To pass the CCNA and / or BCRAN exam, you have to know how to configure Multilink PPP (MLP). The configuration itself is not difficult, but there's one little detail you must not forget!
With BRI, we've got two B-channels to carry data, and both of them have a 64-kbps capacity. You might think it would be a good idea to have both channels in operation when one channel begins to approach capacity, - problem is, it's not a default behavior of ISDN. The second b-channel will not begin to carry traffic until the first one reaches capacity.
With Multilink PPP (MLP), a bandwidth capacity can be set that will allow the second b-channel to bear data before the first channel reaches capacity. The configuration for MLP is simple, but often misconfigured. We'll use our good friend IOS Help to verify the measurement this command uses when we get to the configuration.
Enabling MLP is a three-step process:
Enable PPP on the link
Enable MLP with the command ppp multilink
Define the threshold at which the second b-channel should start carrying data with the dialer load-threshold command. Without this command, MLP will not work.
Let's say you wanted the second b-channel to start carrying data when the first channel reaches 75% of capacity. It would make sense that the command to do so would be dialer load-threshold 75... but it's not.
R1(config)#int bri0
R1(config-if)#ppp multilink
R1(config-if)#dialer load-threshold ?
<1-255> Load threshold to place another call
R1(config-if)#dialer load-threshold 191 ?
either Threshold decision based on max of inbound and outbound traffic
inbound Threshold decision based on inbound traffic only
outbound Threshold decision based on outbound traffic only
R1(config-if)#dialer load-threshold 191 either
The dialer load-threshold value is based on 255, not 100. To have this command bring the line up at a certain percentage, multiply that percentage in decimal format by 255. Above, I multiplied 255 by .75 (75%) to arrive at 191.
As IOS Help shows in the above example, dialer load-threshold has additional options as well. You can configure the interface to consider only incoming, outgoing, or all traffic when calculating when to bring the next B-channel up.
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