Switching

Switching

Switching means to actually send the packet over the predefined route. There are 3 types of switching:

1. Packet switching

2. Circuit switching

3. Message switching

Switches will forward frame (protocol data unit at layer 2) in the same broadcast domain that is the reason why switch can’t communicate with switch or devices in other VLAN, it need router or layer 3 switches.
While router will route the packet (protocol data unit at layer 3) to different network if the other network is known to the router.
Difference between switching and routing:-

1) Switching will be faster as switch uses ASIC technology.
Routing will be slower as it is software based.
2) Switching is done at layer 2.
Routing is done at layer 3.
3) If the destination is not known to switch it will broadcast the frame.
If the destination is not known to router it will drop the packet.
4) Switching is done in same broadcast domain.
Routing is done in different networks.
5) Switching is done by using MAC address.
Routing is done by using IP address.

---->>>>>Protocol data unit at layer 2 is frame
---->>>>> Protocol data unit at layer 3 is packet

As a switch doesnt know the network it just forwards the packet/frame based on the MAC and hence it is possible for multiple networks to exists on the same L2 switch eventhough this is not suggested.

Different types of switching techniques

Different types of switching techniques are employed to provide communication between two computers. These are : Circuit switching, message switching and packet switching.
Circuit Switching
In this technique, first the complete physical connection between two computers is established and then data are transmitted from the source computer to the destination computer. That is, when a computer places a telephone call, the switching equipment within the telephone system seeks out a physical copper path all the way from sender telephone to the receiver’s telephone. The important property of this switching technique is to setup an end-to-end path (connection) between computer before any data can be sent.
Message Switching
In this technique, the source computer sends data or the message to the switching office first, which stores the data in its buffer. It then looks for a free link to another switching office and then sends the data to this office. This process is continued until the data are delivered to the destination computers. Owing to its working principle, it is also known as store and forward. That is, store first (in switching office), forward later, one jump at a time.
Packet Switching
With message switching, there is no limit on block size, in contrast, packet switching places a tight upper limit on block size. A fixed size of packet which can be transmitted across the network is specified. Another point of its difference from message switching is that data packets are stored on the disk in message switching whereas in packet switching, all the packets of fixed size are stored in main memory. This improves the performance as the access time (time taken to access a data packet) is reduced, thus, the throughput (measure of performance) of the network is improved 

Switch is a networking device which generally works on layer2. have 2MB ram, 24ports and cannot stop broadcasting. Switch have single broadcasting domain and multiple collision domains. There are 2 types of switch :

1. Non-managable switch

2. Managable switch

Managable switch have 2 categories:

i)Layer 2 switch

ii)Layer 3 switch

There are 3 functions of switch:

1. It maintain MAC table

2. It create multiple collision domains and stop broadcasting at layer 2.

3. It flood broadcast, multicast and unicast frames out of all ports except the one it was received.

Switch has 3 modes or there are 3 types of switch on the basis of their working:

1. Store and forward switch   It will receive the full frame and passes CRC (Cyclic Redundancy Check) and then send to the destination. It work in full duplex and latency never remain constant. Latency means time to receive the frame bits, join them and passes CRC.

2. Cut through   It only looks at the destination MAC address in an ethernet frame and forward it. It is use in half duplex , only receiving and sending , there is no CRC.

3. Fragment free  It is combination of store and forward switch and cut through switch, it checks the first 64bytes for frame fragmentation before forwarding the frame to the destination.

STP (Spanning Tree Protocol)

STP is a layer 2 protocol use for loop avoidance in the switching process. STP select root bridge by determining which switch has the bridge ID  learn from sending and receiving BPDUs. Bridge Protocol Data Unit is a STP initializing packet that is send at different intervals for the purpose of exchanging information between the different switches/bridges.

NOTE: MAC address with minimum value will become Root bridge.

Functions of STP:

1. Blocking

2. Listening

3. Learning

4. Forwarding

5. Loop Avoidance

RSTP (Rapid STP)

RSTP is Cisco enhancement to STP wihch ensures safe and quick transition to a forwarding state and topology convergence by removing the overdependence of STP timers.

