Supported Platforms
Understanding MAC Address Assignment in an EX Series Switch
This topic describes MAC address assignment for interfaces on standalone Juniper Networks EX Series Ethernet Switches. For information regarding MAC address assignments in a Virtual Chassis, see Understanding MAC Address Assignment on a Virtual Chassis.
MAC addresses are used to identify network devices at Layer 2. Because all Layer 2 traffic decisions are based on an interface’s MAC address, understanding MAC address assignment is important to understanding how network traffic is forwarded and received by the switch. For additional information on how a network uses MAC addresses to forward and receive traffic, see Understanding Bridging and VLANs on EX Series Switches.
A MAC address comprises six groups of two hexadecimal digits, with each group separated from the next group by a colon—for instance, aa:bb:cc:dd:ee:00. The first five groups of hexadecimal digits are derived from the switch and are the same for all interfaces on the switch.
The assignment of a unique MAC address to each network interface helps ensure that functions that require MAC address differentiation—such as redundant trunk groups (RTGs), Link Aggregation Control Protocol (LACP), and general monitoring functions—can properly function.
On switches that use line cards, this MAC addressing scheme differentiates the Layer 2 interfaces on different line cards in the switch.
For EX Series switches, the first five groups of hexadecimal digits are determined when the switch is manufactured. The switch then assigns a unique MAC address to each interface by assigning a unique identifier as the last group of hexadecimal digits. The assignment depends on how the interface is configured. The switch uses a different pattern to distinguish between an interface that is configured as any of a routed VLAN interface (RVI), a virtual management Ethernet (VME) interface, or an aggregated Ethernet interface or is not configured as any of an RVI, a VME, or as an aggregated Ethernet interface.
For aggregated Ethernet interfaces, the MAC address assignment remains constant regardless of whether the configuration of the interface is Layer 2 or Layer 3.
 | Note: In Junos OS Release 11.3 and later releases through Release 12.1, the MAC address assignment for aggregated Ethernet interfaces changes if the interface is changed from Layer 2 to Layer 3 or the reverse. Starting with Junos Release 12.2, the MAC address assignment for aggregated Ethernet interfaces remains constant regardless of whether the interface is Layer 2 or Layer 3. |
| Note: Prior to Junos OS Release 11.3, MAC addresses for Layer 2 interfaces could be shared between interfaces and RVIs on different line cards in the same switch. However, if you upgrade from Junos OS Release 11.2 or earlier to Junos OS Release 11.3 or later on a switch that supports line cards, the MAC addresses of these interfaces will change. |
MAC addresses are assigned to interfaces automatically—no user configuration is possible or required. You can view MAC addresses assigned to interfaces using the show interfaces command.
