CCNA chapter 1
Bootup process
1. POST: Testing the router hardware 2. Loading the bootstrap program 3. Locating Cisco IOS 4. Loading Cisco IOS 5. Locating the configuration file 6. Loading the startup configuration file or entering setup mode
Path Determination Packet forwarding involves two functions:
• Path determination function • Switching function The path determination function is the process of how the router determines which path to use when forwarding a packet. To determine the best path, the router searches its routing table for a network address that matches the packet's destination IP address.
• Directly connected network
: If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the router's interfaces, that packet is forwarded directly to that device. This means that the destination IP address of the packet is a host address on the same network as this router's interface.
• No route determined
: If the destination IP address of the packet does not belong to either a connected or remote network, and the router does not have a default route, the packet is discarded. The router sends an Internet Control Message Protocol (ICMP) Unreachable message to the source IP address of the packet.
equal-cost metric
A metric that has the same value on multiple paths to the same destination. When multiple paths have equal-cost metrics, a router can execute equal-cost load balancing among those paths.
ATM
Asynchronous Transfer Mode (ATM) The international standard for cell relay in which multiple service types (such as voice, video, or data) are conveyed in fixed-length (53-byte) cells. Fixed-length cells allow cell processing to occur in hardware, thereby reducing transit delays. ATM is designed to take advantage of high-speed transmission media, such as E3, SONET, and T3.
Asymmetric Routing
Because routers do not necessarily have the same information in their routing tables, packets can traverse the network in one direction, using one path, and return through another path. This is called asymmetric routing. Asymmetric routing is more common in the Internet, which uses the BGP routing protocol, than it is in most internal networks.
The primary functions of a router are:
Best path selection/ determination and packet forwarding.
Cisco IOS
Cisco IOS Software is responsible for managing the hardware and software resources of the router, including allocating memory, managing processes and security, and managing file systems. Cisco IOS is a multitasking operating system that is integrated with routing, switching, internetworking, and telecommunications functions. As with other operating systems, Cisco IOS has its own user interface. Although some routers provide a GUI (graphical user interface), the CLI (command-line interface) is a much more common method of configuring Cisco routers
Configuration Register
Configuration register is 0x2102 The last line of the show version command displays the current configured value of the software configuration register in hexadecimal. If a second value is displayed in parentheses, this is the configuration register value that will be used during the next reload. The configuration register has several uses, including password recovery. The factory default setting for the configuration register is 0x2102. This value indicates that the router will attempt to load a Cisco IOS Software image from flash memory and load the startup configuration file from NVRAM.
Dynamic routing protocols
Dynamic routing protocols are used by routers to share information about the reachability and status of remote networks. Dynamic routing protocols perform several activities, including the following: • Network discovery • Updating and maintaining routing tables
Interfaces Belong to Different Networks
Every interface on the router belongs to a different network. In other words, each interface is a host on a different IP network, as shown previously in Figure 1-8. Each interface must be configured with an IP address and subnet mask of a different network. Cisco IOS will not allow two active interfaces on the same router to belong to the same network.
Flash Memory
Flash memory is nonvolatile computer memory that can be electrically erased and reprogrammed. Flash is used as permanent storage for the operating system, Cisco IOS. In most models of Cisco routers, the IOS is permanently stored in flash memory and copied into RAM during the bootup process. Flash consists of SIMM or PC cards (PCMCIA cards), which can be upgraded to increase the amount of flash memory. Flash memory does not lose its contents when the router loses power or is restarted.
Frame Relay
Frame Relay A packet-switched data link layer protocol that handles multiple virtual circuits used between connected devices. Frame Relay is more efficient than X.25, the protocol for which it generally is considered a replacement.
• Remote network:
If the destination IP address of the packet belongs to a remote network, the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.
