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ip phone on a different subnet then call manager

Unanswered Question
Mar 16th, 2014
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Hi All

Is it possible to have an ip phone on a different subnet than call manager? What settings need to be done to achieve this and get the ip phone correctly registered?

Thanks

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Overall Rating: 5 (2 ratings)
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Karsten Iwen Sun, 03/16/2014 - 05:56
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It's not only possible, it's the common way to use it. The phone needs to learn it's IP settings and the TFTP-Server via DHCPs, and the config-file on the TFTP-Server includes the info which call-manager to use. The rest is pure IP-routing.

Jonathan Schulenberg Sun, 03/16/2014 - 15:07
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Adding a salient point to Karsten's answer (+5): CUCM has a non-modifiable ARP table size of 1024 entries. As such, it should never be in a subnet mask larger than /24 (assuming IPv4-only). Exceeding this limit will result in performance problems since CUCM will literally have to ARP for every packet it wants to send to devices that don't fit in the ARP table. Even if all of your servers, phones, etc. could fit inside a single /24 I would consider it best practice to isolate the servers into a dedicated subnet from your user access layer devices.

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