Chapter 3: Bridging Configuration Guide60 SmartSwitch Router User Reference ManualFor example, if port 1 belongs to VLAN IPX_VLAN for IPX, VLAN IP_VLAN for IP andVLAN OTHER_VLAN for any other protocol, then an IP frame received by port 1 isclassified as belonging to VLAN IP_VLAN.Trunk ports (802.1Q) are usually used to connect one VLAN-aware switch to another.They carry traffic belonging to several VLANs. For example, suppose that SSR A and Bare both configured with VLANs V1 and V2.Then a frame arriving at a port on SSR A must be sent to SSR B, if the frame belongs toVLAN V1 or to VLAN V2. Thus the ports on SSR A and B which connect the two SSRstogether must belong to both VLAN V1 and VLAN V2. Also, when these ports receive aframe, they must be able to determine whether the frame belongs to V1 or to V2. This isaccomplished by “tagging” the frames, i.e., by prepending information to the frame inorder to identify the VLAN to which the frame belongs. In the SSR switching routers,trunk ports always transmit and receive tagged frames only. The format of the tag isspecified by the IEEE 802.1Q standard. The only exception to this is Spanning TreeProtocol frames, which are transmitted as untagged frames.Explicit and Implicit VLANsAs mentioned earlier, VLANs can either be created explicitly by the administrator (explicitVLANs) or are created implicitly by the SSR when L3 interfaces are created (implicitVLANs).Configuring SSR Bridging FunctionsConfiguring Address-based or Flow-based BridgingThe SSR ports perform address-based bridging by default but can be configured toperform flow-based bridging instead of address-based bridging, on a per-port basis. Aport cannot be configured to perform 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.For example, the following illustration shows an SSR with traffic being sent from port A toport B, port B to port A, port B to port C, and port A to port C.
