1. M.Usman khan

2. Zameer Abbas

3. Sulaiman Ahmad Khan

GROUP MEMBERS

TOPIC

Switching Concepts

OVERVIEW• Routers

• Switches

• Bridges

• Hub

• Repeaters

BRIDGES• A bridge is a Layer 2 device used to divide, or segment, a network.

• A bridge is capable of collecting and selectively passing data frames between two network segments.

• Bridges do this by learning the MAC address of all devices on each connected segment. Using this information, the bridge builds a bridging table and forwards or blocks traffic based on that table.

• This results in smaller collision domains and greater network efficiency.

• Bridges do NOT restrict broadcast traffic.

BRIDGE

SWITCHES• Switches create a virtual circuit between two connected devices,

establishing a dedicated communication path between two devices.

• Switches on the network provide micro segmentation. • This allows maximum utilization of the available bandwidth.

• A switch is also able to facilitate multiple, simultaneous virtual circuit connections.

• Broadcast frames to all connected devices on the network.

SWITCH

ROUTER• A router is a Layer 3 device. • Used to “route” traffic between two or more Layer 3 networks.• Routers make decisions based on groups of network addresses, or

classes, as opposed to individual Layer 2 MAC addresses. • Routers use routing tables to record the Layer 3 addresses of the

networks that are directly connected to the local interfaces and network paths learned from neighboring routers.

• Routers are not compelled to forward broadcasts.

HALF-DUPLEX• Using half-duplex, a host could either transmit or receive at one time,

but not both. • If the network is already in use, the transmission is delayed. • When a collision occurs, the host that first detects the collision will

send out a jam signal to the other hosts. • Upon receiving the jam signal, each host will stop sending data, then

wait for a random period of time before attempting to retransmit. • The back-off algorithm generates this random delay. • As more hosts are added to the network and begin transmitting,

collisions are more likely to occur.

DUPLEX TRANSMISSIONS• Simplex Transmission: One way and one way only.

• One way street• Half-duplex Transmission: Either way, but only one way at a time.

• Two way street, but only one way at a time (land slide).• Full-duplex Transmission: Both ways at the same time.

• Two way street

SENDING AND RECEIVING ETHERNET FRAMES VIA A HUB

• The hub will flood it out all ports except for the incoming port.• Hub is a layer 1 device.• A hub does NOT look at layer 2 addresses, so it is fast in transmitting

data.• Disadvantage with hubs: A hub or series of hubs is a single collision

domain. • A collision will occur if any two or more devices transmit at the same

time within the collision domain.

SENDING AND RECEIVING ETHERNET FRAMES ON A BUS

• When an Ethernet frame is sent out on the “bus” all devices on the bus receive it.

• Each NIC card compares its own MAC address with the Destination MAC Address.

• If it matches, it copies in the rest of the frame.

• If it does NOT match, it ignores the rest of the frame.• Unless you are running a Sniffer program

NEXT

ETHERNET/802.3 LAN DEVELOPMENT• Distance limitations

• Ethernet is fundamentally a shared technology where all users on a given LAN segment compete for the same available bandwidth.

• This situation is analogous to a number of cars all trying to access a one-lane road at the same time.

• Because the road has only one lane, only one car can access it at a time.

• The introduction of hubs into a network resulted in more users competing for the same bandwidth.

• Collisions are a by-product of Ethernet networks.

ELEMENTS OF ETHERNET 802.3 NETWORKS• Broadcast data frame delivery of Ethernet/802.3 • The carrier sense multiple access/collision detect (CSMA/CD) method

allows only one station to transmit at a time. • Multimedia applications with higher bandwidth demand such as video

and the Internet, coupled with the broadcast nature of Ethernet, can create network congestion.

• Normal latency as the frames travel across the layers• Extending the distances and increasing latency of the Ethernet/802.3

LANs by using Layer 1 repeaters.

NETWORK LATENCY• Latency, or delay, is the time a frame or a packet takes to travel

from the source station to the final destination. • It is important to quantify the total latency of the path between the

source and the destination for LANs and WANs. • Latency has at least three sources:

• First, there is the time it takes the source NIC to place voltage pulses on the wire and the time it takes the receiving NIC to interpret these pulses. This is sometimes called NIC delay.

• Second, there is the actual propagation delay as the signal takes time to travel along the cable.

