IOS Gateway Configuration for SIP-Proxy Redundancy

With Cisco IOS gateways, dial-peers are used to match phone numbers, and the destination can be a SIP Proxy Server, DNS SRV, or IP address. The following example shows a Cisco IOS gateway configuration to send calls to a SIP Proxy Server using the SIP Proxy's IP address.
sip-ua
sip-server ipv4:10.4.1.100:5060
dial-peer voice 1000 voip
session target sip-server
...
The sip-server command on the dial-peer tells the Cisco IOS gateway to use the globally defined
sip-server that is configured under the sip-ua settings. In order to configure multiple SIP Proxies for redundancy, you can change the IP address to a DNS SRV record, as shown in the following example. The DNS SRV record allows a single DNS name to be mapped to multiple servers.
sip-ua
sip-server dns:cvp.cisco.com
dial-peer voice 1000 voip
session target sip-server
...
Alternatively, you can configure multiple dial-peers to point directly at multiple SIP Proxy servers, as shown in the following example. This configuration allows you to specify IP addresses instead of relyingon DNS.
dial-peer voice 1000 voip
session target ipv4:10.4.1.100
preference 1
...
dial-peer voice 1000 voip
session target ipv4:10.4.1.101
preference 1
...
In the preceding examples, the calls are sent to the SIP Proxy server for dial plan resolution and callrouting. If there are multiple Unified CVP Call Servers, the SIP Proxy server would be configured with multiple routes for load balancing and redundancy.

DNS SRV records allow an administrator to configure redundancy and load balancing with finer
granularity than with DNS round-robin redundancy and load balancing. A DNS SRV record allows you to define which hosts should be used for a particular service (the service in this case is SIP), and it allowsyou to define the load-balancing characteristics among those hosts. In the following example, the
redundancy provided by the three dial-peers configured above is replaced with a single dial-peer using a DNS SRV record. Note that a DNS server is required in order to do the DNS lookups.
ip name-server 10.4.33.200
dial-peer voice 1000 voip
session target dns:cvp.cisco.com


With Cisco IOS gateways, it is possible to define DNS SRV records statically, similar to static host records. This capability allows you to simplify dial-peer configuration while also providing DNS SRV load balancing and redundancy. The downside of this method is that, if the SRV record needs to be changed, it must be changed on each gateway instead of on a centralized DNS server. The following example shows the configuration of static SRV records for SIP services handled by cvp.cisco.com, and the SIP SRV records for cvp.cisco.com are configured to load-balance across three servers.
ip host cvp4cc2.cisco.com 10.4.33.132
ip host cvp4cc3.cisco.com 10.4.33.133
ip host cvp4cc1.cisco.com 10.4.33.131
(SRV records for SIP/TCP)
ip host _sip._tcp.cvp.cisco.com srv 1 50 5060 cvp4cc3.cisco.com
ip host _sip._tcp.cvp.cisco.com srv 1 50 5060 cvp4cc2.cisco.com
ip host _sip._tcp.cvp.cisco.com srv 1 50 5060 cvp4cc1.cisco.com
(SRV records for SIP/UDP)
ip host _sip._udp.cvp.cisco.com srv 1 50 5060 cvp4cc3.cisco.com
ip host _sip._udp.cvp.cisco.com srv 1 50 5060 cvp4cc2.cisco.com
ip host _sip._udp.cvp.cisco.com srv 1 50 5060 cvp4cc1.cisco.com
Cisco highly recommends the use of UDP instead of TCP for SIP signaling. TCP stack timeout delays can cause significant delays to the caller during failures.