4.7 DAT A LINK LAYER SWITCHING

• In fact, we can do this when the connections are made with devices called bridges.
• In contrast, routers examine the addresses in packets and route based on them, so they only work with the protocols that they were designed to handle.
• The algorithm used by the bridges is backward learning. As mentioned above, the bridges operate in promiscuous mode, so they see every frame sent on any of their ports.
• This design reduces the latency of passing through the bridge, as well as the number of frames that the bridge must be able to buffer. It is referred to as cut-through switching or wormhole routing and is usually handled in hardware.
• c. The result of this broadcast storm is that (1) the entire LAN capacity is occupied by these frames, and (2) all the machines on all the interconnected LANs are crippled just processing and discarding all the frames being broadcast.
• In response to customer requests for more flexibility, network vendors began working on a way to rewire buildings entirely in software. The resulting concept is called a VLAN (Virtual LAN). It has been standardized by the IEEE 802 committee and is now widely deployed in many org anizations. Let us now take a look at it.
• The IEEE 802 committee had this problem thrown into its lap in 1995. After much discussion, it did the unthinkable and changed the Ethernet header. The new format was published in IEEE standard 802.1Q, issued in 1998.
• The main one is the VLAN identifier, occupying the low-order 12 bits
• The last field, CFI (Canonical format indicator), should have been called the CEI (Corporate ego indicator). It was originally intended to indicate the order of the bits in the MAC addresses (little-endian versus big-endian), but that use got lost in other controversies.