MPLS: label operations, LDP, RSVP-TE, L3VPN, L2VPN, and segment routing
What this domain tests
MPLS is the largest SPCOR domain. It tests label data-plane operations, LDP, RSVP-TE, L3VPN, L2VPN, and segment routing. You need to separate control-plane label distribution from the forwarding path a packet actually takes.
Core rules
- Label stack operations are push at ingress, swap in the core, and pop at egress or the penultimate hop. Implicit-null label 3 tells the penultimate router to pop; explicit-null labels 0 and 2 preserve a label for QoS or protocol reasons.
- LDP distributes labels for IGP prefixes and does not provide traffic engineering. RSVP-TE uses CSPF, bandwidth reservations, and explicit paths.
- L3VPN uses VRF, RD, and RT. The RD makes a prefix unique in MP-BGP; the RT controls import and export. The VPN label identifies the VRF at the egress PE.
- L2VPN uses pseudowires, with AToM for point-to-point and VPLS for multipoint. Segment routing uses prefix-SIDs and adjacency-SIDs; MPLS SR can operate without LDP.
Worked example
CE-A sends traffic to CE-B. The ingress PE has VRF RED with RD 65000:1, export RT 65000:100, and import RT 65000:100. It learns CE-B as VPNv4 prefix 65000:1:10.1.2.0/24 with RT 65000:100. The ingress PE pushes VPN label 24000 from the egress PE and transport label 16002 toward the egress loopback. A core router swaps the transport label to 16005. The penultimate router pops the transport label because the egress advertised implicit-null. The egress PE receives VPN label 24000, looks up VRF RED, and forwards to CE-B. If the import RT is missing, the route can remain in BGP but is not installed in the VRF.
Traps that cost marks
- Confusing RD with RT; RD is uniqueness, RT is import/export policy.
- Expecting LDP to perform traffic engineering; that is RSVP-TE or SR policy.
- Forgetting PHP and implicit-null when tracing label stacks.
- Missing an IGP route to the BGP next hop, which breaks VPNv4 recursion.