• Third, latency is added according to which networking devices, whether they are Layer 1, Layer 2, or Layer 3, are added to the path between the two communicating computers.

SENDING AND RECEIVING ETHERNET FRAMES VIA A SWITCH

• Switches are also known as learning bridges or learning switches.• A switch has a source address table in cache (RAM) where it stores

source MAC address after it learns about them.• A switch receives an Ethernet frame it searches the source address

table for the Destination MAC address.• If it finds a match, it filters the frame by only sending it out that port.• If there is not a match if floods it out all ports.

COLLISION DOMAINS• When there is only one device on a switch port, the collision domain is only between

the PC and the switch.

• With a full-duplex PC and switch port, there will be no collision, since the devices and the medium can send and receive at the same time.

TRANSPARENT BRIDGE PROCESS - JEFF DOYLE

LAN SEGMENTATION WITH ROUTERS• Routers provide segmentation of networks, adding a latency factor of

20% to 30% over a switched network. • This increased latency is because a router operates at the network

layer and uses the IP address to determine the best path to the destination node.

• Bridges and switches provide segmentation within a single network or subnetwork.

• Routers provide connectivity between networks and subnetworks.

• Routers also do not forward broadcasts while switches and bridges must forward broadcast frames.

LAN SEGMENTATION WITH ROUTERS

SYMMETRIC AND ASYMMETRIC SWITCHING

NEXT

ETHERNET SWITCH LATENCY• Latency is the period of time from when the beginning of a frame

enters to when the end of the frame exits the switch. • Latency is directly related to the configured switching process and

volume of traffic.

MEMORY BUFFERING• An Ethernet switch may use a buffering technique to store and forward

frames.

• Buffering may also be used when the destination port is busy.

• The area of memory where the switch stores the data is called the memory buffer.

• This memory buffer can use two methods for forwarding frame:

• port-based memory buffering • shared memory buffering

• In port-based memory buffering frames are stored in queues that are linked to specific incoming ports.

• Shared memory buffering deposits all frames into a common memory buffer which all the ports on the switch share.

SWITCHING METHODS1. Store-and-forward – The entire frame is received before any

forwarding takes place. • The destination and source addresses are read and filters

are applied before the frame is forwarded. • CRC Check done

2. Cut-through – The frame is forwarded through the switch before the entire frame is received. • This mode decreases the latency of the transmission, but

also reduces error detection.• In1900 and 2800 series switches, this is configurable,

otherwise depends on the model of the switch.

CUT-THROUGH

• Fast-forward – Offers the lowest level of latency.

• Fast-forward switching immediately forwards a packet after reading the destination address.

• There may be times when packets are relayed with errors.

• Although this occurs infrequently and the destination network adapter will discard the faulty packet upon receipt.

• Fragment-free – Fragment-free switching filters out collision fragments before • forwarding begins.

• Collision fragments are the majority of packet errors. • In a properly functioning network, collision fragments must be smaller than 64 bytes. • Anything greater than 64 bytes is a valid packet and is usually received without

error. • Fragment-free switching waits until the packet is determined not to be a collision

fragment before forwarding.

. Adaptive cut-through

• In this mode, the switch uses cut-through until it detects a given number of errors.

• Once the error threshold is reached, the switch changes to store-and-forward mode.

USING HUBS• Layer 1 devices

• Inexpensive

• In one port, out the others

• One collision domain

• One broadcast domain

SINGLE HUB

USING SWITCHES• Layer 2 devices

• Layer 2 filtering based on Destination MAC addresses and Source Address Table

• One collision domain per port

• One broadcast domain across all switches

INTRODUCING VLANS• VLAN = Subnet

• VLANs create separate broadcast domains within the switch.

• Routers are needed to pass information between different VLANs

INTER-VLAN TRAFFIC

Remember that VLAN IDs (numbers) are assigned to the switch port and not to the host. (Port-centric VLAN switches)

2. Be sure to have all of the hosts on the same subnet belong to the same VLAN, or you will have problems.

• Hosts on subnet 172.30.1.0/24 - VLAN 1

• Hosts on subnet 172.30.2.0/24 - VLAN 2

INTER-VLAN ROUTING NEEDS A ROUTER

• A router is need to route traffic between VLANs (VLAN = Subnet).

• There are various methods of doing this including Router-on-a-stick with trunking (more than one VLAN on the link).

• This will be discussed later when we get to the chapter on VLANs and Inter-VLAN Routing.

END