Cabletron Systems SmartSwitch manual Ports, VLANs, and L3 Interfaces

Chapter 3: Bridging Configuration Guide

the SSR as a result of creating L3 interfaces for IP and/or IPX. However, these implicit VLANs do not need to be created or configured manually. The implicit VLANs created by the SSR are subnet-based VLANs.

Most commonly, an SSR is used as a combined switch and router. For example, it may be connected 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-subnet packets be routed. In other words, traffic between two workstations that belong to the same subnet should be bridged, and traffic between two workstations that belong to different subnets should be routed.

The SSR switching routers use VLANs to achieve this behavior. This means that a L3 subnet (i.e., an IP or IPX subnet) is mapped to a VLAN. A given subnet maps to exactly one and only one VLAN. With this definition, the terms VLAN and subnet are almost interchangeable.

To configure an SSR as a combined switch and router, the administrator must create VLANs 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 which VLAN maps to which IP/IPX subnet.

Ports, VLANs, and L3 Interfaces

The term port refers to a physical connector on the SSR, such as an ethernet port. Each port must belong to at least one VLAN. When the SSR is unconfigured, each port belongs to a VLAN called the “default VLAN”. By creating VLANs and adding ports to the created 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 than physical interfaces. An L3 interface is a logical entity created by the administrator. It can contain more than one physical port. When an L3 interface contains exactly one physical port, it is equivalent to an interface on a traditional router. When an L3 interface contains several ports, it is equivalent to an interface of a traditional router which is connected to a layer-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: access ports and trunk ports. By default, a port is an access port. An access port can belong to at most one VLAN of the following types: IP, IPX or bridged protocols. The SSR can automatically 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 port are untagged, meaning that they contain no special information about the VLAN to which they belong. Untagged frames are classified as belonging to a particular VLAN based on the protocol of the frame and the VLAN configured on the receiving port for that protocol.

SmartSwitch Router User Reference Manual 59