Cabletron Systems SmartSwitch Router, Руководство пользователя

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SmartSwitch Router User Reference Manual 59
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.