Polycom CX5500, Administrator'S Manual

Page: 144 / 386

Polycom CX5500 Unified Conference Station Administrator’s Guide
1.1.0
Polycom, Inc. 144
Caution: No DNS Resolution Will Cause Failover
Failure to resolve a DNS name is treated as signaling failure that will cause a failover.
Behavior When the Primary Server Connection Fails
For Outgoing Calls (INVITE Fallback)
When the user initiates a call, the phone will go through the following steps to connect the call:
1
The phone will try to call the working server.
2
If the working server does not respond correctly to the INVITE, the phone will try and make a call
using the next server in the list (even if there is no current registration with these servers). This
could be the case if the Internet connection has gone down, but the registration to the working
server has not yet expired.
3
If the second server is also unavailable, the phone will try all possible servers (even those not
currently registered) until it either succeeds in making a call or exhausts the list at which point the
call will fail.
At the start of a call, server availability is determined by SIP signaling failure. SIP signaling failure
depends on the SIP protocol being used:
●
If TCP is used, then the signaling fails if the connection fails or the Send fails.

If UDP is used, then the signaling fails if ICMP is detected or if the signal times out. If the
signaling has been attempted through all servers in the list and this is the last server, then the
signaling fails after the complete UDP timeout defined in RFC 3261. If it is not the last server in
the list, the maximum number of retries using the configurable retry timeout is used. For more
information, see
<server/> and <reg/> .
Caution: Use Long TTLs to Avoid DNS Timeout Delays
If DNS is used to resolve the address for Servers, the DNS server is unavailable, and the TTL for
the DNS records has expired, the phone will attempt to contact the DNS server to resolve the
address of all servers in its list
before initiating a call. These attempts will timeout, but the timeout
mechanism can cause long delays (for example, two minutes) before the phone call proceeds using
the working server. To prevent this issue, long TTLs should be used. Polycom recommends
deploying an on-site DNS server as part of the redundancy solution.
Phone Configuration
The phones at the customer site are configured as follows:
●
Server 1 (the primary server) will be configured with the address of the service provider call server.
The IP address of the server(s) will be provided by the DNS server, for example:
reg.1.server.1.address=voipserver.serviceprovider.com .