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Example 4, Isolating Network Traffic According to Protocol
VLAN Operation and Network Applications 12-37
The switch checks the Filtering Database for the MAC address of User B. User B’s MAC
address is located, and Port 2 is identified as the location of User B. The frame is checked for
eligibility and frame format for Port 2. Since Port 2 is a 1Q Trunk port, it is eligible to transmit
frames for all VLANs. The frame is tagged and transmitted out port 2.
7. Switch 4 receives the frame on its 1Q Trunk port, Port 4, and examines the frame’s Tag. The
frame maintains its Default VLAN classification. The switch also refers to its Source Address
Table, FID 1, to see if it can locate an entry for User B. User B is found to be located on Port 1.
The switch also updates its Source Address Table, FID 1, with the port and MAC address
combination for the Mail Server.
The switch examines the Filtering Database and locates the MAC address entry for User B and
Port 1. The frame is then checked for eligibility and frame format for Port 1. As Port 1 is
considered eligible to transmit to the Default VLAN, the frame is transmitted out Port 1 without
a VLAN Tag.
8. Bridge 1 receives the frame and recognizes User B’s MAC address. The frame is forwarded to
the correct port and the bridge’s Source Address Table is updated with an entry for the Mail
Server’s MAC address. User B receives the Mail Server’s response. Any further unicast traffic
between the Mail Server and User B will be handled in the same fashion by the switches in the
network.
12.15 EXAMPLE 4, ISOLATING NETWORK TRAFFIC ACCORDING TO
PROTOCOL
This example illustrates how to contain AppleTalk protocol traffic to prevent unwanted multicast
frames from slowing down the whole network and yet be able to send and receive frames
associated with other protocols.
In this example, illustrated in Figure 12-18, the Publications Department is relocating from another
site to the third floor. This network will consist of six computers and a printer using several
protocols including the AppleTalk protocol. A characteristic of the AppleTalk protocol is to send
all frames as multicast frames. These multicast frames will be isolated to a VLAN (Gray VLAN)
to prevent them from slowing down the other networks.
A second VLAN (Yellow VLAN) will be established to handle traffic of other protocols. The
Publications Department users will have access to the mail server on the first floor along with the
Red, Blue, and Green VLANs.