On-Net to Off-Net Calls
When planning a resilient call-routing strategy, you might need to reroute calls through a
secondary path should the primary path fail. An on-net to off-net call, as illustrated in
Figure 3-7, originates on an internal network and is routed to an external network, usually
to the PSTN. On-net to off-net call-switching functionality might be necessary when a
network link is down or if a network becomes overloaded and unable to handle all calls
presented.
130 Authorized Self-Study Guide: Cisco Voice over IP (CVOICE)
V V
Gateway Gateway
IP WAN
PSTN
1
2
3
WAN is down 4
or congested!!
Figure 3-7 On-Net to Off-Net Calls
Note On-net to off-net calls might occur as a result of dial plan configuration, or they
might be redirected by Call Admission Control (CAC).
An example of an on-net to off-net call is one staff member calling another staff member
at a remote office while the WAN link is congested. When the originating voice-enabled
router determines it cannot complete the call across the WAN link, it sends the call to the
PSTN with the appropriate dialed digits to terminate the call at the remote office via the
PSTN network.
The following steps, numbered in Figure 3-7, summarize the call flow of an on-net to offnet
call:
Step 1. A user on the network initiates a call to a remote site.
Step 2. The output of the WAN gateway is either down or congested, so the call is
rerouted.
Step 3. The call connects to the PSTN.
Step 4. The PSTN completes the call to the remote site.