Routers are Computers A router Is a computer, Just like any other computer Including a PC. The very first router, used for the Advanced Research Projects Agency Network (ARPANET), was the Interface Message Processor (IMP). The IMP was a Honeywell 31 6 minicomputer; this computer brought the ARPANET to life on August 30, 1969. Note: The ARPANET was developed by Advanced Research Projects Agency (ARPA) of the united States Department of Defense.
The was the world’s first operational packet switching network and the predecessor of today’s Internet. Routers have many of the name hardware and software components that are found in other computers including: ; CPU ; RAM ; ROOM ; Operating System Routers are at the network center A router connects multiple networks. This means that It has multiple Interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet onto its destination.
The interface that the router uses to forward the packet may be the network of the final destination of the packet (the network with the destination IP dress of this packet), or it may be a network connected to another router that is used to reach the destination network. Each network that a router connects to typically requires a separate interface. These interfaces are used to connect a combination of both Local Area Networks (Lana) and Wide Area Networks (WANT). Lana are commonly Ethernet networks that contain devices such as PC’s, printers, and servers.
Want are used to connect networks over a large geographical area. For example, a WAN connection Is commonly used to connect a LANA to the Internet Service Provider (ISP) network. Routers determine the best path The primary responsibility of a router is to direct packets destined for local and remote networks by: ; Determining the best path to send packets ; Forwarding packets toward their destination The router uses Its routing table to determine the best path to forward the packet.
When the router receives a packet, It examines Its destination IP address and searches for the best match with a network address In the router’s routing table. The routing table also Includes the Interface to be used to forward the packet. Once a match is found, the router encapsulates the IP packet into the data link frame of the s very likely that a router will receive a packet that is encapsulated in one type of data link frame, such as an Ethernet frame and when forwarding the packet, the router will encapsulate it in a different type of data link frame, such as Point-to-Point Protocol (APP).
The data link encapsulation depends on the type of interface on the router and the type of medium it connects to. The different data link technologies that a router connects to can include LANA technologies, such as Ethernet, and WAN serial connections, such as TTL connection using APP, Frame Relay, and Asynchronous Transfer Mode (ATM). Static routes and dynamic routing protocols are used by routers to learn about remote networks and build their routing tables.
Router CPU and Memory Router Components and their Functions Like a PC,a router also includes: ; Central Processing Unit (CUP’) ; Random-Access Memory (RAM) ; Read-only Memory (ROOM) CPU The CPU executes operating system instructions, such as system initialization, routing functions, and switching functions. RAM RAM stores the instructions and data needed to be executed by the CUP]. RAM is used Operating System: The Cisco ISO (Networking Operating System) is copied into RAM ring bottom. Running Configuration File: This is the configuration file that stores the configuration commands that the router ISO is currently using.
With few exceptions, all commands configure on the router are stored in the running configuration file, known as running-confining. IP Routing Table: This file stores information about directly connected and remote networks. It is used to determine the best path to forward the packet. ARP Cache: This cache contains the IPPP address to MAC address mappings, similar to the ARP cache on a PC. The ARP cache is used on routers that have LANA interfaces such as Ethernet interfaces. Packet Buffer: 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 content when the router is powered down or restarted. However, the router also contains permanent storage areas, such as ROOM, flash and NAVAL. ROOM ROOM is a form of permanent storage. Cisco devices use ROOM to store: bootstrap instructions ; Basic diagnostic software ; Scaled-down version of ISO ; The ROOM uses firmware, which is software that is embedded inside the integrated circuit. Firmware includes the software that does not normally need to be modified or upgraded, such as the bottom instructions. ROOM does not lose its contents when the router loses power or is restarted.
Flash Memory Flash memory is nonvolatile computer memory that can be electrically stored and erased. Flash is used as permanent storage for the operating system, Cisco ISO. Flash consists of SIMMs or 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. NAVAL NAVAL (Nonvolatile RAM) does not lose its information when power is turned off. This is in contrast to the most common forms of RAM, such as DRAM, that requires continual power to maintain its information.
NAVAL is used by the Cisco ISO as Operating System The operating system software used in Cisco routers is known as Cisco Networking Operating System (ISO). Like any operating system on any computer, Cisco ISO manages the hardware and software resources of the router, including memory allocation, processes, security, and file systems. Cisco ISO is a multitasking operating system that is integrated with routing, switching, intertwining, and telecommunications functions. Router Boot-up Process There are four major phases to the bottom process: 1. Performing the POST 2.
Loading the bootstrap program 3. Locating and loading the Cisco ISO software 4. Locating and loading the startup configuration file or entering setup mode The Power-on Self Test (POST) is a common process that occurs on almost every computer during bottom. The POST process is used to test the router hardware. When the router is powered on, software on the ROOM chip conducts the POST. During this self-test, the router executes diagnostics from ROOM on several hardware components including the CUP], RAM, and NAVAL. After the POST has been employed, the router executes the bootstrap program. . Loading the Bootstrap Program After the POST, the bootstrap program is copied from ROOM into RAM. Once in RAM, the CPU executes the instructions in the bootstrap program. The main task of the bootstrap program is to locate the Cisco ISO and load it into RAM. 3. Locating and Loading Cisco ISO Locating the Cisco ISO software. The ISO is typically stored in flash memory, but can also be stored in other places such as a TFTP (Trivial File Transfer Protocol) server. If a full ISO image can not be located, a scaled-down version of the ISO is copied from ROOM into RAM.
