UMN:CLI
User Manual
V8102
308
9.1.1
Port-based VLAN
The simplest implicit mapping rule is known as port-based VLAN. A frame is assigned to a
VLAN based solely on the switch port on which the frame arrives. In the example depict-
ed in
, frames arriving on ports 1 through 4 are assigned to VLAN 1, frame from
ports 5 through 8 are assigned to VLAN 2, and frames from ports 9 through 12 are as-
signed to VLAN 3.
Stations within a given VLAN can freely communicate among themselves using either
unicast or multicast addressing. No communication is possible at the Data Link layer be-
tween stations connected to ports that are members of different VLANs. Communication
among devices in separate VLANs can be accomplished at higher layers of the architec-
ture, for example, by using a Network layer router with connections to two or more VLANs.
Multicast traffic, or traffic destined for an unknown unicast address arriving on any port,
will be flooded only to those ports that are part of the same VLAN. This provides the de-
sired traffic isolation and bandwidth preservation. The use of port-based VLANs effective-
ly partitions a single switch into multiple sub-switches, one for each VLAN.
Fig. 9.1
Port-based VLAN
The IEEE 802.1Q based ports on the switches support simultaneous tagged and un-
tagged traffic. An 802.1Q port is assigned a default port VLAN ID (PVID), and all un-
tagged traffic is assumed to belong to the port default PVID. Thus, the ports participating
in the VLANs accept packets bearing VLAN tags and transmit them to the port VLAN ID.
In a VLAN environment, a
frame’s association with a given VLAN is soft; the fact that a
given frame exists on some physical cable does not imply its membership in any particu-
lar VLAN. VLAN association is determined by a set of rules applied to the frames by
VLAN-aware stations and/or switches.
There are two methods for identifying the VLAN membership of a given frame:
•
Parse the frame and apply the membership rules (implicit tagging).
•
Provide an explicit VLAN identifier within the frame itself.