NVRAM
NVRAM is nonvolatile random-access memory, which does not lose its information when the power is turned off. This is in contrast to the most common forms of RAM such as DRAM, which requires continual power to maintain its information. NVRAM is used by Cisco IOS Software as permanent storage for the startup configuration file (startup-config). All configuration changes are stored in the running-config file in RAM and, with few exceptions, are implemented immediately by the IOS. To save those changes in case the router is restarted or loses power, the running-config file must be copied to NVRAM, where it is stored as the startup-config file. NVRAM retains its contents even when the router is powered off.
Point to Point Protocol (PPP)
Point-to-Point Protocol (PPP) Successor to SLIP that provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Whereas SLIP was designed to work with IP, PPP was designed to work with several network layer protocols, such as IP, IPX, and ARA. PPP also has built-in security mechanisms, such as CHAP and PAP. PPP relies on two protocols: LCP and NCP.
ROM
ROM is a form of permanent storage. Cisco devices use ROM to store • Bootstrap instructions • Basic diagnostic software • Scaled-down version of IOS ROM uses firmware, which is software embedded inside the integrated circuit. Firmware, such as the bootup instructions, does not normally need to be modified or upgraded. Many of these features, including ROM monitor software, will be discussed in a later course. ROM does not lose its contents when the router loses power or is restarted.
Management ports
Routers have management ports, which are physical connectors used to manage the router. Management ports are not used for packet forwarding like Ethernet and serial interfaces. The most common of the management ports is the console port. The console port is used to connect a terminal, or most likely a PC running terminal emulator software, to configure the router without the need for network access to that router. The console port must be used during initial configuration of the router.
Hardware that makes a router
Similar to a PC, a router also includes • CPU • RAM • ROM • Flash memory • NVRAM
RAM
Similar to other computers, RAM stores the instructions and data needed to be executed by the CPU. RAM is used to store • Operating system: Cisco IOS (Internetwork Operating System) is copied into RAM during bootup. • Running configuration file: This is the configuration file that stores the configuration commands that the router's IOS is currently using. With few exceptions, all commands configured on the router are stored in the running configuration file known as the running-config. • IP routing table: This is the file that stores information about directly connected and remote networks. • ARP cache: This cache stores IP address-to-MAC address mappings, similar to the ARP cache on a PC. ARP cache would be used on routers that have Ethernet interfaces. • Packet buffering: Packets are temporarily stored in a buffer when received on an interface or before they exit an interface. RAM is volatile memory and loses its contents when the router is powered down or restarted. For this reason, the router also contains permanent storage areas such as ROM, flash, and NVRAM.
1) POST (Fingers and toes test)
Step 1: Performing the POST A power-on self test (POST) The POST process is used to test the router hardware. When the router is powered on, software on the ROM chip conducts the POST. During this self test, the router executes diagnostics from ROM on several hardware components, including the CPU, RAM, and NVRAM. After the POST has been completed, the router executes the bootstrap program.
2) Bootstrap program
Step 2: Loading the Bootstrap Program After the POST the bootstrap program is copied from ROM into RAM. When the bootstrap program is in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM. At this point, if you have a console connection to the router, you will begin to see output on the screen.
3) Locating Cisco IOS
Step 3: Locating Cisco IOS The bootstrap program is responsible for locating the Cisco IOS and copying it into RAM. The IOS is typically stored in flash memory, but it can be stored in other places such as a TFTP server. If a full IOS image cannot be located, a scaled-down version of the IOS is copied from ROM into RAM. This version of IOS is used to help diagnose any problems and can be used to load a complete version of the IOS into RAM.
4) Loading Cisco IOS
Step 4: Loading Cisco IOS Some of the older Cisco routers ran the IOS directly from flash, but current models copy the IOS into RAM for execution by the CPU. When the IOS begins to load, you might see a string of pound signs (#) while the image decompresses.
5) Locating the Config File
Step 5: Locating the Configuration File After the IOS is loaded the bootstrap program searches for the startup configuration file, known as the startup-config file, in NVRAM. This file has the previously saved configuration commands and parameters, including the following: • Interface addresses • Routing information • Passwords • Any other configurations saved by the network administrator If the startup configuration file, startup-config, is located in NVRAM, it is then copied into RAM as the running configuration file, running-config.
