Cisco IP SoftPhone is a PC based IP Telephony endpoint that offers the
ability to make a PC work like a Cisco 7900 Series phone. Remote users who
connect back to their company network through VPN need to configure some
additional settings in order to avoid a one way voice problem. This is a result
of the media stream having to know what the endpoint of the connection is. The
normal topology for this situation looks something like this:
Since the PC uses the VPN tunnel that comes into the corporate network,
all the traffic appears to come through the PC's end of the VPN tunnel, rather
than the regular networking interface that gets its IP address from the ISP
connection. Cisco IP SoftPhone uses the network address of the regular
networking interface by default when it registers its location with Cisco
CallManager. When Cisco IP SoftPhone sets up its media stream with the
corporate end of the connection, the address the corporate end tries to send
the voice data to is the regular (ISP assigned) IP address.
The device on the corporate end of the connection cannot tell its
packets are being dropped, since this is all done with User Datagram Protocol
(UDP). This document discusses how to set up Cisco IP SoftPhone to
automatically set the correct address or allow the user to manually do
Readers of this document should have knowledge of these topics:
Cisco IP SoftPhone
VPN / IPSec
The information in this document is based on Cisco IP SoftPhone Version
1.2 and 1.3.
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Technical Tips Conventions for more information on document
In order to get Cisco IP SoftPhone to have two-way audio when it uses
NAT and utilizes VPN Client software, make sure that you have configured the
VPN IP address, instead of the configured network adapter IP address under the
"network audio settings".
Complete these steps:
Open Cisco IP SoftPhone and click the Settings
Select the Audio settings tab.
Click Network Audio Settings.
There are three ways to get the correct IP address advertised.
Choose one of these three ways and click OK when finished.
The Automatic Selection setting uses the file getIP.asp from the
web server that you use to distribute the IP SoftPhone files to clients.
For example, if the IP address of your IP SoftPhone distribution
web server is 10.21.15.10 and the IP SoftPhone distribution files are shared in
a directory named "ipsp", the url to use on your PC to obtain its current IP
address is http://10.21.15.10/ipsp/getip.asp
This web application provides this output when connected through
172.16.240.37 /* tapiaddress.java result = provider.syncTapiObject.
t.getIPAddress(), t.getUDPPort(), t.usesLowBandwidthCodec()); */
The address at the beginning of the response is the IP address
the request came from. In this case the PC's address is 172.16.240.37. This is
the address Cisco IP SoftPhone then uses when it registers with the Cisco
CallManager. By automating this, the need to manually enter the address every
time a new one is assigned should be alleviated. Point the automatic selection
to the getIP.asp page on the Web server. The page should be there by default
when Web Administrator is installed and should be in the same folder.
For more information on this option please refer to the
Automatic Selection for IP Address Setting
Up Cisco IP SoftPhone Using the Customization Tool.
There is a pull-down menu of addresses on the local machine.
Select the one that is most appropriate.
The address to be advertised can be manually entered if needed.
Cisco IP SoftPhone must be restarted for the changes to take
What Software VPN Clients are Supported?
Supported software VPN Clients include Cisco Systems VPN Client 3.x
or 4.x, and the Microsoft PPTP client. Other third-party VPN Clients might be
unsupported. A VPN solution is typically unsupported if it is not a Cisco
product and does not function like a network interface card.
When you configure a SoftPhone to connect through a VPN, it is
important for Cisco CallManager to receive correct information about the IP
address the SoftPhone uses. By default, the Cisco IP SoftPhone uses its
physical network address to register with Cisco CallManager rather than with
the VPN address. Cisco CallManager instructs other IP phones to send to this
address. The result is a one-way voice since the IP phones are able to reach
the VPN address of the PC and not the physical interface of the PC.