Sip Over Tcp; Sip Proxy Redundancy; Configuring Survivable Remote Site Telephony (Srst) Support - Cisco 521SG - Unified IP Phone VoIP Administration Manual

Configuring SIP, SPCP, and NAT
SIP and Cisco IP Phones
Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide

SIP Over TCP

To guarantee state-oriented communications, Cisco IP phones can use TCP as the
transport protocol for SIP. This protocol provides guaranteed delivery that assures
that lost packets are retransmitted. TCP also guarantees that the SIP packages are
received in the same order that they were sent.
TCP overcomes the problem UDP ports have of being blocked by corporate
firewalls. With TCP, new ports do not need to be opened or packets dropped,
because TCP is already in use for basic activities, such as Internet browsing or
e-commerce.

SIP Proxy Redundancy

An average SIP proxy server can handle tens of thousands of subscribers. A
backup server allows an active server to be temporarily switched out for
maintenance. Cisco phones support the use of backup SIP proxy servers to
minimize or eliminate service disruption.
A static list of proxy servers is not always adequate. If your user agents are served
by different domains, for example, you would not want to configure a static list of
proxy servers for each domain into every Cisco IP phone.
A simple way to support proxy redundancy is to configure a SIP proxy server in
the Cisco IP phone configuration profile. The DNS SRV records instruct the phones
to contact a SIP proxy server in a domain named in SIP messages. The phone
consults the DNS server. If configured, the DNS server returns an SRV record that
contains a list of SIP proxy servers for the domain, with their host names, priority,
listening ports, and so forth. The Cisco IP phone tries to contact the hosts in the
order of their priority.
If the Cisco IP phone currently uses a lower-priority proxy server, the phone
periodically probes the higher-priority proxy and switches to the higher-priority
proxy when available.

Configuring Survivable Remote Site Telephony (SRST) Support

proxy outbound proxy
The and
extension that includes a statically-configured DNS SRV record or DNS A record.
This allows for failover and fallback functionality with a secondary proxy server.
The format for the parameter value is:
FQDN format: hostname[:port][:SRV=host-list OR :A=ip-list]
host-list: srv[|srv[|srv...]]
srv: hostname[:port][:p=priority][:weight][:A=ip-list]
ip-list: ip-addr[,ip-addr[,ip-addr...]]
fields in the Ext tab can be configured with an
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