Overlapping Dial Plan In IOS Gateways And Dial Peer Matching

Published by Keyboard Banger on

Let’s try an overlapping dial plan on gateways that operate in a digit by digit fashion. We will use the following network topology:

dial-peer-matching--2016-04-03 11_23_35

Remember that digit by digit operation is performed on these devices:

  • CUCME router
  • gateways connected to PSTN configured for Overlap Sending and Receiving.

We have one CUCME router on which we configured two dial peers with the same value of precedence. Both dial peers are potential matches for the dial string 5124771551.

Overlapping dial plan in IOS gateways: scenario 1

overlapping-dial-plan-ios-gateways--2016-04-04 18_11_48-

Dial peer 512 has more explicit digits than dial peer 5122.

We make a call from Mongi shop to Bicycle Sport shop and observe which outbound dial peer is matched on Mongi voice gateway.

overlapping-dial-plan-ios-gateways--2016-04-04 18_15_18

Although the full PSTN number is dialed, the voice gateway still waits for more digits. That’s the effect of the terminating character. After no more digits are sent and the interdigit timeout expires, the IOS finds an outbound matching dial peer: dial peer 512 is a match. Dial peer 5122 is a potential match:

overlapping-dial-plan-ios-gateways--2016-04-04 18_17_10

I repeated the experiment and had the same results: dial peer 512 is the match and dial peer 5122 is a potential match. So, dial peer 5122 is never matched as long as there is dial peer 512.

This demonstrates that the most specific dial peer (the dial peer with the more explicit digits in the pattern) always wins.

Compare this result with page 457 of the CVOICE Foundation Learning Guide:

dial-peer-matching--2016-04-03 11_31_08

According to the author, dial peer 90110 will never be used, although it is the dial peer with the most explicit pattern. This statement contradicts what we just found.

Overlapping dial plan in IOS gateways: scenario 2

overlapping-dial-plan-ios-gateways--2016-04-04 18_19_42

We make a call from Mongi phone to PSTN. As soon as I finish dialing “512”, it displays “unknown number” and the call fails.

dial-peer-matching--2016-04-03 12_08_08

In the output of the debug, we clearly see the reason: dial peer 5122 was the match and not dial peer 512.

overlapping-dial-plan-ios-gateways--2016-04-04 18_21_39

So as soon as the IOS finds a matching dial peer, it uses it and immediately processes the call for routing.

Categories: CCNP Collaboration

Keyboard Banger

Keyboard Banger is a network engineer from Africa. He has been working in network support and administration since 2008. He started writing study notes about certification exams and technology topics a couple of years ago. When he's not writing articles, he can be found wandering on technical forums.

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