3b,
but with a 0 in the VLAN ID. A 0 VLAN ID means, ???I don??™t know???; the bridge assigns
the frame to the PVID. This enables a station to indicate a priority in the frame??™s VLAN
tag. The bridge attached to such a station may be configured either to transmit frames
VLAN-tagged with the PVID or to transmit untagged frames. Bridges cannot transmit
frames with a 0 VID.
On a LAN that connects two (or more) bridges, the bridges typically include the
VLAN tag, as in Figure 13.3b and 13.3c. This allows frames on as many as 4094
VLANs to be carried on a single LAN. Typically, the only VLAN-aware stations are
routers. (To a bridge, a router is a station, just like a PC or a printer is a station.) In
an enterprise network, each VLAN typically corresponds to an IP subnetwork, and
the router is required to route packets from one subnetwork (VLAN) to another subnetwork
(VLAN).
VLAN and MAC Address Pruning
If a bridge has not learned a particular MAC address, then when a frame destined for
that address is received, the bridge floods the frame on all ports on the spanning tree.
Similarly, by default, any broadcast or multicast frame is flooded to all ports on the
spanning tree.
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