A VLAN bridge associates every received data frame with a VLAN, either implicitly
by configuring the port on which it was received or explicitly by a VLAN tag, as shown
in Figure 13.3. (Figure 13.3 calls this a C-VLAN tag, for reasons that will become apparent
in the next section.) Most stations are not VLAN-aware. They transmit and receive
untagged Ethernet frames, as shown in Figure 13.3a. Each port of a VLAN bridge
is configured with a port VLAN ID (PVID). Untagged frames received by the bridge
are assigned to the PVID. The bridge may be configured to discard tagged frames or
to discard frames not tagged with the PVID, in order to enforce its configuration upon
the stations on the LAN. Similarly, all frames transmitted by the bridge to non-VLANaware
stations are untagged; otherwise, a non-VLAN-aware station would be unable
to decode the frame.
Figure 13.3 C-tag or Q-tag frame format for 802.1Q VLAN bridges
destination_address source_address data FRC
destination_address source_address
0x8100 Priority
C
F
I
VLAN ID (VID)
6 6 2 46-1498 4
Type/
length data FRC
2 46-1498 4 6 6
16 3 1 12
C-VLAN tag
(a) Untagged
frame
(b) VLAN-
tagged frame
(c) Format of
C-VLAN tag
bits
bytes
bytes
Type/
length
4
382 Chapter 13
If a station knows about VLAN tags, but does not know what particular VID its LAN
has been assigned by the bridge, then it may transmit a tagged frame as in Figure 13.
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