System information

Troubleshooting ISDN Connections 17-391

ISDN: Second B Channel Comes Up Too Early

Symptom: When using a second B channel as a backup connection to a single destination, the

second B channel comes up before the load on the ﬁrst B channel is high enough.

Table 17-8 outlines the problem that might cause this symptom and describes solutions to that

problem.

Table 17-8 ISDN: Second B Channel Comes Up Too Early

Possible Problem Solution

Misconﬁgured dialer

load-threshold

command

Step 1 Use the show running-conﬁg privileged exec command to view the router

conﬁguration. Check the 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 Make sure that the value conﬁgured by this command is not too low. 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 load value speciﬁed is too low, increase the speciﬁed load in increments of

25 or 50 to allow the load on the ﬁrst B channel to reach a greater value before

the second B channel dials.