Router ECMP Path Selection Using Source Interface Consistency
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Solution Overview
Problem
Inconsistent routing of locally sourced packets in networks using Equal Cost Multi-Path (ECMP) strategies leads to issues when network characteristics differ, causing packets with private IP addresses to be unable to receive replies from public networks.
Innovation Solution
Implementing a system where a router performs destination and source address lookups to identify ECMP paths and ensures consistency between the source interface in the control plane and the egress interface in the data plane by using an auxiliary header or database mappings to select appropriate paths, preventing packets from being forwarded to inappropriate networks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If ECMP provides multiple paths for routing packets, then network bandwidth utilization and redundancy are improved, but routing consistency between control plane and data plane deteriorates
Solution Approach 1:
The system performs a source address lookup in the FIB before packet forwarding to pre-determine the source interface. This preliminary action ensures that when ECMP path selection is needed, the system has already identified the correct source interface, allowing it to select only those ECMP paths that are consistent with the source interface, thereby maintaining routing consistency while utilizing multiple paths for bandwidth.
Solution Approach 2:
The system uses feedback from the source address lookup results to guide ECMP path selection. By checking which ECMP paths are consistent with the determined source interface, the system creates a feedback loop that ensures only appropriate paths are selected, resolving the contradiction between utilizing multiple paths and maintaining routing consistency.
2Speed
If packets are forwarded using ECMP paths without interface consistency checking, then forwarding speed is improved, but packet delivery reliability deteriorates
Solution Approach 1:
The system performs interface consistency checking as a preliminary action during the packet forwarding process. By determining the source interface and filtering ECMP paths based on consistency before actual packet transmission, the system ensures reliable packet delivery without significantly impacting forwarding speed, as the checking is integrated into the existing lookup process.
3Measurement precision
If source interface determination is performed through additional lookups, then routing accuracy is improved, but processing complexity increases
Solution Approach 1:
The system merges the source address lookup operation with the existing destination address lookup in the FIB. By performing both lookups in a single database query operation, the system determines the source interface without adding significant processing complexity, while still achieving high routing accuracy through interface consistency verification.
Data Source
AI summary
In one embodiment, a method includes performing, by a router, a destination address lookup of an IP packet in a Forwarding Information Base (FIB) and identifying, by the router, an equal cost multi-path (ECMP) object from the destination address lookup. The ECMP object includes a plurality of paths for forwarding the IP packet to a destination associated with a destination address. The method further includes determining, by the router, a source interface associated with the IP packet, determining, by the router, that the source interface matches an egress interface associated with a path among the plurality of paths, and communicating, by the router, the IP packet based on the path to the destination using the egress interface.