Chapter 1 Introduction to Routing and Packet Forwarding

1. Chapter 1Introduction to Routing and Packet Forwarding CIS 82 Routing Protocols and Concepts Rick Graziani Cabrillo College graziani@cabrillo.edu Last Updated: 2/16/2009

2. This Presentation • For detailed information see the notes section within this PowerPoint. • 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. 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.

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. Topics • 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 • 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 • 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. Routers are Computers Leonard Kleinrock and the first IMP. • 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.

8. Routers forwarding packets (packet switching): • From the original source to the final destination. • Selects best path based on destination IP address • A router connects multiple networks: • Interfaces on different IP networks

9. Router interfaces: • LAN • WAN

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

11. Routers Determine the Best Path IP Packet enters router’s Ethernet interface. Router examines the packet’s destination IP address. • Routing table • Determines best path. • Best match between destination IP address and network address in routing table Router searches for a best match between packet’s destination IP address and network address in routing table. Using the exit-interface in the route, the packet is forwarded to the next router or the final destination.

12. Router CPU and Memory • CPU - Executes operating system instructions • Random access memory (RAM) • 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

13. Router physical characteristics

14. Cisco IOS - Internetwork Operating System • Responsible for managing the hardware and software resources: • Allocating memory • Managing processes • Security • Managing file systems • Manydifferent IOS images. • An IOS image is a file that contains the entire IOS for that router. • Routermodel • IOS features • Example IPv6 or a routing protocol such as Intermediate System–to–Intermediate System (IS-IS).

15. Router Bootup Process (more in later course)

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

17. Where is the permanent configuration file stored used during boot-up? NVRAM Where is the diagnostics software stored executed by hardware modules? ROM Where is the backup (partial) copy of the IOS stored? ROM Where is IOS permanently stored before it is copied into RAM? FLASH Where are the bootsystem commands stored which are used to locate the IOS? NVRAM running-config startup-config IOS Bootup program IOS (running) ios (partial)

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

19. startup-config running-config Bootup program IOS (running) IOS ios (partial) running-config IOS startup-config Bootup program IOS (running) ios (partial)

20. Router Boot Process – Details (later) 1. ROM 1. POST 2. Bootstrap code executed 3. Check Configuration Register value (NVRAM) 0 = ROM Monitor mode 1 = ROM IOS 2 - 15 = startup-config in NVRAM 2. Check for IOS boot system commands in startup-config file (NVRAM) If boot system commands in startup-config a. Run boot system commands in order they appear in startup-config to locate the IOS b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM) 3.Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config a. Flash (sequential) b. TFTP server (netboot) - The router uses the configuration register value to form a filename from which to boot a default system image stored on a network server. c. ROM (partial IOS) or keep retrying TFTP depending upon router model - If no IOS located, get partial IOS version from ROM 4.Locate and load startup-configconfiguration a. If startup-config found, copy to running-config b. If startup-config not found, prompt for setup-mode c. If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)

21. Verify the router boot-up process • show version command is used to view information about the router during the bootup process (later).

22. 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.

23. Management Ports Console port • Terminal • PC running terminal emulator software • No need for network access • Used for initial configuration 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.

24. Router Interfaces • Interfaces - Receive and forward packets. • Various types of networks • Different types of media and connectors. • Different types of interfaces. • Fast Ethernet interfaces - LANs • Serial interfaces - WAN connections including T1, DSL, and ISDN

25. Router Interfaces FastEthernet 0/0 MAC: 0c00-41cc-ae12 10.1.0.1/16 FastEthernet 0/0 MAC: 0c00-3a44-190a 192.168.1.1/24 • Router Interface: • Different network • IP address and subnet mask of thatnetwork • Cisco IOS will not allow two active interfaces on the same router to belong to the same network. Serial 0/0 Serial 0/1 172.16.1.1/24 172.16.1.2/24

26. LAN Interfaces • Ethernet and Fast Ethernet interfaces • Connects the router to the LAN • Layer 2MAC address • Participates in the Ethernet • 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: crossover cable

27. WAN Interfaces • Point-to-Point, ISDN, and Frame Relay interfaces • Connects routers to external networks. • The Layer 2 encapsulation can be different types including: • PPP • Frame Relay • HDLC (High-Level Data Link Control). • Note: MAC addresses are used only on Ethernet interfaces and are not on WAN interfaces. • Layer 2 WAN encapsulation types and addresses are covered in a later course.

28. Routers at the Network Layer • Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet (destination IP address). • This is known as routing.

29. Routers Operate at Layers 1, 2, and 3

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

31. 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

32. Ethernet Frame IPv4 (Internet Protocol) • Layer 2 addresses: • Interface-to-Interface on the same network. • Changes as packet is decapsulated and encapsulated 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)

33. Best Path • Router’s best-path to a network: • optimum or “shortest” path • Routing protocol dependent • Dynamic routing protocols use their own rules and metrics. • 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. • Example, a router will prefer a path that is one hop away over a path that is two hops away.

34. Best Path 1.5 Mbps 1.5 Mbps • Comparing Dynamic Routing Protocols: RIP and OSPF • 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

35. Equal Cost Load Balancing To reach the 192.168.1.0/24 network it is 2 hops via R2 and 2 hops via R4. ? ? 192.168.1.0/24 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.

36. Equal-Cost Paths Versus Unequal-Cost Paths T1 T3 192.168.1.0/24 Can a router use multiple paths if the paths (cost, metric) to reach the destination network are not equal? Yes, if the routers are using the EIGRP routing protocol which supports unequal cost load balancing.

37. Path Forwarding • Packet forwarding involves two functions: • Path determination function • Switching function

38. Path Forwarding Router receives packet. Destination IP address matches a network on one of its directly connected networks. Packet is forwarded out that network. Directly connected network • 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 • Remote network • No route determined

39. Path Forwarding Router receives packet. Destination IP address matches a remote network which can only be reached via another router. Packet is forwarded out that network to the next-hop router. Remote network • 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 • Remote network • No route determined

40. Path Forwarding Router receives packet. Destination IP address does NOT match any network in the router’s routing table. Packet is dropped. No route determined • 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 • Remote network • No route determined Does this mean the network does not exist? No, only that the router does not know about that network. (later)

41. 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.

42. 192.168.4.10 Path Forwarding 192.168.1.10 Layer 2 Data Link Frame Layer 3 IP Packet What does a router do with a packet received from one network and destined for another network? • Decapsulates the Layer 3 packet by removing the Layer 2 frame header and trailer • Examines the destination IP address of the IP packet to find the best path in the routing table • Encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface

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

44. Layer 2 Data Link Frame Layer 3 IP Packet • This is just a summary. • The details will be shown next! • Now for the details…

45. Layer 2 Data Link Frame Layer 3 IP Packet 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

46. Layer 2 Data Link Frame Layer 3 IP Packet 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.

47. Layer 2 Data Link Frame Layer 3 IP Packet 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.

48. Layer 2 Data Link Frame Layer 3 IP Packet 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.

49. Layer 2 Data Link Frame Layer 3 IP Packet 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?

50. Layer 2 Data Link Frame Layer 3 IP Packet • The summary once again!