Ethernet Switching on EX Series Switches - Juniper Networks

<strong>Ethernet</strong> <strong>Switching</strong> on EX Series Switches
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setting a low MAC notification interval is that the network management system is better
synchronized with the switch.
MAC notification is disabled by default. When MAC notification is enabled, the default
MAC notification interval is 30 seconds.
•
Configuring MAC Notification (CLI Procedure) on page 163
• Configuring SNMP (J-Web Procedure)
Understanding MAC Address Aging
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Juniper Networks EX Series <strong>Ethernet</strong> Switches store MAC addresses in the <strong>Ethernet</strong>
switching table, also called the MAC table. When the aging time for a MAC address in the
table expires, the address is removed.
You can configure the MAC table aging time on all VLANs on the switch or on a per-VLAN
basis. You can also configure aging time to be unlimited, either on all VLANs or per-VLAN,
so that MAC addresses never age out of the table.
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 <strong>Ethernet</strong> 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 <strong>Ethernet</strong> 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—if traffic is received on
an interface that is associated with VLAN v-10 and there is no entry in the <strong>Ethernet</strong>
switching table for VLAN v-10 (the <strong>Ethernet</strong> 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 <strong>Ethernet</strong>
switching table. If a particular destination MAC address is not in the <strong>Ethernet</strong> 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 <strong>Ethernet</strong> switching table.
The switch uses a mechanism called aging to keep the <strong>Ethernet</strong> switching table current.
For each MAC address in the <strong>Ethernet</strong> 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 <strong>Ethernet</strong> switching table, it updates the
timestamp of that MAC address. A timer on the switch periodically checks the timestamp,
and if it is older than the value set for mac-table-aging-time, the switch removes the
node's MAC address from the <strong>Ethernet</strong> 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 <strong>Ethernet</strong> switching table MAC addresses that are no longer available.
You configure how long MAC addresses remain in the <strong>Ethernet</strong> switching table using the
mac-table-aging-time statement in either the edit ethernet-switching-options or the vlans
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