Cisco CCNA Certification Training:

ISDN BRI And PPP Multilink

By Chris Bryant, CCIE #12933

NOTE: As of the exam version 640-802, ISDN is no longer on the CCNA exams. However, it is still on the 640-801 exam, which many Cisco Academy students are still eligible to take as of the writing of this note (January 31, 2009). For that reason, I'm leaving this tutorial on the website!

Earning your CCNA certification means you have to notice details about Cisco routers that others might miss, and that's true in the exam room and on the job. In today's tutorial, we'll take a look at multilink PPP - a topic that seems simple enough, but has details that trip up some CCNA certification candidates.

ISDN BRI (Basic Rate Interface) has two B-channels that both carry data, and they each have a capacity of 64 KBPS. Interestingly enough, they don't share the load by default - the first channel has to be at capacity before the second channel starts taking some of the load.

It's more efficient to have the channels share the load before then, and we can do just that by configuring PPP multilink. (Note that it's called PPP multilink for a reason. We can't configure it if the BRI interface is running at the default of HDLC.)

Configuring PPP multilink is simple enough. PPP must be running on the interface to begin with, and then you simply configure the interface with the ppp multilink command.

R1(config)#int bri0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp multilink

Here's where knowing the details comes in handy. An additional command, dialer load-threshold, is needed to define the capacity level of the first b-channel that will bring up the second b-channel.

For example, let's say we want the second channel to come up when the first channel hits 50% of its capacity. It's easy to think that we would put "50" at the end of the dialer load-threshold command, but that's wrong! Look at what IOS Help tells us about this command.

R1(config-if)#dialer load-threshold ?

<1-255> Load threshold to place another call

This value is based on 255, not 100. To bring the second channel up as described earlier, we need to enter a value that is 50% of 255:

R1(config-if)#dialer load-threshold 127

The dialer load-threshold command has other options not shown here. The interface can be configured to consider only incoming or outgoing traffic for this command, or both.

This is just one of the many details you must master in order to earn your CCNA certification. Take it from me - it's worth it. Keep studying - and there's plenty of free Cisco study material for you on my Cisco Certification Tutorial Page!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com