Cisco Cisco TelePresence Video Communication Server Expressway
Appendix 11 – Cluster name and DNS SRV records
Appendix 11 – Cluster name and DNS SRV
records
records
Using DNS SRV to convert a domain to an IP address has a number of benefits:
The structure of the lookup includes service type and protocol as well as the domain, so that a
common domain can be used to reference multiple different services which are hosted on
different machines (e.g. html, sip, h.323).
common domain can be used to reference multiple different services which are hosted on
different machines (e.g. html, sip, h.323).
The DNS SRV response includes priority and weighting values which allow the specification of
primary, secondary, tertiary etc groups of servers, and within each priority group, the weighting
defines the proportion of accesses that should use each server.
primary, secondary, tertiary etc groups of servers, and within each priority group, the weighting
defines the proportion of accesses that should use each server.
Because the DNS SRV response contains details about priorities and weights of multiple servers,
the receiving device can use a single lookup to search for an in-service server (where some
servers are in-accessible) without the need to repeatedly query the DNS server (this is in contrast
to using round robin DNS which does require repeated lookups into the DNS server if initial
servers are found to be in-accessible).
the receiving device can use a single lookup to search for an in-service server (where some
servers are in-accessible) without the need to repeatedly query the DNS server (this is in contrast
to using round robin DNS which does require repeated lookups into the DNS server if initial
servers are found to be in-accessible).
The generic format of a DNS SRV query is:
_service._protocol.<fully.qualified.domain>
The format of DNS SRV queries for sip (RFC 3263) and h.323 used by Cisco VCS are:
_sips._tcp.<fully.qualified.domain>
_sip._tcp.<fully.qualified.domain>
_sip._udp.<fully.qualified.domain>
- not recommended for video calls, only for audio-only
calls
calls
_h323ls._udp.<fully.qualified.domain> - for udp RAS messaging, e.g LRQ
_h323cs._tcp.<fully.qualified.domain> - for H.323 call signaling
The format of DNS SRV queries for sip (RFC 3263) and h.323 typically used by an endpoint are:
_sips._tcp.<fully.qualified.domain>
_sip._tcp.<fully.qualified.domain>
_sip._udp.<fully.qualified.domain>
- not recommended for video calls, only for audio-only
calls
calls
_h323ls._udp.<fully.qualified.domain> - for udp RAS messaging, e.g LRQ
_h323cs._tcp.<fully.qualified.domain> - for H.323 call signaling
_h323rs._udp.<fully.qualified.domain> - for H.323 registrations
The DNS SRV response is a set of records in the format:
_ service. _ protocol.<fully.qualified.domain>. TTL Class SRV Priority Weight Port Target
Where Target is an A-record defining the destination
Note: UDP is not a good transport medium for video – SIP messaging for video systems is too large
to be carried on a packet based (rather than stream based) transport. UDP is often used for audio
only devices.
to be carried on a packet based (rather than stream based) transport. UDP is often used for audio
only devices.
Further details on DNS SRV can be found in the Cisco VCS Administrator Guide and RFC 2782.
The Cisco VCS Cluster name (configured on the VCS configuration > Clustering page) should be
the <fully.qualified.domain> specified in the DNS SRV records that point to the Cisco VCS cluster.
the <fully.qualified.domain> specified in the DNS SRV records that point to the Cisco VCS cluster.
Cisco VCS Deployment Guide: Cluster creation and maintenance (Cisco VCS X5)
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