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Chapter 4: Basic Concepts and Management
Router
1a 2a
Ethernet switch
Ethernet switch
Ethernet switch
BACKBONE
Ethernet switch
Figure 4-9. Virtual LAN.
By applying VLAN technology, you can configure the system shown in Figure 4-9. You can partition the users into the different
logical networks that have their own broadcast domain. The traffic will not disturb these logical networks. The users 1x (x denotes
a – d) are members of VLAN 1. Any traffic within VLAN 1 does not flow to VLAN 2 and others. This helps configure the
network easily according to the criteria needed, for example, financial, accounting, R&D, and whatever you think is necessary. You
can also easily move a user to a different location or join a new user somewhere in the building to VLAN. Without VLAN, it is
very hard to do. Basically, VLAN can be used to move and change users, reduce broadcast traffic, and increase performance.
VLANs can greatly reduce the traffic congestion and increase total performance, because there are no longer too many users in
the same broadcast domain.
There are many types of VLAN. Most popular are port-based VLAN, tag-based VLAN, and protocol-based VLAN.
• Port-based VLAN: Some physical ports are configured as members of a VLAN. All stations attached on these ports can
communicate with each other.
• Tag-based VLAN identifies the membership by VLAN ID, no matter where the packet comes from. It is also referred to as
802.1Q VLAN.
• Protocol-based VLAN identifies the VLAN membership by layer 3 protocol types, for example IPX, Appletalk, IP, etc.
Other VLAN technologies not mentioned above are MAC-based VLAN, and IP-based VLAN.
Terminology
Tagged Frame:
A frame carrying a tag field with the source MAC address is four bytes long and contains VLAN protocol ID and tag control
information composed of user priority, Canonical Format Indicator (CFI), and optional VLAN identifier (VID). Normally, the
maximum length of a tagged frame is 1522 bytes.