1. Chapter 1 Introduction to Routing and Packet Forwarding CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College graziani@cabrillo.edu Last Updated: 1/16/2008

2. 2 This Presentation This presentation is based on the Exploration course/book, Routing Protocols and Concepts. For a copy of this presentation and access to my web site for other CCNA, CCNP, and Wireless resources please email me for a username and password.  Email: graziani@cabrillo.edu  Web Site: www.cabrillo.edu/~rgraziani

3. 3 Note This chapter contains mostly introductory material. Most of not all of this information will be explained in more detail in later chapters or later courses.  The bootup process and the IOS are examined in a later course. Do not worry or focus too much on the details for now. This will all be examined and explained in the following chapters. The audio of the lecture for this presentation will be available on my web site after February 11, 2008 My web site is www.cabrillo.edu/~rgraziani. For access to these PowerPoint presentations and other materials, please email me at graziani@cabrillo.edu.

4. 4 For further information This presentation is an overview of what is covered in the curriculum/book. For further explanation and details, please read the chapter/curriculum. Book:  Routing Protocols and Concepts  By Rick Graziani and Allan Johnson  ISBN: 1-58713-206-0  ISBN-13: 978-58713- 206-3

5. 5 Topics Inside the Router  Routers are computers  Router CPU and Memory  Internetwork Operating System  Router Bootup Process  Router Ports and Interfaces  Routers and the Network Layer CLI Configuration and Addressing  Implementing Basic Addressing Schemes  Basic Router Configuration Building the Routing Table  Introducing the Routing Table  Directly Connected Networks  Static Routing  Dynamic Routing  Routing Table Principles Path Determination and Switching Function  Packet Fields and Frame Formats  Best Path and Metrics  Equal Cost Load Balancing  Path Determination  Switching Function

6. Inside the Router Routers are computers Router CPU and Memory Internetwork Operating System Router Bootup Process Router Ports and Interfaces Routers and the Network Layer

7. 7 Routers are Computers A router is a computer:  CPU, RAM, ROM, Operating System The first router: used for the Advanced Research Projects Agency Network (ARPANET):  IMP (Interface Message Processor)  Honeywell 516 minicomputer that brought the ARPANET to life on August 30, 1969. Leonard Kleinrock and the first IMP.

8. 8 Routers forwarding packets:  From the original source  To the final destination. A router connects multiple networks:  Interfaces on different IP networks  Receives a packet on one interface and determines which interface to forward it towards its destination. The interface that the router uses to forward the packet can be:  The network of the final destination of the packet  The destination IP address of this packet  A network connected to another router

9. 9 Router interfaces:  LAN  WAN

10. 10 Routers Determine the Best Path The router’s primary responsibility:  Determining the best path to send packets  Forwarding packets toward their destination

11. 11 Routers Determine the Best Path The routing table is used to determine the best path. Examines the destination IP address  searches for the best match with a network address in the router’s routing table. The routing table includes the exit interface to forward the packet.  Router encapsulates the IP packet into the data-link frame of the outgoing or exit interface Packet is the forwarded toward its destination

12. 12 Routers Determine the Best Path R1 receives the packet encapsulated in an Ethernet frame. After decapsulating the packet, the router uses the destination IP address of the packet to search the routing table for a matching network address. R1 (typo: R2 in book) found the static route 192.168.3.0/24, which can be reached out its Serial 0/0/0 interface. R1 (typo: R2 in book) will encapsulate the packet in a frame format appropriate for the outbound interface and then forward the packet. Note: More later on static and dynamic routes.

13. 13 Router CPU and Memory CPU - Executes operating system instructions  Random access memory (RAM) (RAM contents lost when power is off)  running copy of configuration file.  routing table  ARP cache Read-only memory (ROM)  Diagnostic software used when router is powered up.  Router’s bootstrap program  Scaled down version of operating system IOS Non-volatile RAM (NVRAM)  Stores startup configuration. (including IP addresses, Routing protocol) Flash memory - Contains the operating system (Cisco IOS) Interfaces - There exist multiple physical interfaces that are used to connect network. Examples of interface types:  Ethernet / fast Ethernet interfaces  Serial interfaces  Management interfaces

14. 14 Router physical characteristics

15. 15 Cisco IOS - Internetwork Operating System Responsible for managing the hardware and software resources of the router, including:  Allocating memory  Managing processes  Security  Managing file systems There are many different IOS images. An IOS image is a file that contains the entire IOS for that router.  depending on the model and the features within the IOS. For example, some features can include the ability to run Internet Protocol version 6 (IPv6) or a routing protocol such as Intermediate System–to–Intermediate System (IS-IS).

