ETHERNET SWITCHING (MẠNG máy TÍNH cơ bản SLIDE)

Chương 8: ETHERNET SWITCHING Overview • Shared Ethernet works extremely well under ideal conditions When the number of devices trying to access the network is low, the number of collisions stays well within acceptable limits Computer Network • when the number of users on the network increases, the increased number of collisions can cause intolerably bad performance Bridging was developed to help ease performance problems that arose from increased collisions Switching evolved from bridging to become the key technology in modern Ethernet LANs Computer Network • The concept of collision domains and broadcast domains is concerned with the ways that networks can be designed to limit the negative effects of collisions and broadcasts This module explores the effects of collisions and broadcasts on network traffic and then describes how bridges and routers are used to segment networks for improved performance Computer Network • Students completing this module should be able to: – Define bridging and switching – Define and describe the content-addressable memory (CAM) table – Define latency – Describe store-and forward and cut-through switching modes – Explain Spanning-Tree Protocol (STP) – Define collisions, broadcasts, collision domains, and broadcast domains – Identify the Layer 1, 2, and devices used to create collision domains and broadcast domains – Discuss data flow and problems with broadcasts – Explain network segmentation and list the devices used to create segments Computer Network 8.1 Ethernet Switching 8.1.1 Layer bridging • As more nodes are added to an Ethernet physical segment, contention for the media increases Ethernet is a shared media, which means only one node can transmit data at a time The addition of more nodes increases the demands on the available bandwidth and places additional loads on the media Computer Network • By increasing the number of nodes on a single segment, the probability of collisions increases, resulting in more retransmissions A solution to the problem is to break the large segment into parts and separate it into isolated collision domains • To accomplish this a bridge keeps a table of MAC addresses and the associated ports The bridge then forwards or discards frames based on the table entries The following steps illustrate the operation of a bridge: Computer Network Computer Network – The bridge has just been started so the bridge table is empty The bridge just waits for traffic on the segment When traffic is detected, it is processed by the bridge – Host A is pinging Host B Since the data is transmitted on the entire collision domain segment, both the bridge and Host B process the packet – The bridge adds the source address of the frame to its bridge table Since the address was in the source address field and the frame was received on port 1, the frame must be associated with port in the table Computer Network Computer Network – The destination address of the frame is checked against the bridge table Since the address is not in the table, even though it is on the same collision domain, the frame is forwarded to the other segment The address of Host B has not been recorded yet as only the source address of a frame is recorded – Host B processes the ping request and transmits a ping reply back to Host A The data is transmitted over the whole collision domain Both Host A and the bridge receive the frame and process it Computer Network 10 • The probability of broadcast storms increases as the switched network grows • The three sources of broadcasts and multicasts in IP networks are workstations, routers, and multicast applications Computer Network 64 8.2.5 Broadcast domains • A broadcast domain is a grouping of collision domains that are connected by Layer devices Computer Network 65 Computer Network 66 • Breaking up a LAN into multiple collision domains increases the opportunity for each host in the network to gain access to the media This effectively reduces the chance of collisions and increases available bandwidth for every host But broadcasts are forwarded by Layer devices and if excessive, can reduce the efficiency of the entire LAN Computer Network 67 • Broadcasts have to be controlled at Layer 3, as Layer and Layer devices have no way of controlling them The total size of a broadcast domain can be identified by looking at all of the collision domains that the same broadcast frame is processed by • In other words, all the nodes that are a part of that network segment bounded by a layer three device Broadcast domains are controlled at Layer because routers not forward broadcasts.  Computer Network 68 • Routers actually work at Layers 1, 2, and They, like all Layer devices, have a physical connection to, and transmit data onto, the media They have a Layer encapsulation on all interfaces and perform just like any other Layer device It is Layer that allows the router to segment broadcast domains Computer Network 69 • In order for a packet to be forwarded through a router it must have already been processed by a Layer device and the frame information stripped off Layer forwarding is based on the destination IP address and not the MAC address For a packet to be forwarded it must contain an IP address that is outside of the range of addresses assigned to the LAN and the router must have a destination to send the specific packet to in its routing table Computer Network 70 8.2.6 Introduction to data flow • Data flow in the context of collision and broadcast domains focuses on how data frames propagate through a network It refers to the movement of data through Layer 1, and devices and how data must be encapsulated to effectively make that journey Remember that data is encapsulated at the network layer with an IP source and destination address, and at the data-link layer with a MAC source and destination address Computer Network 71 Computer Network 72 • Layer devices no filtering, so everything that is received is passed on to the next segment The frame is simply regenerated and retimed and thus returned to its original transmission quality Any segments connected by Layer devices are part of the same domain, both collision and broadcast Computer Network 73 • Layer devices filter data frames based on the destination MAC address A frame is forwarded if it is going to an unknown destination outside the collision domain The frame will also be forwarded if it is a broadcast, multicast, or a unicast going outside of the local collision domain The only time that a frame is not forwarded is when the Layer device finds that the sending host and the receiving host are in the same collision domain A Layer device, such as a bridge, creates multiple collision domains but maintains only one broadcast domain Computer Network 74 • Layer devices filter data packets based on IP destination address The only way that a packet will be forwarded is if its destination IP address is outside of the broadcast domain and the router has an identified location to send the packet A Layer device creates multiple collision and broadcast domains • Data flow through a routed IP based network, involves data moving across traffic management devices at Layers 1, 2, and of the OSI model Layer is used for transmission across the physical media, Layer for collision domain management, and Layer for broadcast domain management Computer Network 75 8.2.7 What is a network segment? • In the context of data communication, the following definitions are used: – Section of a network that is bounded by bridges, routers, or switches – In a LAN using a bus topology, a segment is a continuous electrical circuit that is often connected to other such segments with repeaters Computer Network 76 – Term used in the TCP specification to describe a single transport layer unit of information The terms datagram, frame, message, and packet are also used to describe logical information groupings at various layers of the OSI reference model and in various technology circles • To properly define the term segment, the context of the usage must be presented with the word If segment is used in the context of TCP, it would be defined as a separate piece of the data If segment is being used in the context of physical networking media in a routed network, it would be seen as one of the parts or sections of the total network Computer Network 77 Computer Network 78 ... segments Computer Network 8.1 Ethernet Switching 8.1.1 Layer bridging • As more nodes are added to an Ethernet physical segment, contention for the media increases Ethernet is a shared media, which... other bit rate This is known as asynchronous switching Store-andforward mode must be used for asynchronous switching Computer Network 34 • Asymmetric switching provides switched connections between... ease performance problems that arose from increased collisions Switching evolved from bridging to become the key technology in modern Ethernet LANs Computer Network • The concept of collision domains