Cisco SPA 301 - SIP Proxy Redundancy; Configuring Survivable Remote Site Telephony (SRST) Support

Configuring SIP, SPCP, and NAT
Session Initiation Protocol and Cisco IP Phones
Cisco Small Business SPA 300 Series, SPA 500 Series, and WIP310 IP Phone Administration Guide 89
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TCP overcomes the problem with UDP ports 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 may 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 on. 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
The proxy and outbound proxy fields in the Ext tab can be configured with an
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 as follows:
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…]]
The default priority is 0 and default weight is 1. The default port is 0, and the
application substitutes the proper port value (for example, port 5060 for SIP).