This version of ISO is used to help diagnose any problems and can be used to load a complete version of the ISO into RAM. Router Interfaces Management Ports Routers have physical connectors that are used to manage the router. These connectors are known as management ports. Unlike Ethernet and serial interfaces, management ports are not used for packet forwarding. The most common management port is the console port. The console port is used to connect a terminal, or most often a PC running terminal emulator software, to configure the router during initial configuration of the router.
Another management port is the auxiliary port. Not all routers have auxiliary ports. At times the auxiliary port can be used in ways similar to a console port. It can also be used to attach a modem. Auxiliary ports will not be used in this curriculum. 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 that are used to connect to multiple networks. Typically, the interfaces connect to various types of networks, which means that different types of media and connectors are required.
Often a router will need to have different types of interfaces. For example, a router usually has Fasteners interfaces for connections to different Lana and various types of WAN interfaces to connect a variety of serial links including T 1, DSL and KIDS. The figure shows the Fasteners and serial interfaces on the router. Like interfaces on a PC, the ports and interfaces on a router are located on the outside of the router. Their external location allows for convenient attachment to the appropriate network cables and connectors.
Interfaces Belong to Different Networks Router interfaces can be divided into two major groups: LANA interfaces – such as Ethernet and Fasteners WAN interfaces – such as serial, KIDS, and Frame Relay LANA Interfaces As the name indicates, LANA interfaces are used to connect the router to the LANA, similar to how a PC Ethernet INC is used to connect the PC to the Ethernet LANA. Like a PC Ethernet INC, a router Ethernet interface also has a Layer 2 MAC address and participates in the Ethernet LANA in the same way as any other hosts on that LANA.
For example, a router Ethernet interface participates in the ARP process for that LANA. The outer maintains an ARP cache for that interface, sends ARP requests when needed, and responds with ARP replies when required. A router Ethernet interface usually uses an RAG-45 Jack that supports unshielded twisted-pair (HTTP) cabling. When a router is connected to a switch, a straight-through cable is used. When two routers are connected directly through the Ethernet interfaces, or when a PC INC is connected directly to a router Ethernet interface, a crossover cable is used.
WAN Interfaces WAN interfaces are used to connect routers to external networks, usually over a such as APP, Frame Relay, and HADE (High-Level Data Link Control). Similar to LANA interfaces, each WAN interface has its own IP address and subnet mask, which identifies it as a member of a specific network. Routers and the Network Layer The main purpose of a router is to connect multiple networks and forward packets destined either for its own networks or other networks. A router is considered a Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet, specifically the destination IP address.
This process is known as routing. Routers Operate at Layers 1, 2, and 3 1. 3 Building the Routing Table Introducing the Routing Table A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. The routing table contains network/next hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the “next hop” on the way to the final destination.
The next hop association can also be the outgoing or exit interface to the final destination. The network/exit-interface association can also represent the destination network address of the IP packet. This association occurs on the router’s directly connected networks. A directly connected network is a network that is directly attached to one of the router interfaces. A remote network is a network that is not directly connected to the router. In other words, a remote network is a network that can only be reached by sending the packet dynamic routing protocol or by configuring static routes.
Dynamic routes are routes to remote networks that were learned automatically by the router, using a dynamic routing protocol. Static routes are routes to networks that a network administrator manually configure. When to Use Static Routes Static routes should be used in the following cases: ; A network consists of only a few routers. ; A network is connected to the Internet only through a single ISP. Network is configured in a hub-and-spoke topology. Dynamic Routing ; A large Dynamic routing protocols are used by routers to share information about the respectability and status of remote networks.
Dynamic routing protocols perform several activities, including: ; Network discovery ; Updating and maintaining routing tables Routing Table Principles At times in this course we will refer to three principles regarding routing tables that ill help you understand, configure, and troubleshoot routing issues. These principles are from Alex Sizing’s book, Cisco IP Routing. 1 . Every router makes its decision alone, based on the information it has in its own routing table. 2. The fact that one router has certain information in its routing table does not mean that other routers have the same information. 3.
Routing information about a path from one network to another does not provide routing information about the reverse, or return, path. Best Path Comparing Hop Count and Bandwidth Metrics Two metrics that are used by some dynamic routing protocols are: Hop count-Hop count 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 four 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. Peed of the link. For example, Coco’s implementation of the OSPF routing protocol uses bandwidth as its metric. The best path to a network is determined by the path with an accumulation of links that have the highest bandwidth values, or the fastest inks. Equal Cost Load Balancing You may be wondering what happens if a routing table has two or more paths with the same metric to the same destination network. When a router has multiple paths to a destination network and the value of that metric (hop count, bandwidth, etc. Is the same, this is known as an equal cost metric, and the router will perform equal cost load balancing. The routing table will contain the single destination network but will have multiple exit interfaces, one for each equal cost path. The router will forward packets using the multiple exit interfaces listed in the routing table. 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. One of three path determinations results from this search: ; 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, hat 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. Remote Network – If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router. ; No Route Determined – If the destination IP address of the packet does not belong to either a connected or remote network, and if the router does not have a default route, then the packet is discarded. The router sends an IGMP unreachable message Switching Function The switching function is the process used by a router to accept a packet on one interface and forward it out another interface.
A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link. What does a router do with a packet received from one network and destined for another network? The router performs the following three major steps: 1 . Decapitates the Layer 3 packet by removing the Layer 2 frame header and trailer. 2. Examines the destination IP address of the IP packet to find the best path in the routing table.