Example 2, VLANs Across Multiple Switches
Examples
4-3
4.1.2
Frame Handling
This section describes the operations of the switch when two frames are received. The first frame
is a broadcast sent by station R1.
1.
Station R1 transmits the broadcast frame. The switch receives this frame on Port 1. As the frame
is received, the switch classifies it. The frame is untagged, so the switch classifies it as belonging
to the VLAN that Port 1 is assigned to, the Red VLAN.
2.
At the same time, the switch adds the source MAC address of the frame and the VLAN
associated with port 1 to its Source Address Table in FID 2. In this fashion it learns that station
R1 is located out Port 1.
3.
Once the frame is classified, its destination MAC address is examined. The switch discovers that
the frame is a broadcast, and treats it as it would any other unknown destination MAC address.
The switch forwards the frame out all ports in the Red VLAN’s Forwarding List except for the
one that received the frame. In this case, the frame is sent to Ports 2 and 3.
The second frame is a unicast, where station R2 responds to station R1’s broadcast.
4.
Station R2, having received and recognized the broadcast from R1, transmits a unicast frame as
a response. The switch receives this frame on Port 2. The switch classifies this new untagged
frame as belonging to the Red VLAN.
5.
The switch adds the source MAC address and VLAN for station R2 to its Source Address Table
in FID 2, and checks the Source Address Table for the destination MAC address given in the
frame. The switch finds the MAC address and VLAN in this table, and recognizes that the MAC
address and VLAN match for R1 is located out Port 1.
6.
The switch examines its VLAN configuration information and determines that the frame for Red
VLAN is allowed to be forwarded out Port 1 and that it must be sent in an untagged format.
7.
The switch forwards the frame out Port 1. Any other unicast transmissions between stations R1
and R2 will be handled identically.
4.2
EXAMPLE 2, VLANs ACROSS MULTIPLE SWITCHES
This second example investigates the steps that must be taken to set up VLANs across multiple
802.1Q VLAN switches. This includes the configuration and operation of 1Q Trunks between
802.1Q VLAN switches.
As shown in
Figure 4-3
, two companies, “Redco” and “Blue Industries”, share floors 2 and 4 in a
building where the network infrastructure is supplied by the building owner. The objective is to
completely isolate the network traffic of the two companies by limiting the user’s traffic through
the ports of two switches, thus maintaining security and shielding the network traffic from each
company. This example will show the use and configuration of a 1Q Trunk connection and the
creation of VLANs across multiple switches.
