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Virtual LANs

Virtual LANs. In an organization, does it matter which users are on which LAN?. LAN administrators prefer to group users to reflect organizational structure, rather than physical layout.

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Virtual LANs

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  1. Virtual LANs In an organization, does it matter which users are on which LAN? • LAN administrators prefer to group users to reflect organizational structure, rather than physical layout. • Departments want to isolate their activities from Ethernet stations acting in promiscuous mode in other departments. • Load distribution affects LAN organization • Limiting broadcasting to a single LAN saves bandwidth But, in an organization people’s responsibilities change, and the organizational structure changes even when their physical location does not change.

  2. 16 machines organized into 3 VLANs A S1 S2

  3. A network administrator will determine the number of VLANs needed for the organization and typically designate them by color. Using color to name the VLANs makes it easy to associate the physical layout with the members of each VLAN A S1 S2

  4. VLANs are based on specially designed VLAN aware switches – Switches capable of recognizing the IEEE 802.1Q protocol. A S1 S2

  5. If machine A broadcasts a frame it need only go to other “yellow” hosts A S1 S2

  6. How are VLANs Implemented? The 802 committee charged with implementing VLANs did the “unthinkable”!! They changed the Ethernet header!!! Does this make all the several hundred million old Ethernet cards obsolete? If not, who generates the new fields? What happens to frames that are already the maximum size? Only the bridges or switches need to be VLAN aware (not the user machines) The first VLAN aware switch receiving a legacy frame builds a new tagged frame based upon knowledge of sender’s VLAN (using either the port, MAC address, or IP address). A switch that needs to deliver a tagged frame to a legacy machine has to reformat the frame in the legacy format before delivering it. Gigabit Ethernet cards will all be 802.1Q compliant

  7. VLAN aware switch Tagged frame Legacy frame VLAN aware PC Legacy PC VLAN aware end domain Legacy end domain VLAN aware core domain

  8. VLAN protocol ID (0x8100) CFI Pri VLAN Id. The IEEE 802.1Q Standard DestinationAddress Check-sum 802.3 frame Source Address Length Data Pad Destination Address Check-sum Source Address Tag 802.1Q frame 0x8100 Length Data Pad 3 bits 1 bit 12 bits • The 802.1Q Committee added a pair of 2-Byte fields. • The first field replaces the Length field in the legacy header. It contains a value greater than an allowed MAX_LENGTH = 1500 • The second field contains a 12-bit VLAN Identifier field and a 3-bit priority field (for improving quality of service for real-time apps. • The maximum frame length is extended to a total of 1522 bytes (data+header)

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