Trunk Conditioning

Cisco IOS supports a function called a permanent trunk connection, sometimes called a connection trunk. A connection trunk creates a permanent trunk connection across the VoIP part of the network. To accomplish this, the connection trunk command is configured on a voice port to emulate a permanent connection across a packet network. The bandwidth required by the connection trunk is allocated at the creation of the trunk and remains reserved until the trunk is torn down. Figure 8-11 illustrates this concept.

Figure 8-11 Trunk Conditioning

Figure 8-11 Trunk Conditioning

Trunk conditioning is used to monitor the connection trunk state. If the connection trunk becomes unavailable, the originating gateway has the capability to signal the origination PBX and indicate that an alternate route must be found.

A unique attribute of trunk conditioning, compared to other CAC features, is that trunk conditioning has visibility into the condition of the POTS connection on the terminating side of the network and the condition of the WAN. In Figure 8-11, if there is a failure in either the WAN or the remote-side TDM connection, the originating gateway can detect this and signal the origination PBX, indicating that an alternate route must be found. This information is carried as part of the keepalive messages that are generated on connection trunk configurations.

You can tune the precise bit pattern that will be generated to the originating PBX. The ABCD bits can be configured to specific busy or out-of-service (OOS) indications that the originating PBX will recognize and act upon.

Trunk conditioning is therefore not a call-by-call feature, as are those discussed so far. It is a PBX trunk busy-back (or OOS) feature. If there is a failure in the WAN, the trunk to the originating PBX is taken out of service so that no calls can be made across that trunk until the WAN connectivity is recovered.

The following example demonstrates how a connection trunk is configured between the host and remote sites. Example 8-9 shows the configuration of the master for the trunk.

Example 8-9 Connection Trunk Host Site

controller T1 1/0

framing esf

linecode b8zs

ds0-group 1 timeslots

1 type e



ds0-group 2 timeslots

2 type e



clock source line

!— The ds0-group command creates the logical voice-ports: !— voice-port 1/0:1 and voice-port 1/0:2.

voice-port 1/0:1 connection trunk 2000

!"master side"

!This starts the Trunk connection using digits 2000 to match !a VoIP dial-peer. The digits are generated internally by the

!router and are not received from the voice-port. !

voice-port 1/0:2

connection trunk 2001

dial-peer voice 100 voip destination-pattern 200.

!matches connection trunk string 2000 and 2001 dtmf-relay h245-alphanumeric session target ipv4:

ip qos dscp cs5 media

dial-peer voice 1 pots destination-pattern 1000 port 1/0:1

!This dial-peer maps to the remote site's voice-port 1/0:1. !

dial-peer voice 2 pots destination-pattern 1001 port 1/0:2

!This dial-peer maps to the remote site's voice-port 1/0:2. !

interface Serial0/1

ip address

Example 8-10 shows the configuration of the slave for the trunk.

Example 8-10 Connection Trunk Host Site

Example 8-10 Connection Trunk Host Site

Trunk conditioning is limited in scope because it applies to connection trunk networks only.

Table 8-8 evaluates the trunk conditioning mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-8 Trunk Conditioning CAC Evaluation Criteria

Evaluation Criteria


VoX supported

VoIP/H.323, VoFR, VoATM (connection trunk configurations only)

Toll bypass or IP telephony

Toll bypass applications only

Platforms and Releases

Cisco 2600 and 3600 series routers, and Cisco MC3810 multiaccess concentrators; Cisco IOS Release 12.1(3)T

PBX trunk types supported

Analog and CAS

End to end, local, or IP cloud


Per call, interface, or endpoint

Per telephony interface

Topology awareness


Guarantees QoS for duration of call


Postdial delay


Messaging network overhead

None; uses preexisting connection trunk keepalives

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