Yealink Yealink SIP-T28P, Administrator'S Manual

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Configuring Advanced Features
185
Parameters are explained in the following table:
Parameter Description
Priority
Specify preferential treatment for the specific host entry. Lower
priority is more preferred.
Weight
When priorities are equal, weight is used to differentiate the
preference. The preference is from highest to lowest. Keep the
same to load balance.
Port Identify the port number to be used.
Target Identify the actual host for an A query.
SRV query returns two records. The two SRV records point to different hosts and have
the same priority 0. The weight of the second record is higher than the first one, so the
second record will be picked first. The two records also contain a port “5060”, the IP
phone uses this port. If the Target is not a numeric IP address, the IP phone performs an
A query. So in this case, the IP phone uses “server1.yealink.pbx.com" and
“server2.yealink.pbx.com" for the A query.
A (Host IP Address)
The IP phone performs an A query for the IP address of each target host name. Example
of A records:
Server1.yealink.pbx.com IN A 192.168.1.13
Server2.yealink.pbx.com IN A 192.168.1.14
The IP phone picks the IP address “192.168.1.14” first.
Outgoing Call When the Working Server Connection Fails
When a user initiates a call, the phone will go through the following steps to connect the
call:
1. Sends the INVITE request to the primary server.
2. If the primary server does not respond correctly to the INVITE, then tries to make the
call using the secondary server.
3. If the secondary server is also unavailable, the IP phone will try the fallback server
until it either succeeds in making a call or exhausts all servers 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 as described below:

If TCP is used, then the signaling fails if the connection 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.