Chapter 7: Troubleshooting MPLS-IP

Master the art of MPLS troubleshooting with systematic approaches to common issues, LDP problems, VPN connectivity challenges, and essential debugging techniques for maintaining robust MPLS networks.

Common Issues

MPLS troubleshooting requires a systematic approach. Understanding common failure patterns and their symptoms helps network engineers quickly identify and resolve problems.

Troubleshooting Methodology

Follow the OSI model: Physical → Data Link → Network → MPLS → Application. Verify each layer before moving to the next.

Issue Category Common Symptoms Primary Causes First Steps
Label Distribution No labels, black-holing traffic LDP session down, IGP issues Check LDP neighbors, IGP status
VPN Connectivity Customer sites can't communicate RT mismatch, BGP problems Verify VRF config, BGP sessions
PE-CE Routing Routes missing from VRF Protocol misconfiguration Check PE-CE routing protocol
Performance High latency, packet loss Congestion, misrouting Monitor interfaces, check paths
Critical Issues
  • Complete service outage
  • Core network failure
  • BGP session flapping
  • Multiple site isolation
  • Label allocation failure
Performance Issues
  • Intermittent packet loss
  • High latency periods
  • QoS violations
  • Suboptimal routing
  • Convergence delays

LDP Problems

LDP issues are among the most common MPLS problems. Understanding LDP state machines and troubleshooting neighbor relationships is crucial for MPLS operations.

LDP Neighbor Issues
Common Problems:
  • Hello packets not received
  • TCP connection failure
  • Authentication mismatch
  • Router ID conflicts
Troubleshooting Steps:
  1. Verify interface MPLS enablement
  2. Check LDP router ID configuration
  3. Confirm IP connectivity
  4. Validate authentication settings
Label Advertisement Problems
Common Problems:
  • Missing label bindings
  • Label allocation failures
  • Filter policy blocking
  • Platform limitations
Troubleshooting Steps:
  1. Check LDP bindings database
  2. Verify label range allocation
  3. Review filter policies
  4. Monitor platform resources
LDP Troubleshooting Commands
! Check LDP neighbor status
show mpls ldp neighbors
show mpls ldp neighbors detail

! Verify LDP discovery
show mpls ldp discovery
show mpls ldp discovery detail

! Check label bindings
show mpls ldp bindings
show mpls ldp bindings 10.1.1.0 255.255.255.0

! Monitor LDP sessions
show mpls ldp session
show mpls ldp session detail

VPN Connectivity Issues

VPN connectivity problems often involve complex interactions between VRFs, BGP, and PE-CE routing protocols. Systematic troubleshooting is essential.

VPN Troubleshooting Flow

1. Check VRF → 2. Verify BGP sessions → 3. Check route propagation → 4. Test end-to-end connectivity → 5. Validate forwarding

Problem Area Symptoms Verification Commands Common Fixes
VRF Configuration Routes not in VRF table show ip vrf, show ip route vrf Fix RT import/export
BGP Sessions No VPNv4 routes show bgp vpnv4 unicast all Fix neighbor configuration
PE-CE Routing Customer routes missing show ip route vrf, debug routing Fix redistribution, protocols
Label Distribution Forwarding failures show mpls forwarding-table Check LDP sessions
Route Target Issues
! Check VRF RT configuration
show ip vrf detail CUSTOMER_A

! Verify BGP extended communities
show bgp vpnv4 unicast all
show bgp vpnv4 unicast 10.1.1.0/24

! Check route import/export
show ip route vrf CUSTOMER_A
show bgp vpnv4 unicast vrf CUSTOMER_A
PE-CE Troubleshooting
! Check PE-CE interface
show ip interface brief | include GigE0/1
show ip route vrf CUSTOMER_A

! Verify routing protocol
show ip ospf neighbor vrf CUSTOMER_A
show ip bgp neighbors vrf CUSTOMER_A

! Test connectivity
ping vrf CUSTOMER_A 192.168.1.2
traceroute vrf CUSTOMER_A 10.10.10.1

Debug Commands

Debug commands provide detailed insight into MPLS operations but should be used carefully in production environments due to their impact on system performance.

Debug Command Caution

Always use debug commands with filters and time limits in production. High traffic volumes can overwhelm the router CPU.

LDP Debugging
! LDP session debugging
debug mpls ldp session
debug mpls ldp discovery
debug mpls ldp messages

! LDP binding debugging  
debug mpls ldp bindings
debug mpls ldp advertisements

! Hello debugging
debug mpls ldp hello
BGP VPN Debugging
! BGP session debugging
debug ip bgp vpnv4 unicast
debug ip bgp updates
debug ip bgp events

! Route debugging
debug ip bgp dampening
debug ip bgp keepalives
debug ip bgp vpnv4 unicast updates
Forwarding Debugging
! MPLS forwarding debugging
debug mpls packet
debug mpls forwarding

! VRF debugging
debug ip routing vrf
debug ip bgp vrf

! CEF debugging
debug ip cef
Conditional Debugging

Use conditions to limit debug output:

! Debug specific prefix
debug condition interface GigE0/1
debug condition ip 10.1.1.1
debug condition mac 0012.4567.89ab

! Enable debug with condition
debug ip bgp updates
debug mpls ldp bindings
Debug Best Practices
  • Set time limits for debug sessions
  • Use access-lists to filter output
  • Log to buffer instead of console
  • Turn off debug when done
  • Monitor CPU usage
Debug Category Use Case Impact Level Recommended Filter
debug mpls packet Forwarding issues High Specific destination
debug mpls ldp session LDP neighbor problems Medium Specific neighbor
debug ip bgp updates Route propagation High Specific prefix
debug mpls ldp bindings Label distribution Medium Specific FEC
Next Steps

Now that you've mastered MPLS troubleshooting, continue to Chapter 8: Best Practices & Security to learn about optimal MPLS design principles and security considerations.