A Catalyst 2940 switch may stop receiving traffic on the control plane while it is still able to transmit control traffic. (For example, Bridge Protocol Data Unit (BPDU), Cisco Discovery Protocol (CDP), and so forth). This affects functions that require traffic through the control plane, such as these:
Receiving spanning tree BPDUs
Receiving Cisco Discovery Protocol (CDP) information from neighbors
Receiving pings or ping replies to the switch itself
Receiving Address Resolution Protocol (ARP) requests or replies for the switch itself
VLAN Management Policy Server (VMPS) VLAN resolution
Normal data flow through the switch between the physical interfaces that does not need to be seen by the switch appears to be un-impacted.
This problem is documented in Cisco bug ID CSCsb84292.
For a workaround, soft or hard reset the switch.
In order to resolve this issue, upgrade the Cisco IOS Software on the switch to the latest maintenance release, which you can download from LAN Switching Software.