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SSIM-H2-02 Fast Ethernet - Translational Switch SmartStack Interface Module Installation and User Guide Configuration
TS Virtual Ports and VLAN Tagging
Unlike ordinary Token Ring ports, Translational Switch (TS) ports can function as
trunk ports, which means they can carry traffic belonging to all of the 63 VLANs it
is possible to configure in the switch over the same physical cable connection.
Frames from different VLANs are distinguished by means of standard IEEE
802.1Q frame tags. A frame tag consists of a 2-byte Ethernet Frame Type of value
0x8100 (as described by standard P802.3ac) followed by a 2-byte Tag Control
Information (TCI) field. It is inserted into the Ethernet frame after the frame header
before transmission on the TS, and subsequently recognized and removed when the
frame is received.
To identify the VLAN to which the frame belongs, the TCI field makes use of a 12-
bit VLAN identifier that uniquely identifies the BRF. It also contains a 3 bit priority
field (described in “FC Modification” on page31).
Each TS port can be assigned to several CRFs, but there can be only one TS port
per BRF/VLAN. A virtual port is created for each CRF to which the TS port is
assigned. All virtual ports but one must be tagged (that is a single virtual port can
be left untagged).
Note that a TS 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 trcrf-default.
Virtual Port Restriction
A TS port can have only one virtual port in each VLAN. 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 TS port has virtual ports must have the same ring number. 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 ambiguous and would depend
on the VLAN to which the frame was sent. It would thus be incomprehensible to
equipment that does not interpret VLAN tags.