This also doesn’t affect how many TFTP servers you can have in your cluster just where you turn the service on if your also running Jabber and what high availability. This doesn’t affect how a phone retrieves its information though because a phone as we mentioned earlier is using DHCP option 150. This will ensure that Jabber will never be stuck looking for a third server in the same datacenter and will need to look else where, hopefully in another datacenter where you have a third TFTP running. So what is the best practice? Never have more than two CUCM TFTP servers per cluster enabled per data center for Jabber. Jabber will check to see if the first three TFTP servers are available only and if all three of those TFTP servers happen to be in a data center that is off line the client will not be able to pull down the jabber-config.xml file. Initially this may seem like a good idea to turn TFTP on every CUCM server and you have redundancy but this isn't how Jabber reads the file. See slide below for a more graphical look at the process. So if you have turned on TFTP on ever server in your cluster that’s what it will return. Depending on how many TFTP servers you have enabled in your environment your service profile file will list all of them automatically. Once this is located a service profile is returned which has a number of things, but in our case we are most interested in is the TFTP server addresses. UDS is enabled by default on all your CUCM servers by the way. In most cases Jabber uses a SRV records to locate a CUCM server running the User Data Service (UDS). The underlying PC takes care of addressing and Jabber has to find alternative methods to get to its TFTP server to download its configuration file. Jabber on the other hand works a little differently as it does not use DHCP directly. The phone looks up a TFTP server address based on DHCP option 150 and downloads its configuration and files from the published address. CUCM TFTP server selection for Cisco phones has been a well known architecture for quite some time.
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