You are viewing the Ethernet switching table and notice that a previous entry is no longer listed. What is a valid reason for an entry to be removed from the table?

A.
The learning limit is exceeded.

B.
The filtering threshold is exceeded.

C.
The MAC aging timer expired.

D.
The ARP aging timer expired.

Explanation:
MAC Address in the Switchs Ethernet Switching Table Is Not Updated After a MAC Address Move
Problem
Sometimes a MAC address entry in the switchs Ethernet switching table is not updated after the device with that MAC address has been moved from one interface to another on the switch. Typically, the switch does not wait for a MAC address expiration when a MAC move operation occurs. As soon as the switch detects the MAC address on the new interface, it immediately updates the table. Many network devices send a gratuitous ARP packet when switching an IP address from one device to another. The switch updates its ARP cache table after receipt of such gratuitous ARP messages, and then it also updates its Ethernet switching table. However, sometimes silent devices, such as SYSLOG servers or SNMP Trap receivers that receive UDP traffic but do not return acknowledgement (ACK ) messages to the traffic source, do not send gratuitous ARP packets when a device moves. If such a move occurs when the system administrator is not available to explicitly clear the affected interfaces by issuing the clear ethernet-switching table command, the entry for the moved device in the Ethernet switching table is not updated.

Solution
Set up the switch to handle unattended MAC address switchovers.

* Reduce the system-wide ARP aging timer. (By default, the ARP aging timer is set at 20 minutes. In Junos OS Release 9.4 and later, the range of the ARP aging timer is from 1 through 240 minutes.)

[edit system arp]
user@switch# set aging-timer 3

* Set the MAC aging timer to the same value as the ARP timer. (By default, the MAC aging timer is set to 300 seconds. The range is 15 to 1,000,000 seconds.)

[edit vlans]
user@switch# set vlans sales mac-table-aging-time 180

The ARP entry and the MAC address entry for the moved device expire within the times specified by the aging timer values. After the entries expire, the switch sends a new ARP message to the IP address of the device. The device responds to the ARP, thereby refreshing the entries in the switchs ARP cache table and Ethernet switching table