350-501 Question #576: Real Exam Question with Answer & Explanation

Question

What is the primary purpose of the locator in a segment-routing environment?

Options

• Ato identify the next segment on the forwarding path
• Bto provide path information for routing decisions
• Cto load-balance traffic across multiple paths
• Dto identify the starting point of a segment in the network

Explanation

In a segment routing environment, particularly SRv6, the primary purpose of the locator within a Segment Identifier (SID) is to identify the specific network node where the segment's instruction is to be processed. It functions as the routable IP prefix for that segment endpoint.

Approach. In SRv6, a Segment Identifier (SID) is structured with a Locator as its initial part. The Locator is a routable IPv6 prefix that uniquely identifies the specific segment routing node (router) in the network where the segment's instruction is to be processed. It acts as the "home" address or the starting point for the segment's execution, ensuring the packet is routed to the correct node before the segment function is applied.

Common mistakes.

• A. While a locator is part of an SRv6 SID, its primary role is to identify the node where a segment is anchored, not simply the "next segment" in a list, which is determined by the Segment Routing Header.
• B. The locator provides reachability to a specific node. Full path information is provided by a complete segment list, where each SID in the list contains a locator and a function.
• C. Load balancing (e.g., via ECMP) typically occurs when routing to the locator prefix, but the locator's fundamental purpose is identification of the node, not load balancing itself.

Concept tested. SRv6 locator purpose

Reference. https://www.cisco.com/c/en/us/products/collateral/ios-xe/segment-routing/white-paper-c11-741270.html

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