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4.3 | |
DDR Configuration | |
| 4.3.9 | |
Troubleshooting the DDR configuration |
| There are two major types of DDR
problems. Either a router is not dialing when it should, or it is
constantly dialing when it should not. Several
debug
commands can be used to help troubleshoot problems with a DDR
configuration. The
debug isdn q921
command is useful for viewing Layer 2 ISDN call setup exchanges.
The
debug isdn q931
command is useful for observing call setup exchanges for both outgoing
and incoming calls.
The
debug dialer [events | packets]
command is useful for troubleshooting DDR connectivity. The
debug dialer events
command sends a message to the console indicating when a DDR link has
connected and what traffic caused it to connect.
Not all DDR problems result in an interface failing to dial. Routing protocols can cause an interface to continuously dial, even if there is no user data to send. An interface that is constantly going up and down is said to be flapping. The debug dialer packet command sends a message to the console every time a packet is sent out a DDR interface. Use this debug command to see exactly what traffic is responsible for a flapping DDR interface. If a router is not connecting when it
should, then it is possible that an ISDN problem is the cause, as
opposed to a DDR problem. The remote router may be incorrectly
configured, or there could be a problem with the ISDN carrier network.
Use the isdn call interface
command to force the local router to attempt to dial into the remote
router. In some cases it is useful to reset the connection between the router and the local ISDN switch. The clear interface bri command clears currently established connections on the interface and resets the interface with the ISDN switch. This command forces the router to renegotiate its SPIDs with the ISDN switch, and is sometimes necessary after making changes to the isdn spid1 and isdn spid2 commands on an interface.
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The “i =”
field in the Q.921 payload field is the hexadecimal value of a Q.931
message. In the following lines, the seventh and eighth most significant
hexadecimal numbers in the “i =” field indicate the type of Q.931
message: 
If a router
is not configured correctly for DDR, then the output of the command will
usually indicate the source of the problem. If there is no debug output,
then the router is not aware of any interesting traffic. An incorrectly
configured dialer or access list may be the cause. 
If
the routers cannot communicate using this command, then the lack of
connectivity is an ISDN problem, not a DDR problem. However, if the
routers can communicate, then both the toll network and the ISDN
configurations on the routers are working properly. In this case, the
problem is most likely an error in the DDR configuration on either
router.
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