Functions of RSTP:

1. Blocking

2. Learning

3. Forwarding

Etherchannel

It is a Cisco method of bundling redundant links between switches to act as a single aggregated link. this allows utilization of all the bandwidth because STP will treat the link as a single interface in the case of link failure. . Etherchannel automatically distributes the traffic load over the remaining links in milliseconds.

VLAN (Virtual LAN)

VLAN logically divide a switch into multiple broadcast domains at layer 2. Each VLAN represents a logical grouping of users by their function or departmentwise. User from a VLAN cannot communicate with users of another VLAN without using the Inter VLAN routing or without using Layer 3 device.

Inter VLAN routing

It requires a layer 3 device, the connection between the layer 3 device and the switch must be atleast fastethernet speed and must be a trunk. The router interface consists of subinterfaces to assign an IP Gateway for each VLAN. The VLAN is associated with  a subinterface using dot1q encapsulation types.

Voice VLAN

Voice VLAN is use to separate VOIP traffic from data on an access port for QOS and to manage traffic.

Trunk

VLAN can span multiple switches  using trunk. Trunk multiplex traffic from all VLANs over  a single connection. The VLAN identifier is tagged  over the trunk using one of the following tagging method:

1. IEEE802. 1q It is standard based VLAN tagging that insert 4byte tag in the original ethernet frame. Traffic originating from the native VLAN (by default VLAN 1) is not tagged over the trunk. If native VLAN configuration does not match on both sides , then this could cause VLAN leakage.

2. ISL (Inter Switch Link) It is a Cisco proprietory trunk that encapsulates the original ethernet frame  with a 26byte header and 4 byte CRC. ISL is a single ethernet switch which can participate in multiple VLANs.ISL also available on ethernet card, it allows a  single ethernet card to have multiple logical addresses that reflects the VLAN which it belongs.

VLAN process:

VLAN is a group of switch ports which act as separate isolated LAN. Their can be several VLAN on a single switch. VLAN can also have multiple switches. VLAN will never encounter traffic  or share bandwidth from other VLAN unless the data is routed. Each port represent a single collision domain and divide share bandwidth only with other device on the same switch port unless the switch is segmented into VLANs. However, all the devices in a switch re still in the single broadcast domain. All broadcast are sent to each port through the switching. When we create VLAN, we are just creating boundary for broadcast traffic.

Frame Tagging

It is possible for a device to participate more than oneVLAN by using a special network card which perform ISL.

Frame Tagging Process

A frame enter the switch, switch encapsulate the frame with a header that tags the frame with VLAN ID. Any time a frame need to leave the switch for another, the tag frame send throughout switching. When frame arrive to the destination switch, the tag is stripped off from the frame and send to the destination device.

VTP (VLAN Trunking Protocol)

Cisco created VTP to minimize the amount of VLAN administration in switches by enabling  a VTP serverto multicast VTP advertisements to other switches in the same VTP domain. Switches receving the advertisements syncronize their VLAN database with the VLAN information advertised from the server assuming that the revision number is higher.

VTP has 4 modes:

1. VTP server mode  It is default VTP mode that enables to create, modify and delete VLANs. These VLANs are advertised to other switches and saved in the VLAN database.

2. VTP client mode It cannot create , modify and delete VLANs, it forward advertisements received from the VTP server but does not save the VLAN configuration in the VLAN database.

3. VTP transparent mode  It create, modify and delete VLAN only on local switch, it does not participate in VTP but forwards VTP advertisements received from the VTPserver. It also save the VLAN configuration in the
VLAN database.

VTP pruning  It increase available bandwidth by restricting flooded traffic to those trunk links which  traffic must use to access the appropriate network devices.

NOTE:VTP is a layer 2 messaging protocol which maintain VLAN configuration consistency throughout a common administrative domain. VTP accomplishes this goal by managing the addition, deletion and name change of VLAN accross network. VTP advertisements are flooded throughout the management domain every 5 minutes or whenever there is change in VLAN configuration.

Step to configure VLAN:

Step1  Create VLAN

Step2  Give name to VLAN

Step3  Add desire interface in VLAN

Example of VLAN: