Nortel Networks NN46110-602 - Manual (Page 80)

80 Chapter 4 Troubleshooting
NN46110-602
• Start from the top down to go in the opposite direction, looking at PPP first
and working down to the physical connection. An important point to
remember when taking this approach is that at the higher protocol layers,
there are more options to misconfigure, but changing them is easier and
generally involves less effort.
A key point to remember when diagnosing WAN link problems is to involve the
T1 service provider in the troubleshooting effort. This is not only because they can
help diagnose the problem, but also because an ISP can bring down a link if it
detects errors on the line. Notify the ISP administrator if you are planning to work
on the link.
Check the T1/V.35 interface
To diagnose a problem at the WAN physical layer, use the following steps to
verify that the T1/V.35 interface to the public data network (PDN) is operating
correctly, and that the T1 line is properly connected:
1 Have your ISP run a loopback test from their end to the CSU/DSU to verify
that the external line is working correctly.
2 Check the connections between the VPN Router and the CSU/DSU. Make
sure that the V.35 cable is a straight-through cable and firmly seated, that the
CSU/DSU is configured to use internal clocking, and that NRZ is encoded
with CCITT CRC for the checksum.
3 Make sure that all the control signals are asserted (CTS, DCD, DSR, RTS,
and DTR). You can check these signals on the VPN Router from the Manager
WAN Statistics window. If any of these signals are incorrect, you can try
disabling or enabling the link from the Manager WAN Interfaces window, or
unplugging and plugging in the link. If these steps do not resolve the problem,
try switching ports on the same card, switching cables, or switching to a new
card, if available.
4 If the previous steps fail to resolve the problem, and you still suspect a
problem with the physical connection, try rebooting the VPN Router to
reinitialize the WAN interface.