16. 16 Router Bootup Process (more in later course)

17. 17 Bootup Process Step 1: POST (Power On Self Test) Executes diagnostics from ROM on several hardware components, including the CPU,RAM, NVRAM Step 2: Loading Bootstrap Program Copied from ROM into RAM Executed by CPU Main task is to locate the Cisco IOS and load it into RAM Step 3: Locating the IOS 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 This version of IOS is used to help diagnose any problems and to try to load a complete version of the IOS into RAM. Step 4: Loading the IOS Some of the older Cisco routers ran the IOS directly from flash Current models copy the IOS into RAM for execution Might see a string of pound signs (#) while the image decompresses. Step 5: Locating the Config File Bootstrap program searches for the startup configuration file (startup- config), in NVRAM. This file has the previously saved configuration commands and parameters, Step 6: Loading the Config File If a startup configuration file is found in NVRAM, the IOS loads it into RAM as the running-config file and executes the commands. If the startup configuration file cannot be located, prompt the user to enter setup mode If setup mode not used, a default running-config file is created

18. 18 Bootup Process running-config IOS (running) startup-configIOS ios (partial) Bootup program

19. 19 Verify the router boot-up process show version command is used to view information about the router during the bootup process. Information includes:  IOS version  ROM bootstrap program  Location of IOS  CPU and amount of RAM  Interfaces  Amount of NVRAM  Amount of flash  Configuration register information

20. 20 Verify the router boot-up process

21. 21 Ports and Interfaces Port - normally means one of the management ports used for administrative access Interface normally refers to interfaces that are capable of sending and receiving user traffic. Note: However, these terms are often used interchangeably in the industry and even with IOS output.

22. 22 Management Ports Console port - Most common of the management ports  Used to connect a terminal,  Or most likely a PC running terminal emulator software, No need for network access to that router. The console port must be used during initial configuration of the router. Auxiliary (AUX) port Not all routers have auxiliary ports.  At times, can be used similarly to a console port  Can also be used to attach a modem. Note: Auxiliary ports will not be used in this curriculum.

23. 23 Router Interfaces 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 which may mean:  Various types of networks  Different types of media and connectors.  Different types of interfaces. For example, Fast Ethernet interfaces for connections to different LANs and also have different types of WAN interfaces used to connect a variety of serial links, including T1, DSL, and ISDN.

24. 24 Router Interfaces Every interface on the router:  Belongs to a different network  Is a host on a different IP network  Have 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. Note: A single interface on a router can be used to connect to multiple networks; however, this is beyond the scope of this course and is discussed in a later course.

25. 25 LAN Interfaces Examples: Ethernet and Fast Ethernet interfaces. Used to connect the router to the LAN, similar to how a PC’s Ethernet NIC.  Layer 2 MAC address  Participates in the Ethernet LAN the same way as any other hosts on that LAN.  Example: Address Resolution Protocol (ARP):  Maintains ARP cache for that interface  Sends ARP requests when needed  Responds with ARP replies when required Typically an RJ-45 jack (UTP).  Router to switch: straight-through cable.  Router to router via Ethernet interfaces, or PC’s NIC to router’s Ethernet interface: crossover cable.

26. 26 WAN Interfaces Example: serial, ISDN, and Frame Relay interfaces. Used to connect routers to external networks, usually over a larger geographical distance. The Layer 2 encapsulation can be different types including:  PPP  Frame Relay  HDLC (High-Level Data Link Control). Similar to LAN interfaces, each WAN interface has its own IP address and subnet mask, making it a member of a specific network. Note: MAC addresses are used only on Ethernet interfaces and are not on WAN interfaces. However, WAN interfaces use their own Layer 2 addresses depending on the technology. Layer 2 WAN encapsulation types and addresses are covered in a later course.

27. 27 Routers at the Network Layer 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 is known as routing. When a router receives a packet, it  examines the destination IP address.  If the destination IP address does not belong to any of the router’s directly connected networks, the router must forward this packet to another router.

28. 28 Routers at the Network Layer R1 receives the packet Examines the packet’s destination IP address Searches the routing table Forwards the packet onto R2. R2 receives the packet Examines the packet’s destination IP address Searches its routing table Forwards the packet out its directly connected Ethernet network to PC2 Sequence of events is explained in more detail later in this chapter.

