Cisco Cisco Expressway
Appendix 4: Cluster name and DNS SRV records
Using DNS SRV to convert a domain to an IP address has a number of benefits:
n
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).
domain can be used to reference multiple different services which are hosted on different machines (e.g.
html, sip, h.323).
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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.
secondary, tertiary etc groups of servers, and within each priority group, the weighting defines the
proportion of accesses that should use each server.
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As 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 inaccessible)
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 inaccessible.)
device can use a single lookup to search for an in-service server (where some servers are inaccessible)
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 inaccessible.)
The generic format of a DNS SRV query is:
n
_service._protocol.<fully.qualified.domain>
The format of DNS SRV queries for sip (RFC 3263) and h.323 used by Expressway are:
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_sips._tcp.<fully.qualified.domain>
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_sip._tcp.<fully.qualified.domain>
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_sip._udp.<fully.qualified.domain> - not recommended for video calls, only use for audio-only calls
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_h323ls._udp.<fully.qualified.domain> - for UDP location (RAS) signaling, such as LRQ
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_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:
n
_sips._tcp.<fully.qualified.domain>
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_sip._tcp.<fully.qualified.domain>
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_sip._udp.<fully.qualified.domain> - not recommended for video calls, only use for audio-only calls
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_h323ls._udp.<fully.qualified.domain> - for UDP location (RAS) signaling, such as LRQ
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_h323cs._tcp.<fully.qualified.domain> - for H.323 call signaling
The DNS SRV response is a set of records in the format:
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_service._ protocol.<fully.qualified.domain>. TTL Class SRV Priority Weight Port Target
where Target is an A-record defining the destination.
where Target is an A-record defining the destination.
Note that UDP is not a recommended transport medium for video signaling; SIP messaging for video system
is too large to be reliably carried on datagram-based (rather than stream-based) transports.
is too large to be reliably carried on datagram-based (rather than stream-based) transports.
Further details on DNS SRV can be found in Expressway Administrator Guide and RFC 2782.
The Expressway Cluster name (configured on the
System > Clustering
page) should be the
<fully.qualified.domain> specified in the DNS SRV records that point to the Expressway cluster.
Example
DNS SRV records for 2 peers of an Expressway-E cluster for company.com
where:
Cisco Expressway Cluster Creation and Maintenance Deployment Guide (X8.6)
Page 34 of 40
Appendix 4: Cluster name and DNS SRV records