Troubleshooting ADSL at Layer 2
If the interface status shows that it is up, and the line protocol is up, move to
troubleshooting Layer 2 issues. Try the following to look for PVC or PPP
problems:
1. Use the command ping atm interface atm interface_number vpi vci
seg-loopback to check that your PVC is configured on the next-hop
ATM switch, which is typically the DSLAM. This command sends
management traffic called Operation, Administration, and Maintenance
(OAM) packets to the DSLAM. You should receive a normal ping
response if the PVC is configured.
2. Debug the events occurring on the interface processor with the debug
atm events command. This should show no output when everything is
working well; when there are problems, however, it can show useful
information such as the VPI/VCI number that the DSLAM expects.
The ISCW course recommends beginning a continuous ping over the
Internet (not over the internal network) to the router’s IP address
before giving this command.
3. Verify that the router is receiving data by using the show interfaces
atm interface_number command. Look for packets input and output.
4. If the previous procedures show that everything is working, check
for PPP problems. PPP should go through three phases: Link Control
Protocol (LCP) negotiation, authentication, and Network Control
Protocol (NCP) negotiation. The IP address is assigned by IPCP during
the NCP phase. Use the commands debug ppp negotiation and debug
ppp authentication to see whether there is a failure at any of these
phases.
When debugging PPP, look first for a lack of response from the aggregation
router. If data link parameters cannot be negotiated, LCP will
not open. If the authentication parameters are incorrectly configured,
CHAP authentication will fail. If IPCP fails, the IP parameters are
likely configured incorrectly either on the CPE or on the aggregation
router.