System information

Troubleshooting ISDN Connections 17-389
ISDN: Second B Channel Does Not Come Up
Possible Problem Solution
Missing or misconfigured
dialer load-threshold
command
Step 1 Use the show running-config privileged exec command to view the router
configuration. Check for a dialer load-threshold interface configuration
command entry.
The following is the syntax for the dialer load-threshold command:
dialer load-threshold load [outbound | inbound | either]
This command defines the load level that must be exceeded on the first
ISDN B channel before the router attempts to bring up a second B channel
for a multilink PPP connection. The load is a value between 1 and 255 and
defines a fraction taken over 255. The load can be calculated based on
outbound, inbound, or either inbound or outbound traffic on the interface.
For instance, the sample configurations use “dialer load-threshold 200
either.” If the combination of both inbound and outbound traffic levels
reaches 200/255 (about 80%, or roughly 50 kbps) capacity of the first
ISDN B channel, the router attempts to bring up a second B channel to
assist with the traffic load. Thus, the second
B channel is activated only when the traffic demands exceed the capacity
of one B channel.
Note: In scenarios where it is desired to have both B channels all the time,
use the value 1. This causes both
B channels to be brought up whenever the router dials.
Step 2 If the command is not present, configure the router interface with the
dialer load-threshold load command. This command specifies what the
load on the first B channel must be before the second B channel is
activated.
Note: The range is 1 to 255.
Step 3 If the command is already configured, make sure that the load value
specified is not too high. Reduce the specified load in increments of 25 or
50 to see whether the second channel comes up.
No dialer map configured for
second B channel
Step 1 Use the show running-config privileged exec command to view the router
configuration. Look for dialer map interface configuration command
entries.
Step 2 If there is not a dialer map configured for the second remote ISDN
telephone number, configure one for the missing telephone number.
In some topologies (PRIs, or a BRI with a 5ESS switch), one telephone
number refers to both B channels. If this is the case, you will not be able to
add a second dialer map statement.
For detailed information on configuring dialer maps, refer to the Cisco IOS
Wide-Area Networking Configuration Guide and Wide-Area Networking Command
Reference.
No service profile handler
(SPID) specified for second
B channel (BRI only)
Step 1 Use the show running-config privileged exec command to view the router
configuration. Look for isdn spid1 interface configuration command
entries.
Step 2 Make sure that there are isdn spid1 command entries for each of the
B channels. Make sure that the specified SPIDs are those assigned to you
by your service provider.
No second B channel on
remote router
The remote ISDN router must have at least two B channels for the local router to be
able to use its second B channel. Contact your service provider to find out whether
the remote ISDN router has at least two B channels.