29. 29 Routers Operate at Layers 1, 2, and 3 A router makes its primary forwarding decision at Layer 3, But also participates in Layer 1 and Layer 2 processes. After a router has examined the destination IP address and consulted its routing table to make its forwarding decision, then  forward that packet out the appropriate interface toward its destination. Encapsulate the Layer 3 IP packet into the data portion of a Layer 2 data-link frame appropriate for the exit interface. The Layer 2 frame will then be encoded into the Layer 1 physical signals used to represent these bits over the physical link.

30. 30 Routers Operate at Layers 1, 2, and 3 R1 receives the stream of bits on its interface. The bits passed up to Layer 2. R1 examines data-link frame’ s destination address to determine whether it matches the receiving interface.  If match, the data portion of the frame, the IP packet, is then passed up to Layer 3  R1 makes its routing decision. R1 then reencapsulates the packet into a new Layer 2 data-link frame and forwards it out the outbound interface (bits). The new Layer 2 data-link address is associated with that of the interface of the next-hop router (or final destination IP address).

31. CLI Configuration and Addressing Implementing Basic Addressing Schemes Basic Router Configuration

32. 32 CLI Configuration This is a review from CIS 81 (Networking Fundamentals Exploration 1) Basic Router Configuration:  Naming the router  Setting passwords  Configuring interfaces  Configuring a banner  Saving changes on a router  Verifying basic configuration and router operations

33. 33 Establishing a HyperTerminal session (next week) Take the following steps to connect a terminal to the console port on the router: Connect the terminal using the RJ-45 to RJ-45 rollover cable and an RJ-45 to DB-9 or RJ-45 to DB-25 adapter. Configure the terminal or PC terminal emulation software for 9600 baud, 8 data bits, no parity, 1 stop bit, and no flow control. Rollover cable Console port Com1 or Com2 serial port Terminal or a PC with terminal emulation software Router

34. 34 Establishing a HyperTerminal session Important: A console connection is not the same as a network connection! =

35. 35 NetLab

36. 36 Command Overview Router>user mode Router> enable Router#privilege mode Router# configure terminal Router(config)# exit Router# config t Router(config)# hostname name Router(config)# enable secret passwordprivilege password Router(config)# line console 0console password Router(config-line)# password password Router(config-line)# login Router(config)# line vty 0 4telnet password Router(config-line)# password password Router(config-line)# login Router(config)# banner motd # message #banner Router(config)# interface type numberconfigure interface Router(config-if)# ip address address mask Router(config-if)# description description Router(config-if)# no shutdown

37. 37 Other Commands Router# copy running-config startup-config Router# show running-config Router# show ip route Router# show ip interface brief Router# show interfaces

38. 38 Example

39. 39 Hostname and Privilege Password Router# config t Router(config)# hostname R1 R1(config)# enable secret class

40. 40 Passwords R1(config)# line console 0 R1(config-line)# password cisco R1(config-line)# login R1(config-line)# exit R1(config)# line vty 0 4 R1(config-line)# password cisco R1(config-line)# login

41. 41 Banner R1(config)# banner motd # Enter TEXT message. End with the character ‘#’. ****************************************** WARNING!! Unauthorized Access Prohibited!! ****************************************** # R1(config)#

42. 42 WAN Interface Configuration R1(config)# interface Serial0/0/0 R1(config-if)# ip address 192.168.2.1 255.255.255.0 R1(config-if)# description Link to R2 R1(config-if)# clock rate 64000DCE Only R1(config-if)# no shutdown

43. 43 LAN Interface Configuration R1(config)# interface FastEthernet0/0 R1(config-if)# ip address 192.168.1.1 255.255.255.0 R1(config-if)# description R1 LAN R1(config-if)# no shutdown

44. 44 Each Interface Belongs to a Different Network R1(config)# interface FastEthernet0/1 R1(config-if)# ip address 192.168.1.2 255.255.255.0 192.168.1.0 overlaps with FastEthernet0/0 R1(config-if)# no shutdown 192.168.1.0 overlaps with FastEthernet0/0 FastEthernet0/1: incorrect IP address assignment

45. 45 Each Interface Belongs to a Different Network R1# show ip interface brief Interface IP-Address OK? Method Status Protocol FastEthernet0/0 192.168.1.1 YES manual up up Serial0/0 192.168.2.1 YES manual up up FastEthernet0/1 192.168.1.2 YES manual administratively down down Serial0/1 unassigned YES unset administratively down down

