Understanding CISCO Switches

Switches Operates at Layer 2

Operating Modes of a Cisco Switch’s Port

1. Access – ports configured with this mode is meant to connect to end devices
   
   ex. Switch(config-if)#switchport mode access
2. Trunk – ports configured with this mode is meant to connect to other switches to propagate                vlan-tagged information
    ex. Switch(config-if)#switchport mode trunk
3. Dynamic Desirable – ports configured with this mode will actively try to  from trunk link with other switches
    ex. Switch(config-if)#switchport mode dynamic desirable
4. Dynamic Auto – ports configured with this mode will passively form trunk link with other switches. (The port on the other end should operate either in Dynamic Desirable or Trunk in              order to form trunk link)
    ex. Switch(config-if)#switchport mode dynamic auto

Switch Operation: Learning

• When a frame enters a switch, the switch learns its source mac address and the port it entered to.

Switch Operation: Filter Or Forward Decision

• When the switches doesn’t know about the destination mac address, if floods the frame.

 Summary of a Switch’s Logic

1.  Whenever a frame enters a switch, its source mac address and entered port is learned.
2.  The switch decides whether to forward or filter the frame by looking for the destination mac address  from the mac address table. 

•   When it is found, the port where it entered and the port where that mac address was learned from are compared.

               a.  If they are the same, the frame will be dropped.
               b. If they are not the same, the frame will be forwarded to the destination port.

• When it is not found, the frame is flooded. (This is why switches would always flood layer 2 broadcast messages, because ff:ff:ff:ff:ff:ff would never end up in MAC table.)

 Virtual LANs: Ultimate Solution to Broadcast Messages

• Segmentation
• Flexibility
• Security

 VLAN = Broadcast Domain = Logical Network (Subnet)