In the CCIE Collaboration lab, understanding dial-peers is extremely important. Lack of knowledge in this area can yield devastating results in your lab score report since they can be found in so many different sections of the exam. We must be thoroughly prepared to tackle every aspect of this technology should we be presented with it at some point.
I recently got a great question in our forums about digit manipulation within POTS dial-peers and how they interact with translation rules and profiles. I figured that since this is such an important topic, my answer to his question bears repeating so it can reach a wider audience.
Let’s begin with the simple example of dialing the number “123” from a CUCME phone. Of course, the POTS dial-peer must be created to support the desired behavior.
When this pattern is selected, all digits will be stripped automatically since they are explicitly defined. This is due to the “automatic POTS dial-peer digit strip” feature in IOS. See below for the ISDN Q.931 debug output (no Called Party Number).
Since we are not currently sending a Called Party Number, we’ll need some way to add the digits back to the string to route the call. Let’s add a translation rule and profile to accomplish this.
This configuration will match the original dialed string of “123” and replace it with “345″. It should be applied to the dial-peer as shown below.
After applying the translation profile, as expected, the Q.931 debug shows that the Called Party Number is now “345”.
Next, to test the available dial-peer digit manipulation techniques, add the forward-digits command to the dial-peer.
Since this manipulation occurs after the translation profile, it will keep the “rightmost” 2 digits for the “345” called number, as shown in the Q.931 debug. Take note that it is using the number that was applied in the translation profile as opposed to the original called number that matched the dial-peer.
Now, add the prefix command to the dial-peer to understand how it affects the number.
With both the forward-digits and prefix commands applied, this results in the number “745″, per the Q.931 debug command.
Based on the above, what we can conclude is that translation profiles are applied as the first step in IOS digit manipulation. All other dial-peer digit manipulation techniques are applied after this point. In other words, digit manipulation in IOS is considered to be “additive” since it can modify the number after the translation profile is applied. This is in contrast to what is normally seen on CUCM, where digit manipulations performed at different configuration levels still reference the original called number instead of adhering to manipulations that were previously performed. For example, Called Party Transformations in CUCM take precedence over called party digit manipulations in both Route Lists and Route Patterns and use the original called number instead of any manipulated called number from the Route Lists/Patterns.
I hope this has been helpful to all that are studying for the CCIE Collaboration lab exam. Please keep your eye out for many updates to come for both our workbooks and videos. Also, if you’re ever feeling like you need an extra push to get ready for the lab, are hitting roadblocks in your preparation, or just need some direction on how tackle the CCIE Collaboration Lab, give us a call and speak with an iPexpert Training Adviser about attending one of my bootcamps.
Thanks again for reading and good luck in your preparation!
- Andy