© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.23-1 Frame-Mode MPLS Implementation on Cisco IOS Platforms Troubleshooting Frame-Mode MPLS on Cisco

© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.2—3-1

Frame-Mode MPLS Implementation on Cisco IOS Platforms

Troubleshooting Frame-Mode MPLS on Cisco IOS Platforms


Outline

• Overview

• What Are Common Frame-Mode MPLS Issues?

• Solving LDP Session Startup Issues

• Solving Label Allocation Issues

• Solving Label Distribution Issues

• Solving Packet-Labeling Issues

• Solving Intermittent MPLS Failures

• Solving Packet Propagation Issues

• Summary


• The LDP session does not start.

• Labels are not allocated.

• Labels are not distributed.

• Packets are not labeled, although the labels have been distributed.

• MPLS intermittently breaks after an interface failure.

• Large packets are not propagated across the network.

Symptoms of Common Frame-Mode MPLS Issues


LDP Session Startup Issues

• Symptom

– LDP neighbors are not discovered.

• The show mpls ldp discovery command does not display expected LDP neighbors.

• Diagnosis

– MPLS is not enabled on the adjacent router.

• Verification

– Verify with the show mpls interface command on the adjacent router.


LDP Session Startup Issues (Cont.)

• Symptom


• Diagnosis

– There is a label distribution protocol mismatch—TDP on one end, LDP on the other end.

• Verification

– Verify with the show mpls interface detail command on both routers.



• Symptom


• Diagnosis

– Packet filter drops LDP neighbor discovery packets.

• Verification

– Verify access list presence with the show ip interface command.

– Verify access list contents with the show access-list command.



• Symptom

– LDP neighbors are discovered; the LDP session is not established.

• The show mpls ldp neighbor command does not display a neighbor in operational state.

• Diagnosis

– The connectivity between loopback interfaces is broken; the LDP session is usually established between loopback interfaces of adjacent LSRs.

• Verification

– Verify connectivity with the extended ping command.


Label Allocation Issues

• Symptom

– Labels are not allocated for local routes.

• The show mpls forwarding-table command does not display any labels.

• Diagnosis

– CEF is not enabled.

• Verification

– Verify with the show ip cef command.


Label Distribution Issues

• Symptom

– Labels are allocated, but not distributed.

• Using the show mpls ldp bindings command on the adjacent LSR does not display labels from this LSR.

• Diagnosis

– There are problems with conditional label distribution.

• Verification

– Debug label distribution with the debug mpls ldp advertisements command.

– Examine the neighbor LDP router IP address with the show mpls ldp discovery command.

– Verify that the neighbor LDP router IP address is matched by the access list specified in the mpls advertise command.


Packet Labeling Issues

• Symptom

– Labels are distributed, but packets are not labeled.

• Using the show interface statistic command does not show labeled packets being sent.

• Diagnosis

– CEF is not enabled on the input interface (potentially because of a conflicting feature being configured).

• Verification

– Verify with the show cef interface command.


Packet Labeling Issues: show cef interface

Router#show cef interfaceSerial1/0.1 is up (if_number 15) Internet address is 192.168.3.5/30 ICMP redirects are always sent Per packet loadbalancing is disabled IP unicast RPF check is disabled Inbound access list is not set Outbound access list is not set IP policy routing is disabled Interface is marked as point to point interface Hardware idb is Serial1/0 Fast switching type 5, interface type 64 IP CEF switching enabled IP CEF VPN Fast switching turbo vector Input fast flags 0x1000, Output fast flags 0x0 ifindex 3(3) Slot 1 Slot unit 0 VC -1 Transmit limit accumulator 0x0 (0x0) IP MTU 1500

Router#show cef interface

IP CEF switching enabled


Intermittent MPLS Failures After Interface Failure

• Symptom

– The overall MPLS connectivity in a router intermittently breaks after an interface failure.

• Diagnosis

– The IP address of a physical interface is used for the LDP (or TDP) identifier. Configure a loopback interface on the router.

• Verification

– Verify the local LDP identifier with the show mpls ldp neighbors command.


Packet Propagation Issues

• Symptom

– Large packets are not propagated across the network.

• Use of the extended ping command with varying packet sizes fails for packet sizes close to 1500 packets.

– In some cases, MPLS might work, but MPLS VPN will fail.

• Diagnosis

– There are label MTU issues or switches that do not support jumbo frames in the forwarding path.

• Verification

– Issue the traceroute command through the forwarding path; identify all LAN segments in the path.

– Verify the label MTU setting on routers attached to LAN segments.

– Check for low-end switches in the transit path.


Summary

• Some common frame-mode issues are as follows: LDP session does not start, labels are not allocated or distributed, and MPLS intermittently breaks after an interface failure.

• One LDP session startup issue is when LSP neighbors are not discovered.

• A label allocation issue is one in which the labels are not allocated for local routes.

• Labels may be allocated but not distributed correctly.

• Ensure that there are no conflicts between CEF and any other configured features; otherwise, packets might not be labeled.

• Use loopback IP addresses, not a configured interface IP address, to avoid MPLS connectivity intermittently breaking down.

• Large packets are not propagated across the network.


Documents

© 2006 Cisco Systems, Inc. All rights reserved. MPLS v2.23-1 Frame-Mode MPLS Implementation on Cisco IOS Platforms Troubleshooting Frame-Mode MPLS on Cisco