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Configuring DNS Records

[In progress]

SIP Cluster SIP connectivity relies on SIP Cluster Fully Qualified Domain Name (FQDN) with SRV-based name resolution. SRV-type records for the SIP Cluster are registered in a geo-aware DNS zone. The same SIP Cluster FQDN is used in each data center. The same priority, weight, and port must be specified within the SRV records.

All SIP Proxies in the SIP Cluster use the same port number (sip-port) for SIP signaling. Each SIP Proxy runs on its own machine and uses its own IP address. For a given data center, the SRV records resolve into the IP addresses of the SIP Proxies located in this data center and the port number used by all SIP Proxies. See the SIP Proxy IP address and DNS mapping example below.

SIP Server, Resource Managers, the Session Border Controller (SBC), and SIP Phones rely on this DNS resolution to local devices, which they use when distributing SIP requests across the SIP Proxies in the load-balancing manner that they support. A SIP Phone or SBC can remain with one SIP Proxy as long as the SIP Proxy is operational

Required Provisioning

  • The FQDN specified as the target within the Record Data of the SRV records must be identical to the FQDN specified as the Host name of the Host object. If the Host object uses the IP address and does not use the FQDN, the FQDN must resolve into this IP address within the SRV record.
  • The port number specified within the Record Data of the SRV records must be identical to the port number specified in the "'sip-port'" parameter on the Server Info tab of the SIP Proxy application (?).
  • The FQDN that resolves to the SRV list of the SIP Proxy addresses is specified in the SIP Outbound Proxy DN configuration:
    • contact
    • external-contact
  • If SBC or SIP Phones do not support SRV name resolution, specify another FQDN in the external-contact option. This FQDN should be registered as an Address-type record and should be resolved using A or AAAA type queries.

Example: SIP Proxy IP address and DNS mapping

DNS record mapping example (click to expand)

In data center 1 (DC1), SIP Cluster SIP connectivity as follows:

  • SIP Proxies:
    • DC1 SIP Proxy 1: 192.168.11.121
    • DC1 SIP Proxy 2: 192.168.11.125
  • DNS records for the SIP Proxy machines:
    • dc1-spx1.example.com. IN A 192.168.11.121
    • dc1-spx2.example.com. IN A 192.168.11.125

In data center 2 (DC2), SIP Cluster SIP connectivity as follows:

  • SIP Proxies:
    • DC2 SIP Proxy 1: 192.168.22.121
    • DC2 SIP Proxy 2: 192.168.22.125
  • DNS records for the SIP Proxy machines:
    • dc2-spx1.example.com. IN A 192.168.22.121
    • dc2-spx2.example.com. IN A 192.168.22.125

As shown in the diagram, SBC and GVP Resource Manager send new SIP requests to the local SIP Cluster SIP Proxies. Geo-aware name resolution enables local SIP signaling of the SIP requests delivered to the SIP Cluster.

UDP protocol example -- SRV Records for SIP Cluster

  • SIP Proxies listen on port 5060
  • SIP Cluster FQDN: spx-srv.example.com

For systems that use the UDP protocol for SIP signaling (usually phones and SBC), SRV records would be as follows:

  • In data center 1, the geo-aware DNS zone version:
    • _sip._udp.spx-srv.example.com. IN SRV 10 50 5060 dc1-spx1.example.com.
    • _sip._udp.sipx-srv.example.com. IN SRV 10 50 5060 dc1-spx2.example.com.
  • In data center 2, the geo-aware DNS zone version:
    • _sip._udp.spx-srv.example.com. IN SRV 10 50 5060 dc2-spx1.example.com.
    • _sip._udp.spx-srv.example.com. IN SRV 10 50 5060 dc2-spx2.example.com.

TCP protocol example -- SRV Records for SIP Cluster

  • SIP Proxies listen on port 5060
  • The SIP Cluster FQDN: spx-srv.example.com

For systems that use the TCP protocol for SIP signaling (usually SIP Server and GVP RM), SRV records would be as follows:

  • In data center 1, the geo-aware DNS zone version:
    • _sip._tcp.spx-srv.example.com. IN SRV 10 50 5060 dc1-spx1.example.com.
    • _sip._tcp.spx-srv.example.com. IN SRV 10 50 5060 dc1-spx2.example.com.
  • In data center 2, the geo-aware DNS zone version:
    • _sip._tcp.spx-srv.example.com. IN SRV 10 50 5060 dc2-sipx1.example.com.
    • _sip._tcp.spx-srv.example.com. IN SRV 10 50 5060 dc2-sipx2.example.com.

SIP registration in failover scenarios based on SRV records

The IP address of each SBC is defined by your telecom team. Then, for each IP address, you create an FQDN, which will be resolved into one IP address.

Create DNS SRV records for each SIP Proxy and, if you are using SBC, for each SBC in your environment. Configure priority and weight within the SRV records according to data center priorities in your environment.

SIP phones register with the primary data center. If the primary data center goes down, the phones re-register with the SBCs in the recovery data center using the SRV record of the SBC to which they re-register. The phones stay registered with the recovery data center until one of the following occurs:

  • they are restarted
  • the recovery data center is no longer operational
  • the new agent’s login session is initiated
Failover scenario (click to expand)

IP address and SRV records example

Primary data center 1 connectivity:

  • SBC IP address: 192.0.2.10
  • DNS record for the SBC: sip-dc1.example.com. IN A 192.0.2.10

Recovery data center 2 connectivity:

  • SBC IP address: 203.0.113.10
  • DNS record for the SBC: sip-dc2.example.com. IN A 203.0.113.10

FQDN for SRV-based name resolution: sip-dc.example.com

  • Best priority path (10) is via data center 1 SBC (port 6000) would be:
    _sip._udp.sip-dc.example.com. IN SRV 10 50 6000 sip-dc1.example.com.
  • Lower priority path (20) is via data center 2 SBC (port 6000) would be:
    _sip._udp.sip-dc.example.com. IN SRV 20 50 6000 sip-dc2.example.com.
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