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22
Fast Ethernet
The following sections up to page 32 provides information about Fast Ethernet and
various network scenarios.
Fast Ethernet Ports
The Fast Ethernet ports are configured much like Token-Ring ports, and generally
appear in all tables where Token-Ring ports appear.
Fast Ethernet Virtual Ports and VLAN Tagging
For general information on VLAN support, see page 35.
Unlike Token-Ring ports, the Fast Ethernet ports can function as trunk ports. This
means that they can carry traffic belonging to all of the possible 63 VLANs over
the same physical cable connection.
Frames from different VLANs are distinguished by means of standard IEEE
802.1Q frame tags. The tag is inserted in the Ethernet frame after the frame header
before transmission on the Fast Ethernet port, and subsequently recognized and
removed when the frame is received.
To identify the VLAN to which the frame belongs, the frame tag makes use of a
VLAN identifier that uniquely identifies the BRF. It also contains a 3 bit priority
field (described in section "Virtual Port Configuration" on page 41.)
Each Fast Ethernet port can be assigned to several CRFs, but there can be only one
per BRF/VLAN. A virtual port is created for each CRF to which the Fast Ethernet
port is assigned. All virtual ports but one must be tagged (i.e., a single virtual port
can be left untagged).
Note that a Fast Ethernet port with a single untagged virtual port is functionally
equivalent to an ordinary Token-Ring port. This is also the default configuration,
with the virtual port in ethcrf-default.
Virtual Port Restriction
A Fast Ethernet port can have only one virtual port in each VLAN. This follows
from the nature of VLAN tagging. Otherwise it would not be possible to distinguish
frames from two virtual ports in the same VLAN, since they would have the same
VLAN identifier.
Ring Number Restriction
All CRFs in which a Fast Ethernet port has virtual ports must have the same ring
number. Thus, the ring number is a "per-port" parameter and not a "per-virtual-
port" parameter. If this were not the case, the meaning of the RIF would be
CrossFire 8730 Switch Reference Guide, DOC-7047 v. 1.1
Switch Overview
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