Cisco TelePresence Administrator's Manual page 185

If the domain part of the URI address was resolved using an H.323 Call Signaling SRV record (that is,
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for _ h323cs) then the VCS will send an A/AAAA record query for each name record returned. These will
resolve to one or more IP addresses, and the VCS then routes the call, in priority order to the IP
addresses returned in those records. (An exception to this is where the original dial string has a port
specified - for example, user@example.com:1719 - in which case the address returned is queried
via an LRQ for the full URI address.)
2. If a relevant SRV record cannot be located:
If the Include address record setting for the DNS zone being queried is set to On, the system will fall
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back to looking for an A or AAAA record for the domain in the URI. If such a record is found, the call will
be routed to that IP address and the search will terminate. Note that if the A and AAAA records that are
found at this domain are for systems other than those that support SIP or H.323, the VCS will still
forward the call to this zone, and the call will therefore fail. For this reason, you are recommended to use
the default setting of Off.
If the Include address record setting for the DNS zone being queried is set to Off, the VCS will not
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query for A and AAAA records and instead will continue with the search, querying the remaining lower
priority zones.
SIP
The VCS supports the SIP resolution process as outlined in
implements this process is as follows:
1. The VCS sends a NAPTR query for the domain in the URI. If available, the result set of this query
describes a prioritized list of SRV records and transport protocols that should be used to contact that
domain.
If no NAPTR records are present in DNS for this domain name then the VCS will use a default list of _
sips._tcp.<domain>, _sip._tcp.<domain> and _sip._udp.<domain> for that domain as if
they had been returned from the NAPTR query.
The VCS sends SRV queries for each result returned from the NAPTR record lookup. A prioritized list
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of A/AAAA records returned is built.
The VCS sends an A/AAAA record query for each name record returned by the SRV record lookup.
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The above steps will result in a tree of IP addresses, port and transport protocols to be used to contact the
target domain. The tree is sub-divided by NAPTR record priority and then by SRV record priority. When
the tree of locations is used, the searching process will stop on the first location to return a response that
indicates that the target destination has been contacted.
2. If the search process does not return a relevant SRV record:
If the Include address record setting for the DNS zone being queried is set to On, the system will fall
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back to looking for an A or AAAA record for the domain in the URI. If such a record is found, the call will
be routed to that IP address and the search will terminate. Note that if the A and AAAA records that are
found at this domain are for systems other than those that support SIP or H.323, the VCS will still
forward the call to this zone, and the call will therefore fail. For this reason, you are recommended to use
the default setting of Off.
If the Include address record setting for the DNS zone being queried is set to Off, the VCS will not
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query for A and AAAA records and instead will continue with the search, querying the remaining lower
priority zones.
Cisco VCS Administrator Guide (X7.1)
Dial plan and call processing
RFC 3263. An example of how the VCS
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