MPLS and Advanced WAN/VPN Technologies
This domain covers MPLS label operations (push, swap, pop), the LFIB vs. FIB, LDP and RSVP-TE signaling, and L3VPN concepts: VRFs, route distinguishers (RD), route targets (RT), and MP-BGP VPNv4/VPNv6. Know the label stack: transport label on top, VPN label below. L2VPN (VPLS, VPWS) and GRE/IPsec tunnels round out WAN topics. Candidates should understand PHP (penultimate hop popping) and why implicit-null labels exist.
Traps center on RD vs. RT confusion. The RD makes a prefix unique within MP-BGP; the RT controls import/export into VRFs. A question may give different RDs on two PEs and ask whether routes are exchanged - they are, as long as RTs match. Another trap: assuming LDP distributes labels for all routes; it only labels IGP-learned prefixes unless configured otherwise. Also watch MTU and fragmentation in GRE/IPsec tunnels, and the difference between control-plane and data-plane encryption.
Worked example: a customer site cannot reach a remote site over an L3VPN. Verify the VRF route table (display ip routing-table vpn-instance X), confirm the PE has an MP-BGP VPNv4 session, and check that export RT on one PE matches import RT on the other. If the local VRF has the route but the remote does not, suspect an RT mismatch or a missing vpn-target import.
Rule of thumb: RD = uniqueness, RT = membership. Mismatched RTs silently drop routes from the target VRF.