Cisco Cisco Expressway Maintenance Manual
■
ensure all endpoints are registered with an AOR (SIP) or H.323 ID in the form of a URI
■
configure appropriate DNS records, depending on the protocols and transport types you want to use
section.
Firewall traversal calls
To configure your system so that you can place and receive calls using URI dialing through a firewall, see the
URI Resolution Process Using DNS
When the Expressway attempts to locate a destination URI address using the DNS system, the general process is as
follows:
follows:
H.323
1.
The Expressway sends a query to its DNS server for an SRV record for the domain in the URI. (If more than one
DNS server has been configured on the Expressway, the query will be sent to all servers at the same time, and
all responses will be prioritized by the Expressway with only the most relevant SRV record being used.) If
available, this SRV record returns information (such as the FQDN and listening port) about either the device
itself or the authoritative H.323 gatekeeper for that domain.
DNS server has been configured on the Expressway, the query will be sent to all servers at the same time, and
all responses will be prioritized by the Expressway with only the most relevant SRV record being used.) If
available, this SRV record returns information (such as the FQDN and listening port) about either the device
itself or the authoritative H.323 gatekeeper for that domain.
—
If the domain part of the URI address was resolved successfully using an H.323 Location SRV record (that
is, for _ h323ls) then the Expressway will send an A/AAAA record query for each name record returned.
These will resolve to one or more IP addresses, and the Expressway then sends, in priority order, an LRQ for
the full URI to those IP addresses.
is, for _ h323ls) then the Expressway will send an A/AAAA record query for each name record returned.
These will resolve to one or more IP addresses, and the Expressway then sends, in priority order, an LRQ for
the full URI to those IP addresses.
—
If the domain part of the URI address was resolved using an H.323 Call Signaling SRV record (that is, for _
h323cs) then the Expressway will send an A/AAAA record query for each name record returned. These will
resolve to one or more IP addresses, and the Expressway 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,
h323cs) then the Expressway will send an A/AAAA record query for each name record returned. These will
resolve to one or more IP addresses, and the Expressway 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 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 Expressway 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.
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 Expressway 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 Expressway will not
query for A and AAAA records and instead will continue with the search, querying the remaining lower
priority zones.
query for A and AAAA records and instead will continue with the search, querying the remaining lower
priority zones.
SIP
. An example of how the Expressway
implements this process is as follows:
1.
The Expressway 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 Expressway will use a default list of
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 Expressway 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 Expressway sends SRV queries for each result returned from the NAPTR record lookup. A prioritized list
of A/AAAA records returned is built.
of A/AAAA records returned is built.
—
The Expressway sends an A/AAAA record query for each name record returned by the SRV record lookup.
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
domain. The tree is sub-divided by NAPTR record priority and then by SRV record priority. When the tree of
191
Cisco Expressway Administrator Guide
Dial Plan and Call Processing