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How to troubleshoot ISDNStep 1: Is the router communicating with the ISDN
network at the signalling level?
When the router is connected to the ISDN network, either by plugging it into an NT1 if the
router has an S/T interface, or by plugging it directly into the ISDN network if the router has
a U interface, it establishes a low-level connection to the network. Basically, the router and
the network device exchange a particular set of waveforms. When the router is satisfied that
it has seen the correct waveforms, it considers the low-level connection to be established,
and the link is now ready to send data frames.
There are two ways to determine whether or not the router considers that the connection is
established:
1. Look at the external LED marked "Active". This will be lit when the connection is
established.
2. At the command prompt, enter the command:
show bri=n state
where n is the number of the BRI interface under consideration. Interfaces are numbered
from 0.
An example of the output of the show bri=0 state command is shown in the following
figure.
The important item in this output is the second item, State. This must be shown as Activated.
If it has any other value (Inactive, Awaiting Signal, Synchronized, etc.) then the connection has
not yet been established.
If the BRI interface is Activated, then the problem lies at a higher level. Go to "Step 2: Is the
router communicating with the ISDN network at the LAPD level?"on page 4.
State for BRI instance 0:
Interface type ..... TE
State .............. Activated
Rx INFO ............ INFO 4
Tx INFO ............ INFO 3
Activate request ... no
Activated .......... yes
Synchronised ....... yes
Activation mode .... normal
Mode ............... ISDN
ISDN slots ......... B1, B2
D channel class .... high
B1 enabled ......... no
B2 enabled ......... no
B1, B2 aggregated... no
Rx multiframing .... no
Transceiver mask .. 56