MPLS

What this topic solves

Labels, LSR/LER roles, LFIB, LDP concepts, VPN services, transport and troubleshooting.

Follow state and packet path

Separate underlay, control plane and service. Validate labels/identifiers, per-service tables, MTU and return path.

Input → classification/state → decision → table → next-hop/output → counters

Design before configuration

• Define the failure domain and expected convergence behavior.
• Document addressing, identifiers, timers, MTU and policy boundaries.
• Prefer explicit, observable state over assumptions.
• Plan rollback and out-of-band access before risky changes.

Commands and verification

Command syntax varies by platform and software release. The examples below come from the existing portal content and must be validated against the target platform documentation before production use.

CE -- PE1 ===== P1 ===== P2 ===== PE2 -- CE
       push      swap     swap/pop
             Label Switched Path

show mpls interfaces
show mpls ldp neighbor
show mpls ldp bindings
show mpls forwarding-table
show ip cef
show ip route
show bgp vpnv4 unicast all
show vrf

Evidence-driven workflow

1. Define source, destination, time window and exact symptom.
2. Establish the expected healthy state and compare it with the observed state.
3. Inspect counters, tables, logs and packet captures before resets.
4. Test one hypothesis at a time and prefer reversible changes.
5. Repeat the original test after correction and check side effects.

IETF MPLS RFCs

Use the current standard/RFC and the documentation for the exact software release being operated. Vendor syntax is an implementation detail; protocol behavior should be understood independently.

Prove the behavior in a controlled topology

Build the smallest topology that reproduces the mechanism, capture control and data traffic, introduce one deliberate failure and diagnose it before applying the fix.

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