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An ATA device in H.323 mode loses network connectivity and does not respond after the gratuitous ARP, which is sent 30 seconds after a call is completed

Core Issue

ATA with H.323 signaling image stops responding after sending out gratuitous ARP (Analog Telephone Adaptor (ATA) ARP itself) 30 seconds after a call is finished. Upgrading the firmware to the latest version or replacing the hardware does not help. Power resetting the ATA can clear the problem temporarily, but it comes back later.

This issue happens when the port VLAN and the voice VLAN are configured as the same. When the switchport is configured with voice VLAN and access VLAN/native VLAN (and those VLANs are configured as the same VLAN number), the ATA learns and enters the voice VLAN through Cisco Discovery Protocol (CDP) packets sent by the switch. The ATA expects the traffic coming with the voice VLAN tag. Since the access VLAN/native VLAN is configured to same VLAN number as the voice VLAN, the switch sends out the packets as the normal Ethernet frames without the VLAN ID.

The ATA drops these normal Ethernet frames after sending the gratuitous ARP, but the ATA can respond the untagged traffic before it sends out the gratuitous ARP, even though it has entered the voice VLAN.

Resolution

To resolve the issue, perform one of these procedures:

  • Change the access VLAN to a VLAN number other than the voice VLAN.
  • Or, disable the voice VLAN.

If you have the voice VLAN configured, changing the access VLAN to another one allows the switch to send out the packets with the voice VLAN tag. The    ATA comes back without the power reset.

On the other hand, when you disable the voice VLAN, the ATA does not receive the VLAN information from CDP, and it knows that it is not in a VLAN environment. It sends or receives the normal Ethernet frame without the VLAN tag. The user does not experience problems when removing the voice VLAN.

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