SIP with Clearmode on Cisco ISR

In this blog post i will try to explain how to configure a Cisco ISR device like the ISR 3825 to use the Clearmode codec to for ISDN traffic over SIP to another device.

Clearmode is in simple terms the 64kbit ISDN B channel converted to an RTP stream. Same data but repackaged for transport over IP. The cisco variant of clearmode is called clear-channel and the routers can be configured to use Clearmode/RFC 4040 instead.

A simple unauthenticated SIP trunk to Asterisk

This configuration enables SIP on the interface GigabitEthernet0/0 and routes 7 digit long phone numbers to IP 1.2.3.4. I'm from Europe so I prefer alaw. I use this kind of fonfiguration on my Cisco ISR 3825 running IOS 15.1(4)M9

voice service voip
 no ip address trusted authenticate
 allow-connections sip to sip
 fax protocol none
 modem passthrough nse codec g711alaw
 sip
  bind control source-interface GigabitEthernet0/0
  bind media source-interface GigabitEthernet0/0
!
voice class codec 1
 codec preference 1 g711alaw
 codec preference 2 g711ulaw
!
dial-peer voice 1234 voip
 description Cool peer 1234
 destination-pattern 1234...
 session protocol sipv2
 session target ipv4:1.2.3.4
 voice-class codec 1
 dtmf-relay rtp-nte
 no vad
!

Adding Clearmode

voice service voip
 sip
  bearer-capability clear-channel udi
  encap clear-channel standard
!
voice class codec 1
 codec preference 3 clear-channel
!

bearer-capability clear-channel udi enables clear-channel codec to UDI bearer capability mapping and UDI bearer capability to clear-channel codec mapping.

encap clear-channel standard enables RFC 4040-based clear-channel codec negotiation for SIP calls. Without this line the calls will not work with non cisco SIP connections.

codec preference 3 clear-channel adds clearmode support to the codec list