6) Loading the startup config file
Step 6: Loading the Startup Configuration File or Entering Setup Mode If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config file and executes the commands in the file one line at a time. The running-config commands contain interface addresses, start routing processes, configure router passwords, and define other characteristics of the router. If the startup configuration file cannot be located, the router will prompt the user to enter setup mode. Setup mode is a series of questions prompting the user for basic configuration information. Setup mode is not intended to enter complex router configurations, nor is it commonly used by network administrators.
The CPU
The CPU executes operating system instructions, such as system initialization, routing functions, and network interface control.
Packet forwarding protocols
The data-link encapsulation depends on the type of interface on the router and the type of medium to which it connects. The different data-link technologies that a router connects to can include LAN technologies, such as Ethernet, and WAN serial connections, such as a T1 connection using PPP, Frame Relay, and ATM.
Show version
The output from the show version command includes information about the following: • IOS version • ROM bootstrap program • Location of IOS • CPU and amount of RAM • Interfaces • Amount of NVRAM • Amount of flash • Configuration register information
What is an interface
The term interface on Cisco routers refers to a physical connector on the router whose main purpose is to receive and forward packets. Routers have multiple interfaces used to connect to multiple networks.
IP Routing Protocols
There are several dynamic routing protocols for IP. Here are some of the more common dynamic routing protocols for routing IP packets: • RIP (Routing Information Protocol) • IGRP (Interior Gateway Routing Protocol) • EIGRP (Enhanced Interior Gateway Routing Protocol) • OSPF (Open Shortest Path First) • IS-IS (Intermediate System-to-Intermediate System) • BGP (Border Gateway Protocol)
Two metrics that are used by some dynamic routing protocols are
Two metrics that are used by some dynamic routing protocols are • Hop count: This is the number of routers that a packet must travel through before reaching its destination. Each router is equal to one hop. A hop count of 4 indicates that a packet must pass through four routers to reach its destination. If multiple paths are available to a destination, the routing protocol, such as RIP, picks the path with the least number of hops. • Bandwidth: Bandwidth is the data capacity of a link, sometimes referred to as the "speed" of the link. For example, the Cisco implementation of the OSPF routing protocol uses bandwidth as its metric. The best path to a network is determined by the path that has an accumulation of links with the highest bandwidth values, that is, the fastest links.
equal-cost load balancing
When a router utilizes multiple paths with the same administrative distance and cost to a destination.
Basic Router Configuration
When configuring a router, certain basic tasks are performed, including the following: • Naming the router • Setting passwords • Configuring interfaces • Configuring a banner • Saving changes on a router • Verifying basic configuration and router operations
Automatic Network Discovery Network discovery
is a routing protocol's capability to share information about the networks it knows about with other routers that are also using the same routing protocol. Instead of configuring static routes to remote networks on every router, a dynamic routing protocol allows the routers to automatically learn about these networks from other routers. These networks and the best path to each network are added to the router's routing table and denoted as a network learned by a specific dynamic routing protocol.
Routing Table
routing table A table stored in the memory of a router or some other internetworking device that keeps track of routes to particular network destinations. A router uses this list of networks to determine where to send data.
Serial data transmission
serial Method of data transmission in which the bits of data characters are transmitted sequentially over a single channel.
When to Use Static Routes Static routes
should be used in the following cases: • A network consists of only a few routers: Using a dynamic routing protocol in such a case does not present a substantial benefit. On the contrary, dynamic routing can add more administrative overhead. • A network is connected to the Internet only through a single ISP: There is no need to use a dynamic routing protocol across this link because the ISP represents the only exit point to the Internet. • A large network is configured in a hub-and-spoke topology: A hub-and-spoke topology consists of a central location (the hub) and multiple branch locations (spokes), with each spoke having only one connection to the hub. Using a dynamic routing protocol would be unnecessary because each branch only has one path to a given destination: through the central location.
