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Catalyst Switch Operation

November 24, 2012


Cisco has manufactured two switch series, which are Catalyst 1900 and 2900 switches that are available in two editions. These two editions include Standard Edition and Enterprise Editions. The 1900 series of switches supports nigh-speed server connections using UTP of fiber-optic cables. The 2900 family of switches provides faster data transfer rate compared to the 1900 switch.

Working with LAN Switches

LAN switches forward Ethernet frames. In edition, the LAN switches only have to decide the time to forward the Ethernet frames. Switch logic relates to the source and destination MAC addresses inside the Ethernet frame headers of the frames sent through the LAN. Switch logic is also dependent on the type of MAC address used. A brief review of Ethernet addresses can help shed some light on how LAN switches work.

The IEEE defines three general categories of MAC addresses on Ethernet, which include:

  • Unicast addresses: – Identifies a single LAN interface card. Most cards use the MAC address that is burned into the card. Fames sent to Unicast addresses are destined for a single device.
  • Broadcast addresses: – Implies that all devices on the LAN should receive and process a frame sent to the broadcast address. The broadcast address is the most often used IEEE group MAC address, which has a value of FFFF.FFFF.FFFF in hexadecimal notation.
  • Multicast addresses: – frames sent to multicast addresses are meant for all devices that receive the frame. There can be some devices that receive the frame, none, or some number in between. Some applications need to communicate with multiple other devices. By sending one frame, all the devices that care about receiving the data sent by that application can process the data, and the rest can ignore it.

With these reminders of the three types of Ethernet MAC addresses, you can appreciate the logic used by a LAN switch. A switch listens for frames that enter all its interfaces. After receiving a frame, a switch decides whether to forward a frame. Switches perform three tasks, such as:

  • Learning: – Examines the source MAC address of each frame the bridge receives. This enables the switch to make good forwarding choices in the future.
  • Forwarding or filtering: – The switch decides when to forward a frame or when to filter (not forward) it based on the destination MAC address. The switch looks at the previously learned MAC addresses in an address table to decide where to forward the frames.
  • Loop prevention: – Creates a loop-free environment with other bridges by using Spanning Tree Protocol (STP). Having physically redundant links, helps LAN availability, and STP prevents the switch logic from letting frames loop around the network indefinitely, congesting the LAN.

Forward-Versus-Filter Decision

Switches reduce network overhead by forwarding traffic from one segment to another only when required. To decide whether to forward a frame, the switch uses a dynamically built table called a bridge table or MAC address table. The switch examines the address table to decide whether it should forward a frame.

For example

Bertha first sends a frame to Peter and then one to John. The switch decides to filter the frame that Bertha sends to Peter. In addition, Berta sends a frame with a destination MAC address of 0200.2222.2222, which is Peter’s MAC address. The switch overhears the frame, because it is attached to Hub. The switch then decides what common sense tells you from looking at the figure. It should not forward the frame because Peter attached to Hub as well, has received the frame. Hubs repeat the signal out all ports, for all frames, so that the switch receives everything sent by either Peter or Bertha. However, the switch does not realize that it does not have to forward the frame. The switch decides to filter the frame because it received the frame on port E0, and it knows that Peter’s MAC is also located out E0.

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