What is a Shared Risk Group (SRG)?

A Shared Risk Group (SRG) is a fundamental concept in network resiliency design. It defines a set of network resources, such as links, nodes, or line cards, that are exposed to a common point of failure. If that single point fails (e.g., a fiber cut in a shared conduit, a power outage in a facility), all resources within the SRG are likely to fail simultaneously. By identifying and configuring SRGs, network management and routing protocols (like RSVP-TE with Fast Reroute) can compute and establish physically diverse primary and backup paths, ensuring that a single failure event does not impact both paths, thereby improving overall network availability.

A. A set of fibers that share the same latency risk

SRGs are defined by a common failure risk, not performance metrics like latency. Two paths can have different latencies but still share a failure risk.

B. A set of boards that share the same failure risk

This is an example of an SRG, but it is not the general definition. An SRG can encompass a wider range of resources, including fibers, conduits, and nodes.

C. A set of nodes that share a common risk of hardware failure

This is also too specific. An SRG can be more granular than a set of nodes, such as multiple links sharing a physical conduit between two nodes.

1. Nokia 7750 SR OS Basic System Configuration Guide, Release 22.10.R1, Document Part Number: 3HE 18112 AAAC TQZZA, Edition 01.

Section: "Shared Risk Groups (SRGs)"

Quote/Paraphrase: The guide defines SRG as a mechanism to group objects that share a common risk. It states, "SRG allows the user to specify that certain LAG ports or PW-SAPs share a common risk and to ensure that the primary and secondary paths for a service do not traverse the same SRG." This confirms that an SRG is a set of resources sharing a common failure risk.

2. Nokia 7750 SR OS MPLS Guide, Release 22.10.R1, Document Part Number: 3HE 18116 AAAC TQZZA, Edition 01.

Section: "Shared Risk Link Group (SRLG)"

Quote/Paraphrase: This guide defines the closely related concept of SRLG: "A Shared Risk Link Group (SRLG) is a set of links that share a common resource, and as a result, a single failure of that resource may cause all links in the set to fail." This directly supports the definition of a group of resources sharing a common failure risk.