Switching & Bridging

Switching & Bridging

 The Switch

 Steps of Switch Functioning

 Switching Methods

 The Bridge

The Switch

A switch is a layer 2 network device that forwards frames using MAC addresses in the header of frames. It is used to improve network performance by:-

• segmenting the network and creating separate collision domains.

• reducing competition for bandwidth.

In a switch frame forwarding is handled by specialized hardware called "Application Specific Integrated Circuit" (ASIC). ASIC technology allows a silicon chip to be programmed to perform specific functions much faster than that of a chip programmed by software.

Steps of Switch Functioning

 Learning 
When switch starts, the MAC address table has no entry. When a node transmits data on its wire the MAC address of the node is learned by Switch Port connected to that node. In this way all the MAC addresses are learned by respective ports and these entries remain in the cache for a specific time. If during this specific time no new frame arrives from a node MAC address entry for that node is dropped from cache.

 Forwarding & Filtering

When a MAC address for a port is learnt, packets addressed to that MAC address are forwarded only to the port associated with it, using one of the Switching Methods.

 Loop Avoidance

Switches and Bridges use Spanning Tree Protocol (STP), specified by IEEE 802.1d, to prevent loops.

Switching Methods

Store & Forward:  In this method complete frame is received by the switch. CRC, source address and destination address are checked. This method has following features:-

• Highest latency (delay in forwarding of frame) but may vary depending upon the length of frame.

• Highest error checking.

• Lowest frame forwarding speed.

Catalyst 500 switch uses this method.

Cut Through:  In this method forwarding starts as soon as destination address of the frame is received in header. Also known as WIRE SPEED. This method has following features:-

• Lowest latency.

• Lowest error checking.

• Highest frame forwarding speed.

Fragment Free (Modified Cut Through):  In this method forwarding starts as soon as first 64 bytes of the frame are received as fragmentation occurs usually in first 64 bytes. This method has following features:-

• Latency approx 60m Sec.

• Sufficient error checking.

• Moderate frame forwarding speed.

The Bridge

It is a layer 2 device used to connect different network types or networks of the same type. Packets having destination address on the same network segment are dropped. Bridges use "Store and Forward" method to inspect the whole packet. 

Advantages: Using a bridge to segment network can provide:-

• Reliability.

• Manageability.

• Scalability.

Disadvantages: 

• A bridge cannot filter out broadcast traffic. 

• It introduces 20 to 30 % latency. 

• Only 2 networks can be linked with a bridge.