Troubleshooting WANs

1
Troubleshooting WANs
2
Troubleshooting Method

• Troubleshooting always starts by pinging from a
  host on one LAN to a host on a remote LAN
• If this works then all is well
• If the previous step fails then ping from
  router-to-router
• If this works then probably you have a problem
  with LAN IP addressing or routing protocols
• If the previous step fails then use show
  interfaces to determine the interface status

3
Troubleshooting Serial Links
Interface status codes from Show Interface, show
ip interface brief orshow interfaces description
commands
4
Layer 1 Problems

• Leased line is down (telco problem)
• A cable is unplugged or is faulty
• CSU/DSU is misconfigured or broken
• Clock rate is not set
• Can use show controllers to determine the
  interface type and, if it is a DCE, whether a
  clock rate is set

5
Layer 2 Problems
6
PAP/CHAP Debugging

• Use debug ppp authentication to see the
  authentication process and look for the SUCCESS
  message
• If you dont see the SUCCESS message or see a
  FAILURE message then check your configuration

7
Troubleshooting Frame Relay

• If the problem is Layer 2 and you are using Frame
  Relay then
• Check for access link issues, especially
  encapsulation and LMI
• Check for PVC problems
• Show frame-relay pvc
• Check for mapping problems
• Show frame-relay map
• Check for mismatched encapsulation types (cisco
  or ietf)
• Check for IP addressing problems on subinterfaces

8
LMI Problems

• LMI problems may include
• DCE not informing DTE about DLCIs
• Show frame-relay PVC shows no PVCs
• Keepalives not being sent
• Show frame-relay lmi and make sure Num status
  Enq. Sent equals Num Status msgs Rcvd
• No keepalives set
• LMI type mismatch
• Show frame-relay lmi

9
PVC Problems

• Make sure subnet addresses assigned to
  subinterfaces match up with IP addresses in the
  frame relay map
• Show frame-relay pvc
• Show frame-relay map