SmartSwitch Router User Reference Manual 25Chapter 3: Bridging Configuration Guidethe SSR as a result of creating L3 interfaces for IP and/or IPX. However, these implicitVLANs do not need to be created or configured manually. The implicit VLANs created bythe SSR are subnet-based VLANs.Most commonly, an SSR is used as a combined switch and router. For example, it may beconnected to two subnets S1 and S2. Ports 1-8 belong to S1 and ports 9-16 belong to S2.The required behavior of the SSR is that intra-subnet frames be bridged and inter-subnetpackets be routed. In other words, traffic between two workstations that belong to thesame subnet should be bridged, and traffic between two workstations that belong todifferent subnets should be routed.The SSR switching routers use VLANs to achieve this behavior. This means that a L3subnet (i.e., an IP or IPX subnet) is mapped to a VLAN. A given subnet maps to exactlyone and only one VLAN. With this definition, the terms VLAN and subnet are almostinterchangeable.To configure an SSR as a combined switch and router, the administrator must createVLANs whenever multiple ports of the SSR are to belong to a particular VLAN/subnet.Then the VLAN must be bound to an L3 (IP/IPX) interface so that the SSR knows whichVLAN maps to which IP/IPX subnet.Ports, VLANs, and L3 InterfacesThe term port refers to a physical connector on the SSR, such as an ethernet port. Eachport must belong to at least one VLAN. When the SSR is unconfigured, each port belongsto a VLAN called the “default VLAN”. By creating VLANs and adding ports to thecreated VLANs, the ports are moved from the default VLAN to the newly created VLANs.Unlike traditional routers, the SSR has the concept of logical interfaces rather thanphysical interfaces. An L3 interface is a logical entity created by the administrator. It cancontain more than one physical port. When an L3 interface contains exactly one physicalport, it is equivalent to an interface on a traditional router. When an L3 interface containsseveral ports, it is equivalent to an interface of a traditional router which is connected to alayer-2 device such as a switch or bridge.Access Ports and Trunk Ports (802.1Q support)The ports of an SSR can be classified into two types, based on VLAN functionality: accessports and trunk ports. By default, a port is an access port. An access port can belong to atmost one VLAN of the following types: IP, IPX or bridged protocols. The SSR canautomatically determine whether a received frame is an IP frame, an IPX frame or neither.Based on this, it selects a VLAN for the frame. Frames transmitted out of an access portare untagged, meaning that they contain no special information about the VLAN to whichthey belong. Untagged frames are classified as belonging to a particular VLAN based onthe protocol of the frame and the VLAN configured on the receiving port for that protocol.