46. 46 Verify Router Configuration R1# show running-config ! version 12.3 ! hostname R1 ! interface FastEthernet0/0 description R1 LAN ip address 192.168.1.1 255.255.255.0 ! interface Serial0/0 description Link to R2 ip address 192.168.2.1 255.255.255.0 clock rate 64000 ! banner motd ^C ****************************************** WARNING!! Unauthorized Access Prohibited!! ****************************************** ^C ! line con 0 password cisco login line vty 0 4 password cisco login ! end

47. 47 Save Configuration R1# copy running-config startup-config R1# show startup-config Using 728 bytes ! version 12.3 ! hostname R1 ! interface FastEthernet0/0 description R1 LAN ip address 192.168.1.1 255.255.255.0 ! interface Serial0/0 description Link to R2 ip address 192.168.2.1 255.255.255.0 clock rate 64000 ! banner motd ^C ****************************************** WARNING!! Unauthorized Access Prohibited!! ****************************************** ^C line con 0 password cisco login line vty 0 4 password cisco login ! end

48. 48 Show Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0

49. 49 Verifying Interfaces R1# show interfaces FastEthernet0/0 is up, line protocol is up (connected) Hardware is Lance, address is 0007.eca7.1511 (bia 00e0.f7e4.e47e) Description: R1 LAN Internet address is 192.168.1.1/24 MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, rely 255/255, load 1/255 Encapsulation ARPA, loopback not set ARP type: ARPA, ARP Timeout 04:00:00, Last input 00:00:08, output 00:00:05, output hang never Last clearing of “show interface” counters never Queueing strategy: fifo Output queue :0/40 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 0 packets input, 0 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles Serial0/0 is up, line protocol is up (connected) Hardware is HD64570 Description: Link to R2 Internet address is 192.168.2.1/24 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation HDLC, loopback not set, keepalive set (10 sec) Last input never, output never, output hang never

50. Building the Routing Table Introducing the Routing Table Directly Connected Networks Static Routing Dynamic Routing Routing Table Principles

51. 51 Introducing the Routing Table Routing table is a data file in RAM that is used to store route information about:  Directly connected  Remote networks R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0

52. 52 Introducing the Routing Table The routing table contains network/next-hop associations The “next hop” is the IP address of a next-hop router. (coming) May also include an outgoing or exit interface (more later) R1# show ip route C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Exit Interfaces

53. 53 Introducing the Routing Table directly connected network is a network that is directly attached to one of the router interfaces. When a router’s interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network. Active directly connected networks are added to the routing table. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks

54. 54 Introducing the Routing Table directly connected network is a network that is directly attached to one of the router interfaces. When a router’s interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network. Active directly connected networks are added to the routing table. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks

55. 55 Introducing the Routing Table A remote network is a network that is not directly connected to the router. A remote network is a network that can only be reached by sending the packet to another router. Remote networks are added to the routing table using  a 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 configured. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Remote Network

56. 56 Directly Connected Networks C: Source of the route information, directly connected network, static route, or a dynamic routing protocol.  The C represents a directly connected route. 192.168.1.0/24: The network address and subnet mask of the directly connected or remote network.  In this example, 192.168.1.0/24 is the directly connected network. FastEthernet 0/0: The exit interface and/or the IP address of the next-hop router.  In this example, both FastEthernet 0/0 is the exit interfaces used to reach these networks. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks

57. 57 Directly Connected Networks Before any static or dynamic routing is configured on a router, the router only knows about its own directly connected networks. These are the only networks that are displayed in the routing table until static or dynamic routing is configured. Static and dynamic routes cannot exist in the routing table without a router’s own directly connected networks. The router cannot send packets out an interface if that interface is not enabled with an IP address and subnet mask, just as a PC cannot send IP packets out its Ethernet interface if that interface is not configured with an IP address and subnet mask. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 Directly Connected Networks

58. 58 Static Routes Static route includes the network address and subnet mask of the remote network, along with the IP address of the next-hop router or exit interface. Note: Configuration of the static route is not shown. Static routes are denoted with the code S in the routing table, Static routes are examined in detail in the next chapter. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 S 192.168.3.0/24 [1/0] via 192.168.2.2c Static Route

59. 59 Dynamic Routes R1 has automatically learned about the 192.168.4.0/24 network from R2 through the dynamic routing protocol RIP (Routing Information Protocol). RIP was one of the first IP routing protocols and will be fully discussed in later chapters. Note: Configuration of RIP not shown. R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0/0 S 192.168.3.0/24 [1/0] via 192.168.2.2 R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0/0

60. 60 Routing Table Principles These principles, listed as follows, are from Alex Zinin’s book, Cisco IP Routing:  Every router makes its decision alone, based on the information it has in its own routing table.  The fact that one router has certain information in its routing table does not mean that other routers have the same information.  Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

61. 61 Asymmetric Routing 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. Asymmetric routing is more common in the Internet, which uses the BGP routing protocol, than it is in most internal networks.

