Segment Routing and Modern MPLS Evolution
What this domain tests
Segment Routing and Modern MPLS Evolution tests the SR-MPLS forwarding model and how it changes label distribution. You should understand SIDs, SRGB, node and adjacency SIDs, TI-LFA, and the roles of BGP-LU and EVPN.
Core rules
- SR-MPLS encodes a path as a list of segment identifiers (SIDs) in the label stack. No LDP or RSVP-TE is required for basic prefix forwarding.
- A node SID is globally significant within the SR domain and maps into the SRGB. An adjacency SID is locally significant and identifies a specific link.
- The SRGB is the label range reserved for segment routing. A prefix SID is advertised by IGP and translated to a label within the SRGB.
- TI-LFA provides local repair by computing a post-convergence path and encoding repair segments, often using adjacency SIDs.
- BGP-LU carries labeled IPv4/IPv6 routes between domains; EVPN uses BGP to advertise MAC/IP and provides multitenant Layer 2/3 services.
Worked example
An SR domain uses SRGB 16000–23999. PE2's loopback has prefix SID index 2, so its node SID is 16002. PE1 receives the IGP advertisement and installs a forwarding entry for 16002 with next hop toward PE2. PE1 pushes 16002. A transit router swaps it according to its own SRGB mapping, and the penultimate router may pop it before delivery. If a link fails, TI-LFA can repair using an adjacency SID to a post-convergence next hop.
Traps
Do not call an adjacency SID globally unique. Do not assume SR eliminates BGP policies for VPNs. Do not confuse the SRGB with the global label space, and do not expect LDP sessions in a pure SR-MPLS core.