MAC Table Aging

To learn MAC addresses, the switch reads all packets that it detects on the LAN or on the local VLAN, looking for MAC addresses of sending nodes. It places these addresses into its Ethernet switching table, along with two other pieces of information—the interface on which the traffic was received and the time when the address was learned.

When the switch receives traffic on an interface, it searches the Ethernet switching table for the MAC address of the destination. If the MAC address is not found, the traffic is flooded out all of the other interfaces associated with the VLAN. For example, if traffic is received on an interface that is associated with VLAN v-10 and there is no entry in the Ethernet switching table for VLAN v-10 (the Ethernet switching table is organized by VLAN), then the traffic is flooded to all access and trunk interfaces that are members of VLAN v-10.

Flooding allows the switch to learn about destinations that are not yet in its Ethernet switching table. If a particular destination MAC address is not in the Ethernet switching table, the switch floods the traffic to all interfaces except the interface on which it was received. When the destination node receives the flooded traffic, it sends an acknowledgment packet back to the switch, allowing the switch to learn the MAC address of the node and to add the address to its Ethernet switching table.

The switch uses a mechanism called aging to keep the Ethernet switching table current. For each MAC address in the Ethernet switching table, the switch records a timestamp of when the information about the network node was learned. Each time the switch detects traffic from a MAC address that is in its Ethernet switching table, it updates the timestamp of that MAC address. A timer on the switch periodically checks the timestamp, and if the MAC address of a node is older than the value set, the switch removes that MAC address from the Ethernet switching table. This aging process ensures that the switch tracks only active MAC addresses on the network and that it is able to flush out from the Ethernet switching table MAC addresses that are no longer available.

You configure how long MAC addresses remain in the Ethernet switching table by:

• (On switches that run Junos OS with support for the ELS configuration style) Using the global-mac-table-aging-time statement in the [edit protocols l2-learning] hierarchy.

• (On switches that run Junos OS that does not support ELS) Using the mac-table-aging-time statement in either the [edit ethernet-switching-options] or the [edit vlans] hierarchy, depending on whether you want to configure it for the entire switch or only for specific VLANs.

For example, in a topology with EX switches that run Junos OS that does not support ELS, if you have a printer VLAN, you might choose to configure the aging time for that VLAN to be considerably longer than for other VLANs so that MAC addresses of printers on this VLAN age out less frequently. Because the MAC addresses remain in the table, even if a printer has been idle for some time before traffic arrives for it, the switch still finds the MAC address and does not need to flood the traffic to all other interfaces.

Similarly, in a data center environment where the list of servers connected to the switch is fairly stable, you might choose to increase MAC address aging time, or even set it to unlimited, to increase the efficiency of the utilization of network bandwidth by reducing flooding.