Chapter 2: Bridging Configuration Guide34 SmartSwitch Router User Reference ManualNote: WAN interfaces on the SSR do not currently support Spanning Tree operations.However, future implementations of WAN for the SSR family of routers willsupport Spanning Tree.Bridging Modes (Flow-Based and Address-Based)The SSR provides the following types of wire-speed bridging:Address-based bridging - The SSR performs this type of bridging by looking up thedestination address in an L2 lookup table on the line card that receives the bridge packetfrom the network. The L2 lookup table indicates the exit port(s) for the bridged packet. Ifthe packet is addressed to the SSR's own MAC address, the packet is routed rather thanbridged.Flow-based bridging - The SSR performs this type of bridging by looking up an entry inthe L2 lookup table containing both the source and destination addresses of the receivedpacket in order to determine how the packet is to be handled.The SSR ports perform address-based bridging by default but can be configured toperform flow-based bridging instead, on a per-port basis. A port cannot be configured toperform both types of bridging at the same time.The SSR performance is equivalent when performing flow-based bridging or address-based bridging. However, address-based bridging is more efficient because it requiresfewer table entries while flow-based bridging provides tighter management and controlover bridged traffic.VLAN OverviewVirtual LANs (VLANs) are a means of dividing a physical network into several logical(virtual) LANs. The division can be done on the basis of various criteria, giving rise todifferent types of VLANs. For example, the simplest type of VLAN is the port-basedVLAN. Port-based VLANs divide a network into a number of VLANs by assigning aVLAN to each port of a switching device. Then, any traffic received on a given port of aswitch belongs to the VLAN associated with that port.VLANs are primarily used for broadcast containment. A layer-2 (L2) broadcast frame isnormally transmitted all over a bridged network. By dividing the network into VLANs,the range of a broadcast is limited, i.e., the broadcast frame is transmitted only to theVLAN to which it belongs. This reduces the broadcast traffic on a network by anappreciable factor.The type of VLAN depends upon one criterion: how a received frame is classified asbelonging to a particular VLAN. VLANs can be categorized into the following types:• Port based
