Access Ports And Trunk Ports (802.1Q Support); Explicit And Implicit Vlans - Cabletron Systems SmartSwitch 8-slot User's Reference Manual

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.
For example, if port 1 belongs to VLAN IPX_VLAN for IPX, VLAN IP_VLAN for IP
and VLAN OTHER_VLAN for any other protocol, then an IP frame received by port 1
is classified 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
B are 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
to VLAN V1 or to VLAN V2. Thus the ports on SSR A and B which connect the two
SSRs together must belong to both VLAN V1 and VLAN V2. Also, when these ports
receive a frame, they must be able to determine whether the frame belongs to V1 or to
V2. This is accomplished by "tagging" the frames, i.e., by prepending information to
the frame in order 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 is specified by the IEEE 802.1Q standard. The only exception to this
is Spanning Tree Protocol frames, which are transmitted as untagged frames.

Explicit and Implicit VLANs

As mentioned earlier, VLANs can either be created explicitly by the administrator
(explicit VLANs) or are created implicitly by the SSR when L3 interfaces are created
(implicit VLANs).
SSR User Reference Manual
Chapter 2: Bridging Configuration Guide
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