H.323 And SIP Dial Peers

In this article we will play with dial peers by setting different combinations between a source and a destination gateway.

Here is the network setting:

x5002 — Mongi Shop router — PSTN router — x4002

Before starting the lab, make sure you know the difference between CUCM wildcards and dial peer wildcards.

In case you need a refresher on H323 configuration or packet analysis, then please read this post.

Playing With Dial Peer Session Protocols

case#1: one outbound call leg is H323 and the remote inbound call leg is SIP

sip-h323--2016-03-06 19_31_38

sip-h323--2016-03-06 19_31_33

The call succeeds and it is a H.323 call:
h323 call

wireshark trace in a h323 call

case#2: one outbound call leg is SIP and the remote inbound call leg is H.323

configuring a h323 dial peer

configuring a sip dial peer

The call succeeds and it is a SIP call:

sip-h323--2016-03-06 19_38_32

sip-h323--2016-03-06 19_38_48

So from both cases, we see that the type of session protocol used for outbound call legs follows what is configured on the outbound call leg, despite the difference with the remote inbound call leg.

But, why does the terminating gateway still accepts inbound call legs whether they are H.323 or SIP? That’s because SIP and H.323 services are up on the router.

So to force the router to stop the H.323 service for example, we can do that as follows:

h323-sip--2016-06-13 16_02_02

DTMF Relay

Configuring DTMF support occurs at the voip dial peer level.

Let’s choose which dial peer we will configure DTMF support for.

displaying a summary of the configured dial peers

We examine if any DTMF settings exist already:

h323-sip--2016-03-10 19_57_461

Line 390 says DTMF Relay is disabled. We configure it under the dial peer:

h323-sip--2016-03-10 19_59_06

and we check again with “show dial-peer” command:

h323-sip--2016-03-10 19_59_411

h323-sip--2016-03-10 20_00_04

Now we want to examine the negotiated DTMF method between two voice gateways, when a call is established. The network setting is: x5002 — Mongi Shop router — PSTN router — x4002. We call from x5002 to x4002.

  • The inbound matched dial peer at PSTN router is voip dial peer 111
  • The outbound matched dial peer at Mongi Shop router is voip dial peer 4000

We remove everything related to DTMF relay on both routers.

h323-sip-dtmf-relay--2016-03-13 11_19_42

h323-sip-dtmf-relay--2016-03-13 11_20_51

We make the call and observe the negotiated DTMF relay method with the command “show sip-ua calls”.

h323-sip-dtmf-relay--2016-03-13 11_26_55

Since there is no custom DTMF relay method, both routers fall to inband-voice DTMF relay:

h323-sip-dtmf-relay--2016-03-13 11_28_23h323-sip-dtmf-relay--2016-03-13 11_28_09

Let’s change the method to H245 alphanumeric on the UAC:

h323-sip-dtmf-relay--2016-03-13 11_32_31

We make the call. The same DTMF method (inband-voice) is agreed upon. That’s because the inbound matched dial peer on the UAS does not support the H245 alphanumeric method.

h323-sip-dtmf-relay--2016-03-13 11_32_52

The following table summarizes what I’ve found from experiencing with different combinations of DTMF relay methods, on the UAC and the UAS.


UAC DTMF method,UAS DTMF method,negotiated DTMF method


H245-alphanumeric cisco-rtp h245-signal rtp-nte sip-kpml sip-notify,none,inband-voice





H245-alphanumeric cisco-rtp h245-signal rtp-nte sip-kpml sip-notify,sip-notify,sip-notify


Notice that, in the absence of a common DTMF method between the UAC and the UAS, both fall to the default which is Inband-voice.

Dial Peer Matching when Dial Plans overlap in IOS Gateways

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.

Cisco Dial Peer Hunting

Dial peer hunting is a way to change the default behaviour of outbound dial peer matching.

Dial peer hunt options

cisco-dial-peer-hunting--2016-06-21 06_10_50

Dial peer lab setup

On our Cisco UBE, I’ve set three dial peers for outbound matching. The test pattern will be 911.

Notice the different preference values.

cisco-dial-peer-hunting--2016-06-21 07_17_43

Testing dial peer hunt options

I’ll go through them one by one. To determine which dial peer matched, do a debug voip dialpeer on the CUBE.

Dial-peer hunt 0

cisco-dial-peer-hunting--2016-06-21 07_24_41

this is the default.

cisco-dial-peer-hunting--2016-06-21 07_23_49

Dial-peer hunt 1

cisco-dial-peer-hunting--2016-06-21 07_24_42

Dial-peer hunt 2

cisco-dial-peer-hunting--2016-06-21 07_24_43

Dial-peer hunt 3

cisco-dial-peer-hunting--2016-06-21 07_24_44

Dial-peer hunt 4

cisco-dial-peer-hunting--2016-06-21 07_24_45

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