Register Handsets in DNS - Necessary ?

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Oct 14th, 2009
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I am running DHCP for phone handsets on the router. The DNS server that the handsets reference is a W2003DC. This works fine (service URL's etc.)


However, currently the handsets are not registering in DNS themselves (creating A records).


Is this actually necessary ? It has not caused me any operational issues so far -I am not sure whether there is a reason to configure this (or not).


I am assuming that the router, when issuing the DHCP lease to the handset could update the DNS running on the 2003DC ?

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paolo bevilacqua Wed, 10/14/2009 - 02:03
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I never seen this necessary.


One note, it-s better practice to let cisco gear - router or CM to give DCHP to phones, putting them on a separate VLAN. This way, you are independent from IT servers.



ajenks Wed, 10/14/2009 - 04:24
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Thanks for the quick reply. I didn't know whether there were any circumstances where this may be necessary, but it sounds like there isn't.


On your second point, I am using the router for DHCP (as stated) I am only using a server outside of CISCO for DNS lookups. Do you mean that this is not advisable ?

Rob Huffman Wed, 10/14/2009 - 05:40
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Hi Alistair,


Here are the SRND - DNS "best practices";


Deploying Unified CM with DNS


There are some situations in which configuring and using DNS might be unavoidable. For example, if Network Address Translation (NAT) is required for communications between the IP phones and Unified CM in the IP Communications network, DNS is required to ensure proper mapping of NAT translated addresses to network host devices. Likewise, some IP telephony disaster recovery network configurations rely on DNS to ensure proper failover of the network during failure scenarios by mapping hostnames to secondary backup site IP addresses.


If either of these two situations exists and DNS must be configured, you must deploy DNS servers in a geographically redundant fashion so that a single DNS server failure will not prevent network communications between IP telephony devices. By providing DNS server redundancy in the event of a single DNS server failure, you ensure that devices relying on DNS to communicate on the network can still receive hostname-to-IP-address mappings from a backup or secondary DNS server.




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Note Hostname resolution within the cluster via either the local HOSTS file or DNS queries is performed only at subsystem initialization (that is, when a server is booted up). As a result, in order for a server within the cluster to resolve a DNS name that has been changed in either the HOSTS file or the DNS server, the Cisco CallManager Service must be restarted on all servers within the cluster.



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Unified CM can use DNS to:


•Provide simplified system management


•Resolve fully qualified domain names to IP addresses for trunk destinations


•Resolve fully qualified domain names to IP addresses for SIP route patterns based on domain name


•Resolve service (SRV) records to host names and then to IP addresses for SIP trunk destinations


When DNS is used, Cisco recommends defining each Unified CM cluster as a member of a valid sub-domain within the larger organizational DNS domain, defining the DNS domain on each Cisco MCS server, and defining the primary and secondary DNS server addresses on each MCS server.


From the SRND Guide;


http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/srnd/7x/netstruc.html#wp1043809



Hope this helps!

Rob

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