Troubleshooting ADSL
When troubleshooting ADSL problems, start with checking Layer 1 physical
connectivity. If that checks out okay, progress to checking Layer 2 connectivity.
Finally, check Layer 3.
Troubleshooting ADSL at Layer 1
Try the following procedures when troubleshooting DSL physical layer
problems:
1. Check for the carrier detect light on the router’s ATM interface. If it is
off, use the show interfaces atm interface_number command to check
the interface status. If the interface status is down, try swapping out the
RJ-11 cable connecting to the wall jack. The middle pins are used with
ADSL signaling. If that doesn’t work, contact the provider to make sure
that the DSL service has been started.
2. If the interface status shown in the show interfaces atm command is
“administratively down,” enable the interface with the command no
shutdown.
3. If there is a carrier detect light, check communication with the DSLAM.
The subscriber DSL modem should train to the DSLAM. This allows
them to negotiate settings such as speed. To verify this, use the show
dsl interface atm interface_number command and look for the Modem
Status field. It should say “Showtime.” There should also be a nonzero
value in the Speed field.
4. Check the DSL modulation type. Verify with the provider that your
router’s chipset is supported and find out what operating mode it
should be in to support the correct modulation. Set this on the router
with the command dsl operating-mode {auto | ansi-dmt | itu-dmt |
splitterless}. If the modulation type is unknown, use the auto option.