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Voice quality deteriorates in 7912, 7902 and 7905 IP phones whenever there is heavy traffic on the switch
7912 is susceptible to broadcast and multicast traffic arriving at the switch port connected to the phone. If this traffic approaches 2 Mbps, the voice quality deteriorates.
The same kind of problem is experienced even when unicast traffic on that port exceeds 10 Mbps.
The Cisco IP Phone 7912G is not recommended if a customer is deploying co-located PC applications requiring more than 2 Mbps of multicast and broadcast traffic. This is also true of applications requiring one-way unicast traffic of greater than 10 Mbps. In these scenarios, Cisco IP Phones 7940G, 7960G or 7970G are more suitable.
To resolve this issue, perform these steps:
Take care to minimize the likelihood of network flooding of unicast streams. Flooding to discover a device is valid and would typically cease within a couple of packets as a response was received.
Ensure that continuous flooding never occurs; hence, the path between servers and users needs to be well defined.
To avoid accidental conditions of flooding, failure or reroute scenarios also need to be considered.
To further minimize the extent of an accidental flooding event, a VLAN per switch may be necessary.
Reconfigure the LAN to ensure that the multicast and broadcast flooding are controlled.