Switching Concepts

What it is

A Layer 2 switch moves Ethernet frames between devices in the same network, sending each frame only where it needs to go.

Unlike a hub, which repeats every signal to every port, a switch learns which device is on which port.

The switch keeps this knowledge in its MAC address table, also called the CAM table. This topic covers how the table is built, how long entries last and how the switch decides what to do with each frame.

How it works

The switch learns from the source address and forwards by the destination address.

Learning

A switch builds its table by looking at the source MAC address of every frame that arrives. It records the source address together with the port it came in on and the VLAN.

Suppose PC-A, on port 1, sends a frame to PC-B. When the frame arrives, the switch notes that PC-A's MAC address is reachable through port 1.

This is called MAC learning. The switch never learns from the destination address, only from the source.

Forwarding decisions

Next, the switch looks at the destination MAC address and chooses what to do:

Frame flooding is how the switch copes when it does not yet know where a device is. Once the unknown device replies, the switch learns its address, and later frames are sent directly.

Flooding is therefore normal at first, and it settles quickly.

Here is the whole sequence for a first exchange:

From then on, frames between the two go only to the right port.

Each port forms its own collision domain, while all ports in a VLAN share one broadcast domain. That is why broadcasts reach every port in the VLAN but unicast traffic does not.

Aging

Entries do not last forever. If a device goes quiet, its entry is removed after an aging time.

The default aging time on a Cisco switch is 300 seconds (five minutes). Each time the switch sees a frame from that address, the timer is reset.

Aging keeps the table small and accurate when devices move to another port or are turned off.

You can change it:

mac address-table aging-time 600

Viewing and clearing the table

SW1# show mac address-table dynamic
          Mac Address Table
-------------------------------------------

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    0001.aaaa.aaaa    DYNAMIC     Fa0/1
   1    0001.bbbb.bbbb    DYNAMIC     Fa0/2
Total Mac Addresses for this criterion: 2

show mac address-table
show mac address-table dynamic
show mac address-table address 0050.56aa.bbcc
clear mac address-table dynamic

The output lists the VLAN, the MAC address, the type (dynamic or static) and the port.

A single port with many MAC addresses usually means another switch, a hub or a virtual host sits behind it. A MAC address that keeps moving between ports can signal a loop.

Switching methods

A switch can start forwarding a frame at different points:

• Store-and-forward: receives the whole frame, checks it for errors, then forwards. It is the safest method and the standard on modern switches.
• Cut-through: starts forwarding once it has read the destination address. It has lower delay but may forward damaged frames.
• Fragment-free: a compromise that waits for the first 64 bytes, enough to catch collision fragments, before forwarding. It appears in older Cisco material.