Aastra Telecom 53I - Manual (Page 115)

Other Interoperability Features
RN-001029-00, Rev 03, Release 2.1 109
5i Series IP Phone Release Notes
Other Interoperability Features
DNS Caching
The IP phones have the ability to cache DNS requests according to RFC1035 and
RFC2181. The phone caches DNS lookups according to the TTL field, so that the
phone only performs another lookup for an address when the TTL expires.
Symmetric UDP Signaling Support
By default, Aastra IP phones use symmetric UDP signaling for outgoing UDP SIP
messages. When symmetric UDP is enabled, the IP phone generates, and listens
for UDP messages using port 5060.
This release allows you to manually disable symmetric UDP signaling using the
IP phone’s configuration file. When you disable symmetric UDP signaling, then
the IP phone chooses a random source port for UDP messages.
The IP phone also chooses a random source port for UDP messages if you
configure a backup proxy server, registrar server, or outbound proxy server.
An Administrator can configure symmetric UDP signaling using the configuration
files only.