Day-25: Introduction to Switching.

By | June 9, 2022

The switch is a multiport bridge, It means both bridge and switches divide the collision and broadcast domain, and both work on Layer 2 of the OSI model.

Bridge Collison Domain.
Switch Collison Domain.

Layer 2 Switch is a form of Ethernet switch that switches packets by looking at their physical addresses (MAC addresses). These switches operate at the data-link layer 2 of the OSI model.

Most switches have receive and transmit buffer on each port where it stores the frame and look and forward the frame one after another and meanwhile, the packets are going to store in the buffer, If the packet is received on a port that frame will be stored in the receive buffer and if that packet is going to be sent then also it will store in the TX buffer. So this is the reason there is no collision.

Note:-There is no way where a switch can look up multiple frames at the same time..One frame can be lookup at one time.

3750 SWitch Example.

In switch, each port is a part of the individual collision domain.

Even if the port is half-duplex still there is no collision because the cable we have from switch to pc are having different twisted pair cables and tx and RX are different.

Blocking Unknown Unicast Flooding.

When an Ethernet switch receives a frame destined for a MAC address, if it is not present in its MAC address table, the default behavior is to flood the frame out of all other ports except the one on which port it is received that frame and that’s what we called as a broadcast. Only after learning of the address as the source of an incoming frame will be entered into the address table with its corresponding port, allowing future frames destined for this address to be forwarded normally as unicasts.

PC-A wants to communicate with PC-C.

Process Of Communication Given below between Pc A & C.

In the first picture the switch does not know the mac of PC-C .so it floods the frame to everyone including PC-B,C,D.

in the second picture, PC-C only replied because it has an IP address and computers understand the IP address.

in the third picture, when the switch will be able to know the MAC address of PC-C then the switch will forward the frame as unicast and this time the frame will not be sent to everyone and it will send the frame to PC-C.

Switches Basic Configuration For Management.

Switchports are primarily used for switching Layer-2 Ethernet Frames and switches Don’t natively support IP addressing. Switch Management IP address can be configured on a logical interface which we called as Switched Virtual Interface (SVI).

By default, all the ports of a switch belong to one broadcast domain but if you want to create multiple broadcast domains then you have to use the concept of VLANs. by default, the VLAN 1 is created and all the ports are members of VLAN 1 and that’s why we have one broadcast domain and normally the VLAN 1 used for management purposes and you cant delete the VLAN 1 from the VLAN database.

Switch(config)# interface vlan 1
Switch(config-if)# ip address <address> <subnet mask>
Switch(config-if)# no shutdown
Switch(config-if)# exit

Switch(config)# ip default-gateway <default-gateway>

Basic Show commands in switches.

Switch# Show ip interface brief
Switch# Show running-configuration
Switch# Show version
Switch# Show mac address-table
Switch# Show vlan

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