RSVP SRLG with ISIS IGP

Overview

Shared Risk Link Groups (SRLG) allow the network to compute backup secondary LSPs or FRR (fast-reroute) LSPs that are disjoint from the primary LSP. Links grouped under the same SRLG share a common underlying risk—for example, fibers routed through the same conduit or multiple wavelengths carried on the same fiber. Because a failure on one link in an SRLG is likely to affect all links in that group, SRLG-aware path computation ensures that alternate LSPs avoid shared risks and maintain path diversity.

Characteristics

SRLG groups identify which links share the same risk. As an Interior Gateway Protocol (IGP), IS-IS distributes network topology information—including link attributes—throughout the network. To advertise SRLG membership, IS-IS uses the Shared Risk Link Group TLV (Type 138).

CSPF (Constrained Shortest Path First) computes LSP paths based on multiple constraints, taking into account not only the network topology but also link and LSP attributes. The CSPF engine uses information from both the signaling and routing modules to determine an optimal path while efficiently balancing network load.

SRLG for Secondary Sessions

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The SRLG of an LSP path is the union of SRLGs across all its links.

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When computing a secondary LSP, CSPF attempts to select a path that is SRLG-disjoint from the primary path.

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This avoids correlated failures that could take down both primary and secondary LSPs.

SRLG for Fast Reroute (FRR)

RSVP supports two FRR modes:

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One-to-One Protection (Detours)

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Facility Backup Protection (Bypass Tunnels)

SRLG constraints prevent detours or bypass tunnels from using links with the same SRLG as the protected link.

Detour LSPs

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Each primary LSP has its own detour.

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Detour path selection avoids links sharing SRLGs with the protected link.

Auto Bypass

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Dynamically created bypass tunnels protect multiple LSPs.

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Auto-bypass path computation avoids links that share SRLGs with protected interfaces.

Manual Bypass

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Manually configured bypass tunnels can be mapped to primary sessions.

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Mapping requires SRLG exclusivity between the bypass tunnel and the protected link.

Benefits

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Ensures secondary or FRR LSPs do not share risk with primary paths.

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Reduces probability that a single failure impacts both primary and backup LSPs.

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CSPF automatically avoids links with common risks.

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Enhances primary/secondary and FRR path diversity.