Spectralink 84-Series Series Wireless Telephones Administration Guide
1725-86984-000_P.docx
September 2016
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parameters, the phone will not attempt NAPTR or SRV record lookups. The only lookup will be
an A record. If no port is specified via configuration parameters, and none is found through DNS
mechanisms, port 5060 will be used by default for TCP or UDP transport types, and 5061 will be
used as the port number for TLS.See
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 handset will go through the following steps to connect the call:
1
The handset will try to call the working server.
2
If the working server does not respond correctly to the INVITE, the handset 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 handset 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.
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 handset 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 handset call proceeds using the working server.
To prevent this issue, long TTLs should be used. Spectralink recommends
deploying an on-site DNS server as part of the redundancy solution.