ccna 1 certification exam

ccna 1 certification exam.Cisco added another feature to the ISO standard HDLC protocol by adding an additional field (a type field) to the HDLC header, creating a Cisco-specific version of HDLC, as shown in Figure 12-9. This type field allows Cisco routers to support multiple network layer packet types traversing HDLC links. (The original HDLC standard, which predates the router by many years, did not have a Type field.) For example, an HDLC link between two Cisco routers is capable of forwarding both IPv4 and IPv6 packets because the Type field identifies which type of packet is encapsulated in each HDLC frame.Cisco Proprietary HDLC (with the Type field added)Figure 12-9 Cisco HDLC Frame Composition Today. the address and control fields of HDLC are of little use. For example, there are only two routers on a given link, and when one router256 Chapter 12 Point-to-Point WAN sends a frame, then there is no doubt that the frame must have been sent to the other router on the link. The address and control fields have played an important role for many years in the past. But now they play less of a role.Hint: If you want to know why HDLC has an address field, it's because years ago the phone company offered multiple lines. These lines included more than two devices, so there was more than one destination, thus requiring an address field identifying the correct destination.Routers typically use HDLC as the data link layer protocol on leased lines (as shown in Figure 12-10). Routers use HDLC just as any router uses any other data link layer protocol: to send packets to the next router. Figure 12-10 shows three familiar routing steps, with HDLC's role appearing in Step 2. (Note that this figure does not show the end of the data link message for each frame.)Figure 12-10 General Concept of Router Unpacking and Re-Encapsulation IP Packets The following is a detailed explanation of the figure. (1) To send an IP packet to router Rl,PC1 encapsulates the IP packet into an Ethernet frame. (2) Router R1 unencapsulates (removes) the IP packet and then encapsulates it into an HDLC frame using the HDLC header and tail. (3) Router R2 unseals (removes) the IP packet, encapsulates it in an Ethernet frame, and forwards the Ethernet frame to PC2. In summary, the HDLC leased line establishes a WAN link between the two routers so that they can forward packets for devices on the LAN where they are located. The leased line itself provides the physical means to transmit data in both directions. HDLC frames provide the means to properly encapsulate network layer packets so that it can be transmitted between routers.Configuring HDLCIt may be useful to think back to the router LAN interface. The router does not need to be configured to enable and use the interface (forwarding packets) by configuring it for both Layer 1 and Layer 2. Once the cables are properly installed, the Layer 1 (Layerl) specific tasks begin in the default state. The router Ethernet interface will, of course, use Ethernet as the data link layer protocol in the default state. The router only needs to configure the IP address on the interface and may also need to enable the interface with the noshutdown command (if the interface is in the "administratively off" state).Similarly, serial interfaces on Cisco routers do not require specific Layer 1 and Layer 2 configuration commands. For Layer 1, the cable should of course be properly installed and the router will attempt to use the serial interface once it has been configured using the noshutdown command. As for the Ethernet interface, the router serial interface usually only requires an ip address command and possibly a no shutdown command.