Switching Tables (Part-1)

Layer-2 Forwarding Overview

Layer-2 devices build hardware address tables, which at a minimum contain the following:

• Hardware addresses for hosts (such as Ethernet MAC addresses)
• The port each hardware address is associated with

Using this information, Layer-2 devices will make intelligent forwarding decisions based on the frame (or data-link) header. A frame can then be forwarded out only the appropriate destination port, instead of all ports. Layer-2 forwarding was originally referred to as bridging. Bridging is a largely deprecated term (mostly for marketing purposes), and Layer-2 forwarding is now commonly referred to as switching. 

Switching Queues

Layer-2 switches utilize queues to store incoming and outgoing frames. Consider the following diagram:

1. The switch receives a frame on Port 1, from HostA destined for HostB.

2. The frame is placed in Port 1’s ingress queue.

3. The switch performs a lookup on the destination hardware address - HostB in this example.

4. The switch determines that the appropriate destination port for HostB is Port 2.

5. The frame is placed in Port 2’s egress queue.

If the switch had no knowledge of HostB’s hardware address, the frame would be placed in the egress queue of all ports except for the originating port, and thus flooded to the entire network. A port can contain multiple ingress or egress queues. This allows critical traffic to be prioritized over less important traffic. 

Ref: 

http://www.routeralley.com/guides.html