In aSegment Routing (SR-MPLS)architecture using IS-IS as the control plane, routers exchange labels (segments) to build Label-Switched Paths (LSPs) without the need for traditional signaling protocols like LDP or RSVP. According to Juniper Networks technical documentation, for a router to be reachable via a shortest- path LSP from other nodes in the network, it must advertise aPrefix Segment Identifier (Prefix SID).
A specific type of Prefix SID is theNode SID, which is assigned to a loopback address (typically lo0.0) to uniquely identify the router within the SR domain. In the provided exhibit, routerR10has been configured with aSegment Routing Global Block (SRGB)starting at label 300000. This configuration tells the router which label range to use for global segments, but it does not automatically assign a label to its own loopback interface.
Without aNode SIDconfiguration, R10 is not telling its neighbors which specific index or label within that SRGB corresponds to its own address. Consequently, the other nine routers in the IS-IS area can calculate the shortest path to R10 using standard SPF, but they cannot perform the "label-binding" necessary to push an SR- MPLS label onto the packets.
To solve this, a Node SID must be explicitly configured under the loopback interface within the IS-IS protocol hierarchy, such as:
set protocols isis interface lo0.0 level 2 ipv4-node-sid index <value>
Analysis of incorrect options:
* Binding SID (Option A):This is used to encapsulate or steer traffic into a specific policy or tunnel (like a TE-LSP) and is not required for basic shortest-path reachability.
* Adjacency SID (Option B):These are generated automatically by Junos for each link and represent a specific local hop; they are not used for global "shortest-path" forwarding to a distant node.
* BGP Prefix SID (Option C):This is used for BGP Egress Peer Engineering (EPE) or prefix advertisement via BGP, which is not relevant for building the underlying IS-IS SR-MPLS transport.
Therefore, configuring anexplicit IPv4 node SIDis the mandatory step to enable the rest of the network to build a shortest-path SR-LSP toward R10.