This document explains the differences between inbound and outbound
dial-peers and call legs. Also, this document stresses the importance of
inbound dial peer(s) that match when you use non-default
services, applications, and/or capabilities to setup and complete voice
Readers of this document need to have knowledge of
Dial Peers and Call Legs on Cisco IOS® Platforms.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Technical Tips Conventions.
Dial peers are used for both inbound and
outbound call legs. It is important to remember that these
terms are defined from the perspective of the router/gateway. An
inbound call leg originates when an incoming call comes
into the router orgateway. An outbound call leg originates
when a call is placed or bridged from the router/gateway.
For inbound calls from a plain old telephone service (POTS) interface
that are destined for the packet network, the originating router/gateway
matches an inbound POTS dial peer for the
inbound call leg first. Next, the originating
router/gateway creates an outbound Voice-Network dial peer such as Voice over
IP (VoIP) or Voice over Frame relay (VoFR) for the outbound call
leg. After this, the router/gateway bridges the two call legs.
For inbound calls from a Voice Network interface that are destined for
a POTS interface, the terminating router/gateway matches an
inbound Voice Network dial peer for the
inbound call leg. Next, an outbound POTS dial peer is created for
the outbound call leg.
A common misunderstanding with voice dial peers is that they are only
configured for outbound functionality, that is, to map a
dial string to a remote network device (with the Cisco IOS commands
destination-pattern and session
target) or a POTS voice port (with the Cisco IOS commands
port). However, dial peers need to be configured for
inbound functionality when you deal with scenarios where
non-default services, applications, and/or capabilities are present.
On inbound POTS call legs received at the
originating router/gateway, some non-default services and applications of
incoming calls include:
When you use such services or applications, it is important to ensure
that the correct inbound POTS dial peer configured with
the appropriate service or application is matched. For more information, refer
How Inbound and Outbound Dial Peers are Matched on Cisco IOS Platforms.
When non-default Voice Network capabilities or TCL applications are
requested by the originating router/gateway, the terminating router/gateway
must match those capabilities and applications configured with an
inbound Voice Network dial peer. If the Cisco IOS Software
is unable to match a non-default configured inbound dial
peer, the software uses an internally defined default dial peer to match the
inbound voice calls. The call setup can fail if the
incoming call leg has non-default capabilities, services, or applications, and
is matched to a default dial peer.
Default Voice-Network capabilities include:
codec g729r8 (payload 20 bytes)
fax rate voice
playout-delay 40 ms
register E.164 number with GK
session protocol cisco (for H.323).
Note: Default capabilities are not displayed in the router/gateway IOS
configuration output. Issue the command show dial-peer voice
in order to view the configured
capabilities, services, and applications on POTS and Voice Network dial peers.
Note: The default DSCP for voice is ef codepoint 101110 (RFC 2598) and the
default DSCP for signaling is af31 codepoint 011010 (RFC 2597). The default
dial-peer, PID 0, does not mark packets to DSCP 0. All voice packets on the
routers are marked by default (can be overridden by the dial-peer), signaling
with AF31 and media with EF. Calls that match the default dial-peer 0 should
also have this behavior.
For more information and a practical example, refer to the case study
How Inbound and Outbound Dial Peers are Matched on Cisco IOS