62. Path Determination and Switching Functions Packet Fields and Frame Formats Best Path and Metrics Equal Cost Load Balancing Path Determination Switching Function

63. 63 Path Determination and Switching Functions The following sections focus on exactly what happens to data as it moves from source to destination.  Review the packet and frame field specifications  Discuss in detail how the frame fields change from hop to hop, whereas the packet fields remain unchanged

64. 64 Ethernet Frame Layer 2 addresses:  Interface-to-Interface on the same network.  Used to send to the next hop router or final destination.  Layer 2 source address: sending interface layer 2 address (if applicable)  Layer 3 destination address: destination interface layer 2 address (if applicable).  Changes from network to network. Layer 3 addresses:  Original source layer 3 address (IP)  Final destination layer 3 address (IP)  Does not change (except with NAT, but this is not a concern of IP but an internal network process) IPv4 (Internet Protocol)

65. 65 Router Paths and Packet Switching As a packet travels from one networking device to another  The Source and Destination IP addresses NEVER change  The Source & Destination Layer 2 (MAC) addresses CHANGE as packet is forwarded from one router to the next.  TTL field decrement by one until a value of zero is reached at which point router discards packet (prevents packets from endlessly traversing the network)

66. 66 Best Path Router’s best-path determination involves evaluating multiple paths to the same destination network and selecting the optimum or “shortest” path to reach that network. Depends upon routing protocol. RIP uses hop count whereas OSPF uses bandwidth (Cisco’s implementation of OSPF). Dynamic routing protocols use their own rules and metrics to build and update routing tables. A metric is the quantitative value used to measure the distance to a given route. The best path to a network is the path with the lowest metric. For example, a router will prefer a path that is five hops away over a path that is ten hops away.

67. 67 Best Path RIP uses hop count  R1 to R3  Fewer links but much slower OSPF uses bandwidth  R1 to R2 to R3  More routers but much faster links 1.5 Mbps

68. 68 Equal Cost Load Balancing What happens if a routing table has two or more paths with the same metric to the same destination network? (equal-cost metric) Router will perform equal-cost load balancing. The router will forward packets using the multiple exit interfaces as listed in the routing table. Static routes and all dynamic routing protocols perform equal cost load balancing. (More later)

69. 69 Equal-Cost Paths Versus Unequal-Cost Paths Just in case you are wondering, a router can send packets over multiple networks even when the metric is not the same if it is using a routing protocol that has this capability. This is known as unequal-cost load balancing. EIGRP and IGRP are the only routing protocols that can be configured for unequal-cost load balancing. (More in CCNP courses)

70. 70 Path Forwarding 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: Packet is forwarded directly to the device with the packet’s destination IP address.  Remote network: Packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.  No route determined: If 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. Packet forwarding involves two functions:  Path determination function  Switching function

71. 71 Path Forwarding 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? 1. Decapsulates 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 3. Encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface Packet forwarding involves two functions:  Path determination function  Switching function

72. 72 Remember: Encapsulation Now, let’s do an example… Destination IP Address Source IP Address Other IP fields Data Destination Address Source Address Type DataTrailer Layer 3 IP Packet Layer 2 Data Link Frame Current Data Link Address of Host or Router’s exit interface Next hop Data Link Address of Host or Router’s interface These change from host to router, router to router, and router to host. These addresses do not change!

