Characteristics of the Default Dial Peer
When a matching inbound dial peer is not found, the router resorts to a virtual dial peer
called the default dial peer. The default dial peer is often referred to as dial peer 0.
Chapter 3: Routing Calls over Analog Voice Ports 177
Note Default dial peers are used for inbound matches only. They are not used to match
outbound calls that do not have a dial peer configured.
Dial peer 0 for inbound VoIP peers has the following characteristics:
■ Any codec
■ IP precedence 0
■ VAD enabled
■ No RSVP support
■ fax-rate service
For inbound POTS peers, dial peer 0 is configured with the no ivr application command.
You cannot change the default configuration for dial peer 0. Default dial peer 0 fails to
negotiate nondefault capabilities or services. When the default dial peer is matched on a
VoIP call, the call leg that is set up in the inbound direction uses any supported codec for
voice compression that is based on the requested codec capability coming from the
source router. When a default dial peer is matched, the voice path in one direction might
have different parameters from the voice path in the return direction. This might cause
one side of the connection to report good quality voice while the other side reports poor
quality voice. For example, the outbound dial peer has VAD disabled, but the inbound
call leg is matched against the default dial peer, which has VAD enabled. VAD would be
on in one direction and off in the return direction.
When the default dial peer is matched on an inbound POTS call leg, there is no default
IVR application with the port. As a result, the user gets a dial tone and proceeds with
dialed digits. Interestingly, the default dial peer cannot be viewed using show commands.
In Figure 3-30, only one-way dialing is configured. Example 3-11 and Example 3-12 illustrate
the configuration for this topology. The caller at extension 7777 can call extension
8888 because a VoIP dial peer is configured on Router 1 to route the call across the network.
However, no VoIP dial peer is configured on Router 2 to point calls across the network
toward Router 1. Therefore, no dial peer exists on Router 2 that will match the calling
number of extension 7777 on the inbound call leg. If no incoming dial peer matches
the calling number, the inbound call leg automatically matches to a default dial peer
(POTS or VoIP).
178 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
IP Cloud
10.18.0.1
Router1 Router2 PBX
Dial Peer 1 Dial Peer 3
Dial Peer 2
V V
Figure 3-30 Default Dial Peer 0
Example 3-11 Router 1 Configuration
Router1(config)#dial-peer voice 1 pots
Router1(config-dial-peer)#destination-pattern 7777
Router1(config-dial-peer)#port 1/0/0
Router1(config-dial-peer)#exit
Router1(config)#dial-peer voice 2 voip
Router1(config-dial-peer)#destination-pattern 8888
Router1(config-dial-peer)#session target ipv4:10.18.0.1
Example 3-12 Router 2 Configuration
Router2(config)#dial-peer voice 3 pots
Router2(config-dial-peer)#destination-pattern 8888
Router2(config-dial-peer)#port 1/1/0