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Cisco IP Phones and Multicast

This may be completely off-base, but I was wondering if anyone had experienced conflicts with multicast being disabled in the small office environment using Cisco phones. To clarify- I have a number of users on 504G's and the network is otherwise optimized, from my understanding, for use with their VoIP provider, but we have occasional lock-up issues. The phones will lose registration and freeze, requiring a hard reset. In the past this issue has had some partial resolution by modifying the upgrade enable rule, among other things, but I've been seeing more and more cases that are resolved by either enabling multicast on the router, or disabling multicast on the devices.

My question is- is this reasonable? My understanding of the relationship between multicast and SIP is that any interaction is specific and relatively benign, but anecdotal evidence shows that there may be more to it than I realize. Does anyone have any experience with this?

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