Cisco ASA Series Cli Configuration Manual page 1168

Skinny (SCCP) Inspection
Supporting Cisco IP Phones
Note
For specific information about setting up the Phone Proxy on the ASA, which is part of the Cisco Unified
Communications architecture and supports IP phone deployment, see
Phone Proxy."
In topologies where Cisco CallManager is located on the higher security interface with respect to the
Cisco IP Phones, if NAT is required for the Cisco CallManager IP address, the mapping must be static
as a Cisco IP Phone requires the Cisco CallManager IP address to be specified explicitly in its
configuration. An static identity entry allows the Cisco CallManager on the higher security interface to
accept registrations from the Cisco IP Phones.
Cisco IP Phones require access to a TFTP server to download the configuration information they need
to connect to the Cisco CallManager server.
When the Cisco IP Phones are on a lower security interface compared to the TFTP server, you must use
an access list to connect to the protected TFTP server on UDP port 69. While you do need a static entry
for the TFTP server, this does not have to be an identity static entry. When using NAT, an identity static
entry maps to the same IP address. When using PAT, it maps to the same IP address and port.
When the Cisco IP Phones are on a higher security interface compared to the TFTP server and
Cisco CallManager, no access list or static entry is required to allow the Cisco IP Phones to initiate the
connection.
Restrictions and Limitations
The following are some of the known issues and limitations when using SCCP application inspection:
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Cisco ASA Series CLI Configuration Guide
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PAT does not work with configurations containing the alias command.
Outside NAT or PAT is not supported.
If the address of an internal Cisco CallManager is configured for NAT or PAT to a different IP
address or port, registrations for external Cisco IP Phones fail because the ASA currently does not
support NAT or PAT for the file content transferred over TFTP.
Although the ASA supports NAT of TFTP messages and opens a pinhole for the TFTP file, the ASA
cannot translate the Cisco CallManager IP address and port embedded in the Cisco IP Phone
configuration files that are transferred by TFTP during phone registration.
The ASA supports stateful failover of SCCP calls except for calls that are in the middle of call setup.
When the ASA is running in transparent firewall mode, it blocks SCCP voice traffic because
pinholes are not opened for the connection. Displaying debugging messages for SCCP inspection
indicate that RTP and RTCP channels are not open. This limication affects communication between
IP Phones and the Unified Communications Manager.
To workaround this limitation, perform one of the following actions to open secondary connections:
–
(Preferred) Install static route entries for IP Phones or convert to routed mode.
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Configure static ARP entries for IP Phones
Example:
UC Manager---IP Phone 1---Inside ASA (Transparent)---Outside ASA---IP Phone 2
Chapter 1 Configuring Inspection for Voice and Video Protocols
Chapter 1, "Configuring the Cisco