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Understanding Layer 3 Subinterfaces


A Layer 3 subinterface is a logical division of a physical interface that operates at the network level and therefore can receive and forward 802.1Q VLAN tags. You can use Layer 3 subinterfaces to route traffic among multiple VLANs along a single trunk line that connects a Juniper Networks EX Series Ethernet Switch to a Layer 2 switch. Only one physical connection is required between the switches. This topology is often called a router on a stick or a one-armed router when the Layer 3 device is a router.

To create Layer 3 subinterfaces on an EX Series switch, you enable VLAN tagging, partition the physical interface into logical partitions, and bind the VLAN ID to the logical interface.

You can partition one physical interface into up to 4094 different subinterfaces, one for each VLAN. We recommend that you use the VLAN ID as the subinterface number when you configure the subinterface. Juniper Networks Junos operating system (Junos OS) reserves VLAN IDs 0 and 4095.

VLAN tagging places the VLAN ID in the frame header, allowing each physical interface to handle multiple VLANs. When you configure multiple VLANs on an interface, you must also enable tagging on that interface. Junos OS on EX Series switches supports a subset of the 802.1Q standard for receiving and forwarding routed or bridged Ethernet frames with single VLAN tags and running Virtual Router Redundancy Protocol (VRRP) over 802.1Q-tagged interfaces. Double-tagging is not supported.