemented to efficiently compute node-protecting remote LFAs by using a reverse SPF algorithm rooted at the primary next hop node, selecting nodes from both the source node's extended P-space and the primary next hop's link-protecting Q-space, and traversing the reverse path to assign nodes as remote LFA for destination nodes, thereby avoiding the computation of forward SPF from every PQ node.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If forward SPF is performed from every PQ node to compute node-protecting remote LFA routes, then complete node protection is achieved, but computational overhead increases and scalability deteriorates
Solution Approach 1:
The patent inverts the traditional SPF computation approach by performing reverse SPF from the primary next-hop node instead of forward SPF from every PQ node. This inversion fundamentally changes the computation direction, allowing the algorithm to identify all nodes that can reach the primary next-hop without using the S-E link, thereby finding remote LFA targets efficiently without exhaustive forward SPF computations from each PQ node
Solution Approach 2:
The reverse SPF algorithm serves multiple functions simultaneously: it identifies the Q-space (nodes that can reach primary next-hop without using S-E link), determines reachability from neighbor nodes without transiting the primary next-hop, and finds intersection nodes that serve as remote LFA targets. This multi-functionality eliminates the need for separate computations that would otherwise be required for each of these tasks
2Reliability
If forward SPF is performed from every PQ node to compute node-protecting remote LFA routes, then complete node protection is achieved, but computation time increases
Solution Approach 1:
By inverting the SPF computation direction to run from the primary next-hop node backwards, the algorithm efficiently identifies all potential remote LFA targets in a single execution rather than performing multiple forward SPF computations from each PQ node, dramatically reducing the time required to compute node-protecting routes
3Reliability
If the network uses pre-calculated backup paths for fast reroute, then traffic loss during failures is reduced, but the convergence time of routing protocols increases
Solution Approach 1:
The patent pre-calculates remote LFA backup paths using reverse SPF before failures occur, storing these pre-computed routes in advance. When failures happen, the forwarding plane can immediately switch to these pre-prepared backup paths without waiting for routing protocol convergence, achieving fast reroute while maintaining efficient computation through the reverse SPF approach
Data Source
AI summary
A method is implemented by a network device to improve efficiency of computing a node-protecting remote loop-free alternate (LFA) in a network topology graph. The method computes a reverse shortest path first (SPF) algorithm rooted at the primary next hop node, where the reverse SPF algorithm rooted at the primary next hop maintains a reverse path of a shortest path computed by the reverse SPF algorithm rooted at the primary next hop node. The method selects a node that is in both the source node's node-protecting extended P-space that protects the primary next hop node and the primary next hop node's link-protecting Q-space that protects the S-E link. The method traverses the reverse path of the shortest path computed by the reverse SPF algorithm rooted at the primary next hop node starting from the selected node and towards the primary next hop node and assigns the selected node as a node-protecting remote LFA for each S-E destination node visited along the traversal of the reverse path that has not been assigned a node-protecting remote LFA.