Troubleshooting Ethernet Switching
Problem
Description
Sometimes a MAC address entry in the switch’s 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.
Sometimes silent devices, such as syslog servers or SNMP trap
receivers that receive UDP traffic but do not return acknowledgment
(ACK) messages to the traffic source, fail to 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. 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 60 to 1,000,000 seconds.)
[edit protocols l2-learning] user@switch# set global-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 message, thereby refreshing the entries in the switch’s ARP cache table and Ethernet switching table.