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Layer 2 Bridging Overview

On MX-series routers only, you can configure one or more bridge domains to perform Layer 2 bridging. A bridge domain is a set of logical ports that share the same flooding or broadcast characteristics. Like a virtual LAN (VLAN), a bridge domain spans one or more ports of multiple devices. Thus, MX-series routers can function as Layer 2 switches, each with multiple bridging, or broadcast, domains that participate in the same Layer 2 network. You can also configure Layer 3 routing support for a bridge domain. Integrated routing and bridging (IRB) provides support for Layer 2 bridging and Layer 3 IP routing on the same interface. IRB enables you to route packets to another routed interface or to another bridge domain that has a Layer 3 protocol configured.

You can also group one or more bridge domains within a single instance, or virtual switch. The MX-series routers also support multiple virtual switches, each of which operates independently of other virtual switches on the router. Virtual switches isolate a LAN segment with its STP instance and separate its VLAN ID space. Thus, each virtual switch can participate in a different Layer 2 network.

Beginning with JUNOS Release 9.2, bridge domains provide support for a Layer 2 trunk port. A Layer 2 trunk interface enables you to configure a single logical interface to represent multiple VLANs on a physical interface. You can configure a set of bridge domains and VLAN identifiers that are automatically associated with one or more Layer 2 trunk interfaces. Packets received on a trunk interface are forwarded within a bridge domain that has the same VLAN identifier. A Layer 2 trunk interface also supports IRB within a bridge domain. In addition, you can configure Layer 2 learning and forwarding properties that apply to the entire set of bridge domains.

Beginning with JUNOS Release 9.3, you can configure VPLS ports in a virtual switch instead of a dedicated routing instance of type vpls so that the logical interfaces of the Layer 2 bridge domains in the virtual switch can handle VPLS routing instance traffic. Packets received on a Layer 2 trunk interface are forwarded within a bridge domain that has the same VLAN identifier.

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