SR Policy Hop-Limit

The Segment Routing (SR) Policy Hop-Limit feature introduces an additional constraint to SR policies, allowing users to specify the maximum number of hops a packet can traverse within a defined SR policy path. This ensures that traffic follows an optimized path and prevents potential routing loops.

SR Policy is an ordered list of segments (instructions) representing a source-routed path. The headend node steers traffic into the SR Policy, embedding the ordered segment list into the packet header.

The Hop-Limit parameter acts as a constraint in dynamic path computation, restricting the path length in terms of the number of hops between the source and destination.

Feature Characteristics

• Allows configuration of a maximum hop count for dynamic SR Policy paths.
• Applicable to dynamic candidate paths computed via:
• IS-IS
• OSPF
• Path Computation Element Protocol (PCEP)
• Prevents long routing paths and looping behavior.
• No default hop-limit value is configured.
• Supports both SR Policy and SR ODN Policy types.

Benefits

This feature offers several key advantages:

• Loop Prevention: Restricts packets from traversing excessive hops, mitigating looping scenarios.
• Optimized Path Selection: Ensures that only efficient routes within the defined hop count are selected.
• Improved Stability: Minimizes route instability in networks with multiple path options.
• Operational Flexibility: Provides an additional control parameter to fine-tune SR Policy computations.

Prerequisites

This feature requires the following device capabilities:

• SR-MPLS must be enabled in the network.
• Underlay IGP (IS-IS or OSPF) must be configured and operational.
• All nodes participating in SR Policy computation must advertise Prefix-SIDs.

Limitations

• The maximum SID depth supported in Qumran (Q1 and Q2) series platforms is 6.
• Although the maximum supported hop-limit value is 32, the SR Policy will not become active if the actual number of hops exceeds 6.
• Explicit policy does not support hop limit.