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Chapter 10
Wide Area Networking Protocols
the dialer-group 1 command. That number must match the dialer-list
number. The hold-queue 75 in command tells the router that when it
receives an interesting packet, it should queue up to 75 packets while it is
waiting for the BRI to come up. If there are more than 75 packets queued
before the link comes up, the packets will be dropped.
Optional Commands
There are two other commands that you should configure on your BRI inter-
face: the dialer load-threshold command and the dialer idle-timeout
command.
The dialer load-threshold command tells the BRI interface when to
bring up the second B channel. The option is from 1 to 255, where 255 tells
the BRI to bring up the second B channel only when the first channel is
100 percent loaded. The second option for that command is in, out, or
either. This calculates the actual load on the interface either on outbound
traffic, inbound traffic, or combined. The default is outbound.
The dialer idle-timeout command specifies the number of seconds
before a call is disconnected after the last interesting traffic is sent. The
default is 120 seconds.
RouterA(config-if)#dialer load-threshold 125 either
RouterA(config-if)#dialer idle-timeout 180
The dialer load-threshold 125 tells the BRI interface to bring up the
second B channel if either the inbound or outbound traffic load is 50 percent.
The dialer idle-timeout 180 changes the default disconnect time from
120 to 180 seconds.
DDR with Access Lists
You can use access lists to be more specific about what is interesting traffic.
In the preceding example we just set the dialer list to allow any IP traffic to
bring up the line. That is great if you are testing, but it can defeat the purpose
of why you use a DDR line in the first place. You can use extended access
lists to set the restriction, for example, to only e-mail or Telnet.
Here is an example of how you define the dialer list to use an access list:
804A(config)#dialer-list 1 list 110
804A(config)#access-list 110 permit tcp any any eq smtp
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