SWITCHING

SWITCHING

COMPARISON BETWEEN BRIDGES AND SWITCHES

1: Bridges are considered software based. Switches are hardware based
because they use an ASIC chip to help make filtering decision.-
2: Bridges can have upto only 16 ports. A switch can have more than 100
ports.

SWITCHING FUNCTION

1: MAC ADDRESS LEARING FUNCTION

Layer 2 switches and bridges remember the source hardware address
of each frame received on an interface and enter it into a mac data base.
(mac address table)

2: FORWARDING AND FILTER DECISION

When a frame is received on an interface the switch look at the destination
hardware address and look up the exit interface in the mac database.

3: LOOP AVOIDANCE

If multiple connection between switches are created for redendancy network
loop can occur . STP is used to stop network loop.
STP is Spanning Tree Protocol. It is created by IEEE. Its version is called 802.11d.
To elect a root bridge the switch check priority number. The priority on all
devices running IEEE STP version is 32-768 by default. The lower the bridge id ,
they more likely a device is to become a root bridge . Switches or Bridges running
STP exchange information called BPDU (Bridge Protocol Data Unit)
BPDU are sent every two second by default.

Electing Route Part

Speed New IEEE Old IEEE cost

10 Gbps 2 1
1 Gbps 4 1
100 mbps 19 10
10 mbps 100 100

To determine the port that will be used to communicate with the route bridge the path cost is determined.

Electing Designated Port

A designated port is on that is active and forwarding traffic. A designated port in one switch connect to all the route port on another switch.

STP Port State

1: Blocking

Listen to BPDU all port are in blocking state by default when the switch is powered on.

2: Listening

Listen to BPDU to make sure no loop occur on the network before passing the data frame.

3: Learning Function

Learn MAC address and build a MAC database but does not forward frames.

4: Forwarding

Bridge point is able to send and recieve data.