couchsurfingcook.com Networks EX Series Ethernet Switches store MAC addressesin the Ethernet switching table, also called the MAC table. When the aging time for a MAC address in the table expires, theaddress is removed.

If your switch runs couchsurfingcook.com Networks Junos operating system(Junos OS) for EX Series switches with support for the Enhanced Layer2 Software (ELS) configuration style, you can configure the MAC tableaging time on all VLANs on the switch. If your switch runs Junos OSthat does not support ELS, you can configure the MAC table aging timeon all VLANs on the switch or on specified VLANs, as well as configureaging time to be unlimited, either on all VLANs or on specified VLANs,so that MAC addresses never age out of the table.

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To learn MAC addresses, the switch reads all packets that itdetects on the LAN or on the local VLAN, looking for MAC addressesof sending nodes. It places these addresses into its Ethernet switchingtable, along with two other pieces of information—the interfaceon which the traffic was received and the time when the address waslearned.

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

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

The switch uses a mechanism called aging to keep the Ethernetswitching table current. For each MAC address in the Ethernet switchingtable, the switch records a timestamp of when the information aboutthe network node was learned. Each time the switch detects trafficfrom a MAC address that is in its Ethernet switching table, it updatesthe timestamp of that MAC address. A timer on the switch periodicallychecks the timestamp, and if the MAC address of a node is older thanthe value set, the switch removes that MAC address from the Ethernetswitching table. This aging process ensures that the switch tracksonly active MAC addresses on the network and that it is able to flushout from the Ethernet switching table MAC addresses that are no longeravailable.

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

(On switches that run Junos OS with support for the ELSconfiguration style) Using the global-mac-table-aging-time statement in the hierarchy.

(On switches that run Junos OS that does not support ELS)Using the mac-table-aging-time statement in either the or the hierarchy, depending on whether you want to configure it for theentire switch or only for specific VLANs.

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

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


The following example uses Junos OS for EX Series switcheswith support for the Enhanced Layer 2 Software (ELS) configurationstyle.

The Ethernet switching table (or MAC table) aging process ensuresthat the EX Series switch tracks only active MAC addresses on thenetwork and is able to flush out MAC addresses that are no longerused.

You can configure the MAC table aging time, the maximum timethat an entry can remain in the Ethernet Switching table before it ages out, on all VLANs on the switch. This settingcan influence efficiency of network resource use by affecting theamount of traffic that is flooded to all interfaces because when trafficis received for MAC addresses no longer in the Ethernet switchingtable, the switch floods the traffic to all interfaces.

user
switch# set protocols l2-learning global-mac-table-aging-time secondsThe following example uses Junos OS for EX Series switchesthat do not support the Enhanced Layer 2 Software (ELS) configurationstyle.

The Ethernet switching table (or MAC table) aging process ensuresthat the EX Series switch tracks only active MAC addresses on thenetwork and is able to flush out MAC addresses that are no longerused.

You can configure the MAC table aging time, the maximum timethat an entry can remain in the Ethernet Switching table before it“ages out,” either on all VLANs on the switch or on particularVLANs. This setting can influence efficiency of network resource useby affecting the amount of traffic that is flooded to all interfacesbecause when traffic is received for MAC addresses no longer in theEthernet switching table, the switch floods the traffic to all interfaces.

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To configure the MAC table aging time on all VLANs onthe switch:

user
switch# set ethernet-switching-options mac-table-aging-time secondsTo configure the MAC table aging time on a VLAN:

user
Note: You can set the MAC table aging time to unlimited. Ifyou specify the value as unlimited, entries are never removedfrom the table. Generally, use this setting only if the switch orthe VLAN has a fairly static number of end devices; otherwise thetable will eventually fill up. You can use this setting to minimizetraffic loss and flooding that might occur when traffic arrives forMAC addresses that have been removed from the table.