Layer 2 Features for a Bridging Environment

Configuring Layer 2 features on an MX Series router can vary from the very simple (aggregated Ethernet trunk interfaces, spanning trees), to the more complex (inner and outer VLAN tags, broadcast domains), to the very complicated (integrated bridging and routing, Layer 2 filtering). This chapter offers a fairly complex configuration for Layer 2 processing in a bridged environment.

• Interfaces and virtual LAN (VLAN) tags—Layer 2 interfaces are usually various type of Ethernet links with VLAN tags used to connect to customer devices or other bridges or routers.
• Bridge domains—Bridge domains limit the scope of media access control (MAC) learning (and thereby the size of the MAC table) and also determine where the device should propagate frames sent to broadcast, unknown unicast, and multicast (BUM) MAC addresses.
• Spanning Tree Protocols (xSTP, where the “x” represents the STP type)—Bridges function by associating a MAC address with an interface, similar to the way a router associates an IP network address with a next-hop interface. Just as routing protocols use packets to detect and prevent routing loops, bridges use xSTP frames to detect and prevent bridging loops. (Layer 2 loops are more devastating to a network because of the broadcast nature of Ethernet LANs.)
• Integrated bridging and routing (IRB)—Support for both Layer 2 bridging and Layer 3 routing on the same interface. Frames are bridged if they are not sent to the router's MAC address. Frames sent to the router's MAC address are routed to other interfaces configured for Layer 3 routing.