The design in Figure was called a collapsed backbone because all hosts would need to go to the corporate backbone to reach any network services—both LAN and mainframe.
Unlike bridges, which use software to create and manage a filter table, switches use applicationspecific integrated circuits (ASICs) to build and maintain their filter tables. Layer 2 switches and bridges are faster than routers because they don’t take up time looking at the Network layer header information. Instead, they look at the frame’s hardware addresses before deciding to either forward, flood or drop the frame.
Figure illustrates how a broadcast can be propagated throughout the network. Observe how a frame is continually being flooded through the internetwork’s physical network media.
A device can receive multiple copies of the same frame since that frame can arrive from different segments at the same time. Figure demonstrates how a whole bunch of frames can arrive from multiple segments simultaneously. The server in the figure sends a unicast frame to Router C. Since it’s a unicast frame, Switch A forwards the frame and Switch B provides the same service—it forwards the broadcast. This is bad because it means that Router C receives that unicast frame twice, causing additional overhead on the network
Layer 2 Switching
Layer 2 switching is the process of using the hardware address of devices on a LAN to segment a network
What Does Switching do…?Switching breaks up large collision domains into smaller onesCollision domain is a network segment with two or more devices sharing the same bandwidthEach port on a switch is actually its own collision domain
Before Layer 2 Switching
First Switched Network
Typical Switched Network Design
Switching ServicesSwitches use ASICApplication Specific Integrated CircuitsLayer 2 Switches are faster then RoutersSwitches create private, dedicated collision domains and provide independent bandwidth on each port, unlike hubs.
What makes layer 2 switching so efficient..?no modification to the data packet takes place.The device only reads the frame encapsulating the packet, which makes the switching process considerably faster and less error-prone than routing processes are.
Bridging vs. LAN Switching Bridges are software based, while switches are hardware based because they use ASIC chips to help make filtering decisions. A switch can be viewed as a multiport bridge. There can be only one spanning-tree instance per bridge, while switches can have many. Switches have a higher number of ports than most bridges. Both bridges and switches forward layer 2 broadcasts. Bridges and switches learn MAC addresses by examining the source address of each frame received. Both bridges and switches make forwarding decisions based on layer 2 addresses.
Three Switch Functions at Layer 2Address learningforward/filter decisionsloop avoidance
Loop AvoidanceRedundant links between switches are a good idea because they help prevent complete network failures in the event one link stops working.If no loop avoidance schemes are put in place, the switches will flood broadcasts endlesslythroughout the internetwork. This is sometimes referred to as a broadcast storm.
Multiple Frame Copies
SolutionSpanning Tree Protocol to the Rescue
Layer 3 Switch it has the same A Layer 3 switch works much like a router because IP routing table for lookups and it forms a broadcast domain. However, the “switch” part of “Layer 3 switch” is there because: The layer 3 switch looks like a switch. It has 24+ Ethernet ports and no WAN interfaces. The layer 3 switch will act like a switch when it is connecting devices that are on the same network. The layer 3 switch is the same as a switch with the router’s IP routing intelligence built in. The switch works very quickly to switch or route the packets it is sent. In other words, the Layer 3 switch is really like a high-speed router without the WAN connectivity.
Why we need it ?Routing functionality of the Layer 3 switch is there to route between different subnets or VLANs on a campus LAN or any sort of large LAN.This means that the Layer 3 switch is really for large Ethernet networks that need to subnet into smaller networks. Most of the time, this is done using VLANs.
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