Introduction to Routing and Packet Forwarding g Routing Protocols and Concepts – Chapter ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Objectives Identify a router as a co pu e with a OS a d de y ou e computer an and hardware designed for the routing process Demonstrate the ability to configure devices and apply addresses Describe the structure of a routing table table Describe how a router determines a path and switches packets ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router as a Computer Describe the bas c purpose o a router esc be e basic pu pose of ou e -Computers that specialize in sending packets over the data network They are responsible for interconnecting networks by selecting the best path for a packet to travel and forwarding packets to their destination Routers have many of the same hardware and software components that are found in other computers p p including: –CPU –RAM –ROM –Operating System ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router as a Computer Router components and their functions” p CPU - Executes operating system instructions such as system initialization, routing functions, and switching functions Random access memory (RAM) -RAM stores the instructions and data needed to be executed by the CPU RAM is used to store these components: –Operating System: The Cisco IOS (Internetwork Operating System) is copied g p into RAM during bootup –Running Configuration File: This is the configuration file that stores the configuration commands that the router IOS is currently using –IP Routing Table: This file stores information about directly connected and g y remote networks It is used to determine the best path to forward the packet –ARP Cache: This cache contains the IPv4 address to MAC address mappings, similar to the ARP cache on a PC The ARP cache is used on routers that h h have LAN i interfaces such as E h f h Ethernet i interfaces f –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 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router as a Computer Router co po e s a d their functions” ou e components and e u c o s Read-only memory (ROM) - Holds diagnostic software used when router is powered up Stores the router’s bootstrap p g program –ROM is a form of permanent storage Cisco devices use ROM to store: –The bootstrap instructions –Basic diagnostic software –Scaled-down version of IOS ROM 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 bootup instructions – ROM does not lose its contents when the router loses power or is restarted ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router R t as a C Computer t Router components and their functions” Non-volatile RAM (NVRAM) - Stores startup configuration This may include IP addresses (Routing protocol Hostname of router) protocol, NVRAM (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 NVRAM is used by the Cisco IOS as permanent storage for the startup configuration file file 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 must be copied to NVRAM where it is stored as the startup-config file NVRAM retains its NVRAM, file contents even when the router reloads or is powered off Flash memory - Contains 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, where it is then executed by the CPU Flash consists of SIMMs or PCMCIA cards, which can be upgraded to increase the amount of flash memory memory Interfaces - There exist multiple physical interfaces that are used to connect network Examples of interface types: -Ethernet / fast Ethernet interfaces -Serial interfaces -Management interfaces ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router as a Computer Router co po e s ou e components ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Internetwork Operating System The operating system software used in Cisco routers is known as Cisco Internetwork Operating System (IOS) (IOS) – Cisco IOS is a multitasking operating system that is integrated with routing, switching, internetworking, and telecommunications functions Although the Cisco IOS may appear to be the same on many routers, there are many different IOS images – An IOS image is a file that contains the entire IOS for that router Cisco creates many different types of IOS images, depending upon the model of the router and the features within the IOS – Typically the more features in the IOS, the larger the IOS image, and therefore, the more flash and RAM that is required to store and load the IOS Although some routers provide a graphical user interface (GUI), the command li i t f d line interface (CLI) i a much more common method of is h th d f configuring Cisco routers – The CLI is used throughout this curriculum Upon bootup the startup config file in NVRAM is copied into RAM and bootup, startup-config stored as the running-config file – IOS executes the configuration commands in the running-config Any changes entered by the network administrator are stored in the runningconfig and are immediately implemented by the IOS IOS ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Overview - Managing Cisco IOS Software (cont) ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public Router as a Computer Major p ases to the ajo phases o e router boot-up process Test router hardware Power-On Self Test (POST) Execute bootstrap loader p Locate & load Cisco IOS software -Locate IOS Locate -Load IOS Locate & load startup configuration file or enter setup mode p program looks -Bootstrap p g for configuration file ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 10 Router Paths and Packet Switching R1 Step - R1 receives Ethernet frame R1 sees that destination MAC address matches its own MAC R1 then strips off Ethernet frame R1 Examines destination IP R1 consults routing table looking for destination IP After finding destination IP in routing table, R1 now looks up next hop IP address R1 re-encapsulates IP packet with a new Ethernet frame f the entry is not in the ARP cache, R1 sends an ARP request out its FastEthernet 0/1 interface R2 sends back an ARP reply R1 forwards Ethernet packet out Fa0/1 interface ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 86 Router Paths and Packet Switching Path determination and switching function details PC1 Wants to send something to PC here is part of what happens Step - Packet arrives at R2 R2 receives Ethernet frame R2 sees that destination MAC address matches its own MAC R2 then strips off Ethernet frame R2 Examines destination IP R2 consults routing table looking for destination IP After finding destination IP in routing table, R2 now looks up next hop IP address R2 re-encapsulates IP packet with a new data link frame R2 forwards Ethernet packet out S0/0 interface R2 When the interface is a point-to-point serial connection, R2 encapsulates the IP packet into the proper data link frame format used by the exit interface (HDLC, PPP, etc.) In this case, the Layer encapsulation is PPP; therefore, the data link destination address is set to a broadcast Remember, there are no MAC addresses on serial interfaces ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 87 Router Paths and Packet Switching PC1 Wants to send something to PC here is part of what happens Step - Packet arrives at R3 R3 receives PPP f frame R3 then strips off PPP frame R3 Examines destination IP R3 consults routing table looking for destination IP After finding destination IP in routing table, R3 is directly connected to destination via its fast Ethernet interface If the entry is not in the ARP cache, R3 sends an ARP request out its cache FastEthernet 0/0 interface PC2 sends back an ARP reply with its MAC address R3 re-encapsulates IP packet with a new Ethernet frame p R3 forwards Ethernet packet out Fa0/0 interface Step - IP packet arrives at PC2 Frame is decapsulated & processed by upper layer protocols ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 88 Packet propagation and switching within a router 89 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 89 Packet propagation and switching within a router 90 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 90 Packet propagation and switching within a router 91 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 91 Packet propagation and switching within a router 92 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 92 Packet propagation and switching within a router 93 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 93 Packet P k t propagation and switching within a router ti d it hi ithi t 94 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 94 Packet P k propagation and switching within a router i d i hi i hi 95 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 95 Packet propagation and switching within a router 96 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 96 Packet P k t propagation and switching within a router ti d it hi ithi t 97 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 97 Summary Routers are computers that specialize in sending data over a network Routers are composed of: -Hardware i.e CPU, Memory, System bus, Interfaces -Software used to direct the routing process Software IOS Configuration file Routers need to be configured Basic configuration consists of: -Router name -Router banner Router -Password(s) -Interface configurations i.e IP address and subnet mask ITE PC v4.0 Chapter Routing tables contain the following information -Directly connected networks -Remotely connected networks Remotely -Network addresses and subnet masks -IP address of next hop address © 2007 Cisco Systems, Inc All rights reserved Cisco Public 98 Summary Routers determine a packets path to its destination by doing the following Receiving an encapsulated frame & examining destination MAC address address If the MAC address matches then Frame is de-encapsulated so that router can examine the destination IP address If destination IP address is in routing table or there is a static route then Router determines next hop IP address Router will re encapsulate re-encapsulate packet with appropriate layer frame and send it out to next destination Process continues until packet reaches destination Note - only the MAC addresses will change the source and destination IP addresses not change ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 99 ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 100 ... Addresses R1(config)#interface Serial0/0/0 R1(config-if)#ip address 19 2 .16 8.2 .1 255.255.255.0 R1(config-if)#description Ciruit#VBN3269 6 -1 23 (help desk: 1- 8 0 0-5 5 5 -1 234) R1(config-if)#no shutdown R1(config-if)#clock... l R1(config-line)#password cisco R1(config-line)#login R1(config-line)#exit R1(config)#line vty R1(config-line)#password cisco R1(config-line)#login ( g ) g R1(config-line)#exit ITE PC v4.0 Chapter. .. Types of router interfaces: -Ethernet Ethernet -Fastethernet -Serial -DSL -ISDN -Cable Cable ITE PC v4.0 Chapter © 2007 Cisco Systems, Inc All rights reserved Cisco Public 31 Two major groups of Router