Understanding MAC Address Assignment on a Virtual Chassis

In a Virtual Chassis, multiple switches—each with its own set of interfaces with unique MAC addresses—are connected together to form one chassis that can be managed as a single switch. The MAC address assigned to each network-facing interface on the switch changes when the switch joins a Virtual Chassis. 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 Virtual Chassis. For additional information about how a network uses MAC addresses to forward and receive traffic, see Understanding Bridging and VLANs on Switches.

When a Virtual Chassis is formed, the MAC address of the switch in the primary role becomes the system MAC base address. The Virtual Chassis assigns the system MAC base address as the MAC address for all Layer 3 interfaces within the Virtual Chassis. The Virtual Chassis also assigns the system MAC base address to the virtual management Ethernet (VME) interface and to all of the virtual LANs (VLANs) in the Virtual Chassis.

The system MAC base address does not change in the event of a switchover if the switch that was originally configured in the primary role remains a member of the Virtual Chassis. If the switch that was originally configured in the primary role is removed from the Virtual Chassis, the MAC address of the current member switch in the primary role is assigned as the system MAC base address after the MAC persistence timer interval has expired. You can configure the MAC persistence timer interval.

The Virtual Chassis assigns a root ID to the spanning tree bridge based on the system MAC based address. If the system base MAC address changes, the spanning tree bridge continues to use the same root ID. You must run the restart layer2-control command for the spanning tree bridge to use a new root ID based on the new system base MAC address.

For Layer 2 and aggregated Ethernet interfaces, the Virtual Chassis assigns a unique MAC address that is derived from the member switch MAC address to each interface. 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 function properly.

Note:

Unique MAC address assignment for Layer 2 and aggregated Ethernet interfaces in a Virtual Chassis was introduced in Junos OS Release 11.3. The same MAC address could be assigned to interfaces on different member switches in the same Virtual Chassis prior to this release.

If you reconfigure a Layer 2 interface into a Layer 3 interface, or the reverse, within a Virtual Chassis, the MAC address of that interface changes accordingly.

MAC addresses are assigned to interfaces in a Virtual Chassis automatically—no user configuration is possible or required. You can view the MAC addresses that are assigned to the interfaces by using the show interfaces command.