ccna professional

ccna professional.3.Advantages of Routers Routers operate at the upper layer of the data link layer and use logical addresses. and therefore have many advantages over data link layer devices such as bridges and switches. which include the following. . Logical addressing at Layer 3 allows you to form hierarchical networks that can scale to extremely large sizes. This is discussed in Chapters 7, 19, and 20. . Routers can block the propagation of broadcasts and multicasts. After a broadcast or multicast is received on one interface, the router will not forward it to another interface by default. Routers are commonly used to solve broadcast storm problems (in fact, routers also add bandwidth and conflict domains, but bridges and switches provide cheaper solutions). o These are discussed later in this chapter in the "Layer 2: Data Link Layer" section and in Chapters 3 and 4. . Routers are better able to connect different Layer 2 technologies (such as Ethernet, Token Ring, FDDI, and serial links) without any translation problems. . Routers can use virtual local area networks (VLANs) to exchange packets on the same interface, which is discussed in Chapter 12.. Routers provide advanced features that allow you to use queuing or traffic shaping for quality of service (QoS), use accessACLs are discussed in Chapter 21. Exam Points Keep in mind the above advantages of routers over switches, the most important of which is the ability of routers to limit the propagation of broadcasts: each interface of a router is a separate broadcast domain and conflict domain. By using logical addresses, routers can create hierarchical networks that support thousands of network devices. On the other hand, Layer 2 devices such as bridges and switches do not support hierarchical addressing: Layer 2 MAC addresses support a flat addressing space. In other words, MAC addresses cannot usually be modified to fit a particular network topology or layout. Also, because routers use logical addresses, it is much easier to implement policy decisions (such as traffic filtering or QoS) because these decisions are made based on logical addresses, which are easier to handle than the physical addresses used by bridges and switches. For example, since logical addresses contain a network component, filtering can be done based on network numbers. To use a bridge or switch for this function, the MAC address of every device in the network segment must be filtered out. Another problem with Layer 2 devices is that they do not connect well to different Layer 2 technologies or protocols (such as Ethernet and Token Ring). At Layer 2, this is called translational bridging. There are many reasons why Layer 2 devices must translate between different technologies/protocols, but the main reason is that both topologies are Layer 2 and the bridge must translate Layer 2 information from different protocols. This is a processing-intensive task that can pose many problems. For example, Ethernet supports frames up to 1500 bytes while Token Ring supports frames up to 16KB. therefore. When sending a large Token Ring frame to an Ethernet segment, the bridge or switch must divide it into multiple Ethernet frames. Also, the speeds may differ between media types: Ethernet supports speeds of 10 Mbit/s, 100 Mbit/s, 1 Gbit/s, and 10 Gbit/s, while Token Ring supports speeds of 4 Mbit/s, 16 Mbit/s, and 100 Mbit/s. This difference may cause congestion problems for the bridge or switch. In addition, it is not always easy to convert between different types of frames. For example, some protocols arrange bits in low-to-highbits in order, while others are ordered from high to low, which can present translation problems. Fortunately, routers provide a clean solution to this conversion process. Instead of converting between different frame (Layer 2 protocol) types, the router actually strips the Layer 2 frame header, makes an outgoing routing decision based on the Layer 3 packet, and then encapsulates the Layer 3 packet into the appropriate Layer 2 frame type based on the outgoing interface. This process is described in more detail in the "Encapsulation and Unencapsulation" section later in this chapter and in Chapter 9.Another advantage of routers over Layer 2 devices is their ability to limit the propagation of broadcasts. When a broadcast is received, the router processes it, but does not forward it out of any port by default. This is different from bridges and switches that flood broadcast traffic. If broadcasts are affecting the bandwidth and performance of the network, you should consider dividing the network into multiple broadcast domains and using routers to connect the different broadcast domains. Each broadcast domain in the network must have a unique Layer 3 network number. Broadcasting is discussed in the next section.Common tools used when troubleshooting Layer 3 from practical experience include the tools ping, traceroute, and Address Resolution Protocol (ARP)will be discussed in more depth in Chapter 5.2.2.6 Layer 2: Data Link LayerLayer 2 of the OSI reference model is the data link layer. The network layer gives logical addresses to the device, while the data link layer gives physical (hardware) addresses to the device. These hardware addresses are often referred to as Media Access Control (MAC) addresses. The data link layer defines how a network device accesses the media to which it is attached, and also defines the frame type and transmission method for the media. Frames contain fields that the data link layer uses to communicate with devices in the current cable or layer 2 topology; this communication can only occur between devices on the same data link layer medium (or cable) within the same network segmentbetween devices on the same data link layer medium (or cable) within the same network segment. For example, in order to communicate between different Layer 2 protocols (such as Ethernet and Token Ring), a router is often required. Exam Essentials Most wide area network (WAN) protocols run primarily at the data link layer and the physical layer. The data link layer is also responsible for reconfiguring the bits (binary values 1 and 0) from the physical layer into data link layer frames. The data link layer performs error detection and discards damaged frames; it does not usually perform error correction, as some transport layers do; however, some data link layer protocols do provide error correction capabilities. Data link layer protocols and standards used for LANs include: IEEE 802.2.802.3 and 802.5; EthernetII; and the FDDI standard specified by the American National Standards Institute (ANSI). Data link layer protocols for the WAN include Asynchronous Transfer Mode (ATM), Frame Relay, Advanced Data Link Control (HDLC), Point-to-Point Protocol (PPP), Synchronous Data Link Control (SDLC), Serial Line Internet Protocol (SLIP), and X.25. The major network components operating at the data link layer include bridges, switches, and network interface cards (NICs), which will be used in the fourth phase. This is discussed in more depth in Chapter 4. The data link layer defines the hardware (MAC) addresses and the communication processes within the same medium. Switches and bridges operate at the data