73. 73 This is just a summary. The details will be shown next! Now for the details… Dest. MAC 00-10 Source MAC 0A-10 Type 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet Dest. MAC 0B-31 Source MAC 00-20 Type 800 TrailerDest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields DataDest. Add FF-FF Source AddType 800 Trailer

74. 74 From Host X to Router RTA Host X begins by encapsulating the IP packet into a data link frame (in this case Ethernet) with RTA’s Ethernet 0 interface’s MAC address as the data link destination address. How does Host X know to forward to packet to RTA and not directly to Host Y?  IP Source and IP Destination Addresses are on different networks How does Host X know or get RTA’s Ethernet address?  Checks ARP Table for Default Gateway IP Address and associated MAC Address. What if it there is not an entry in the ARP Table?  Host X sends an ARP Request and RTA sends an ARP Reply Dest. MAC 00-10 Source MAC 0A-10 Type 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet

75. 75 RTA 1. RTA examines Destination MAC address, which matches the E0 MAC address, so it copies in the frame. 2. RTA sees the Type field is 0x800, IP packet in the data field, a packet which needs to be routed. 3. RTA strips off the Ethernet frame. RTA looks up the Destination IP Address in its routing table. 192.168.4.0/24 has next-hop-ip address of 192.168.2.2 and an exit-interface of e1. Since the exit interface is on an Ethernet network, RTA must resolve the next-hop-ip address with a destination MAC address. 4. RTA looks up the next-hop-ip address of 192.168.2.2 in its ARP cache. If the entry was not in the ARP cache, the RTA would need to send an ARP request out e1. RTB would send back an ARP reply, so RTA can update its ARP cache with an entry for 192.168.2.2. 5. Packet is encapsulated into a new data link (Ethernet) frame. Dest. MAC 0B-31 Source MAC 00-20 Type 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet

76. 76 RTB 1. RTB examines Destination MAC address, which matches the E0 MAC address, and copies in the frame. 2. RTB sees Type field, 0x800, IP packet in the data field, a packet which needs to be routed. 3. RTB strips off the Ethernet frame. RTB looks up the Destination IP Address in its routing table. 192.168.4.0/24 has next-hop-ip address of 192.168.3.2 and an exit-interface of Serial0. Since the exit interface is not an Ethernet network, RTB does not have to resolve the next-hop-ip address with a destination MAC address. When the interface is a point-to-point serial connection, (like a pipe), RTB encapsulates the IP packet into the proper data link frame, using the proper serial encapsulation (HDLC, PPP, etc.). The data link destination address is set to a broadcast (there’s only one other end of the pipe). 5. Packet is encapsulated into a new data link (serial, PPP) frame and sent out the link. Dest. Add FF-FF Source AddType 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet

77. 77 RTC 1. RTC copies in the data link (serial, PPP) frame. 2. RTC sees the Type field is 0x800, IP packet in the data field, a packet which needs to be routed. 3. RTC strips off the data link, serial, frame. RTC looks up the Destination IP Address in its routing table. RTC realizes that this Destination IP Address is on the same network as one of its interfaces and it can sent the packet directly to the destination and not another router. Since the exit interface is on an directly connected Ethernet network, RTC must resolve the destination ip address with a destination MAC address. 2. RTC looks up the destination ip address of 192.168.4.10 in its ARP cache. If the entry was not in the ARP cache, the RTC would need to send an ARP request out e0. Host Y would send back an ARP reply, so RTC can update its ARP cache with an entry for 192.168.4.10. 5. Packet is encapsulated into a new data link (Ethernet) frame and sent out the interface. Dest. MAC 0B-20 Source MAC 0C-22 Type 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet

78. 78 Host Y Layer 2: Data Link Frame 1. Host Y examines Destination MAC address, which matches its Ethernet interface MAC address, and copies in the frame. 2. Host Y sees the Type field is 0x800, IP packet in the data field, which needs to be sent to its IP process. 3. Host Y strips off the data link, Ethernet, frame and sends it to its IP process. Layer 3: IP Packet 4. Host Y’s IP process examines the Destination IP Address to make sure it matches its own IP Address..  If it does not, the packet will be dropped. 5. The packet’s protocol field is examined to see where to send the data portion of this IP packet: TCP, UDP or other? Layer 4: TCP, UDP or other? Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet Dest. MAC 0B-20 Source MAC 0C-22 Type 800 Trailer

79. 79 The summary once again! Dest. MAC 00-10 Source MAC 0A-10 Type 800 Trailer Layer 2 Data Link Frame Dest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields Data Layer 3 IP Packet Dest. MAC 0B-31 Source MAC 00-20 Type 800 TrailerDest. IP 192.168.4.10 Source IP 192.168.1.10 IP fields DataDest. Add FF-FF Source AddType 800 Trailer

80. Chapter 1 Introduction to Routing and Packet Forwarding CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College graziani@cabrillo.edu