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The IA can also be used purely as a router, that is, each physical port of the IA is a separate
routing interface. Packets received at any interface are routed and not bridged. In this
case, no VLAN configuration is required. Note that VLANs are still created implicitly by
the IA as a result of creating Layer-3 interfaces for IP. However, these implicit VLANs do
not need to be created or configured manually. The implicit VLANs created by the IA are
subnet-based VLANs.
Most commonly, an IA is used as a combined switch and router. For example, it may be
connected to two subnets: S1 and S2. Ports 1 through 8 belong to S1, and ports 9 through
16 belong to S2. The required behavior of the IA 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 IA switching routers use VLANs to achieve this behavior. This means that a Layer-3
subnet (that is, an IP 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 IA as a combined switch and router, the administrator must create VLANs
whenever multiple ports of the IA are to belong to a particular VLAN/subnet. Then the
VLAN must be bound to a Layer-3 (IP) interface so that the IA knows which VLAN maps
to which IP subnet.
Ports, VLANs, and Layer-3 Interfaces
The term port refers to a physical connector on the IA, such as an Ethernet port. Each port
must belong to at least one VLAN. When the IA 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 IA has the concept of logical interfaces rather than physical
interfaces. A Layer-3 interface is a logical entity created by the administrator. It can
contain more than one physical port. When a Layer-3 interface contains exactly one
physical port, it is equivalent to an interface on a traditional router. When a Layer-3
interface contains several ports, it is equivalent to an interface of a traditional router that 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 IA 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 type: IP or bridged protocols. The IA can automatically
determine whether or not a received frame is an IP frame. 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
Internet Appliance User Reference Manual
Chapter 2: Bridging Configuration Guide
39
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