FXO Voice Port Configuration
An FXO trunk is one of the simplest analog trunks available. Because Dialed Number
Information Service (DNIS) information can only be sent out to the PSTN, no direct
inward dialing (DID) is possible. ANI is supported for inbound calls. Two signaling types
exist, loopstart and groundstart, with groundstart being the preferred method.
For example, consider the topology shown in Figure 3-16. Imagine you have been
assigned to configure a voice gateway to route calls to and from the PSTN through an
FXO port on the router.
Austin
4001 4002
Inbound calls should
be routed to 4001.
PSTN
FXO
0/0/0
Figure 3-16 FXO Configuration Topology
In this scenario, you must set up a PLAR connection using an FXO port connected to
the PSTN.
The configuration requirements are the following:
■ Configure the voice port to use ground-start signaling.
■ Configure a PLAR connection from a remote location to extension 4001 in Austin.
■ Configure a standard dial peer for inbound and outbound PSTN calls.
Because an FXO trunk does not support DID, two-stage dialing is required for all
inbound calls. If all inbound calls should be routed to a specific extension, (for example,
a front desk), you can use the connection plar opx command. In this example, all
inbound calls are routed to extension 4001.
You could then complete the following steps to configure the FXO voice port:
Step 1. Enter voice-port configuration mode.
Router(config)#voice-port 0/0/0
Step 2. Select the access signaling type to match the telephony connection you are
making.
Router(config-voiceport)#signal ground-start
Step 3. Specify a PLAR off-premises extension (OPX) connection.
Router(config-voiceport)#connection plar opx 4001
Chapter 3: Routing Calls over Analog Voice Ports 147
Note PLAR is an autodialing mechanism that permanently associates a voice interface
with a far-end voice interface, allowing call completion to a specific telephone number or
PBX without dialing. When the calling telephone goes off-hook, a predefined network dial
peer is automatically matched. This sets up a call to the destination telephone or PBX.
Using the opx option, the local voice port provides a local response before the remote
voice port receives an answer. On FXO interfaces, the voice port does not answer until the
remote side has answered.
Step 4. Activate the voice port.
Router(config-voiceport)#no shutdown
Step 5. Exit voice port configuration mode.
Router(config-voiceport)#exit
Step 6. Create a standard dial peer for inbound and outbound PSTN calls.
Router(config)#dial-peer voice 90 pots
Step 7. Specify the destination pattern.
Router(config-dialpeer)#destination-pattern 9T
Step 8. Specify the voice port associated with this dial peer.
Router(config-dialpeer)#port 0/0/0
Example 3-2 shows the complete FXO voice port configuration.
FXO Voice Port Configuration
148 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
Note The T control character indicates that the destination-pattern value is a variablelength
dial string. Using this control character enables the router to wait until all digits are
received before routing the call.
Dial-peer configuration is covered in the section, “Introducing Dial Peers.”
Router(config)#voice-port 0/0/0
Router(config-voiceport)#signal groundstart
Router(config-voiceport)#connection plar opx 4001
Router(config)#dial-peer voice 90 pots
Router(config-dialpeer)#destination-pattern 9T
Router(config-dialpeer)#port 0/0/0