How do external phones locate the PBX after failover

The answer depends on the location of your two PBX’s. If the PBX’s are located in the same data center (i.e. using the same external IP address), then no change is necessary as they will connect to the same IP address. If you need to modify your firewall/router internally to direct traffic to the active peer then see the answer to the question on locating the PBX for internal phones. On the other hand, if the PBX’s are located in different data centers (i.e. accessible using different public IP addresses) then your options are:

1. SRV records: Assuming your user agents (phone sets) support SRV records (which most do), then you should create SRV records for your two PBX’s. Most user agents will perform a DNS lookup for SRV records to find available PBX’s, and try them in order of priority until they successfully register with a PBX. For example, if you have PBX’s located in data centers dc1 and dc2, then create to DNS entries (in your public DNS server) as follows:

   
type=srv
name=_sip._udp.mydomain.com
priority= 10
weight=0
port=5060
hostname=dc1.mydomain.com

   
   and
   
type=srv
name=_sip._udp.mydomain.com
priority= 20
weight=0
port=5060
hostname=dc2.mydomain.com

2. DNS update: Use the pre/post asterisk start/stop event handlers to update a public DNS entry. Be sure to set the TTL value low enough that phones will lookup the new IP in a reasonable timeframe.
3. MPLS: If you use MPLS then you can simply move the label (to move IP between routers of your two DC’s). We don’t provide any further detail on this option (i.e. if you don’t understand how to do this with MPLS, then there’s too much to explain in one post)

Note: Using SRV records or DNS entries makes it easy for users with softphones to move on and off LAN and resume a PBX connection without manual intervention.