1. 1 Sybex CCNA 640-802 Chapter 6: IP Routing

2. Chapter 6 Objectives Understanding IP routing Static routing Default routing Dynamic routing –RIP –RIPv2 –IGRP –Verifying routing –[Oddly, the exam topics covered in this chapter (6) are listed at the beginning of the chapter. Some of the topics listed are not really covered in this chapter at all. For example, OSPF and EIGRP are covered in chapter 7, not chapter 6. ] 2 2

3. What is Routing? 3 - 329 In order to “route”, a router needs to know: –Remote Networks –Neighbor Routers –All Possible routes to remote network –The absolute best route to all remote networks –Maintain and verify the routing information –Remember: a router does not deal with hosts! –A router only deals with networks, and the best path to them –An IP address allows packets to move from network to network –Hardware (Mac) addresses move the packets to specific hosts CB A D

4. Basic Path Selection 4 On what interface will the router send out a packet if it has destination address of 10.10.10.18?

5. Simple IP Routing 5 172.16.1.0 B A 172.16.2.0 172.16.2.2 172.16.1.2 172.16.2.1 172.16.1.1 e0 >ping 172.16.1.2 B 172.16.3.1 172.16.3.2 s0 Host A Host B

6. Routing/PDU Example: Host A Web browses to the HTTP Server…. 6 3. The destination port number in a segment header will have a value of 80 (the port number used by HTTP) 1. The destination address of a frame will be the: Host A address 2. The destination IP address of a packet will be the IP address of the: Destination Router

7. Idea of routing (5 guest slides) Routers forward datagrams between connected networks They need to know via which interface to send a datagram Routing decisions are based on the information stored in the routing table

8. Routing table Tells where to send datagram for a particular network Network Next-Hop Port Metric 194.181.200.0 194.181.208.1 Eth0 1 193.2.1.0 194.181.208.320 Eth1 14 153.5.0.0 194.181.214.25 Fddi0 8 0.0.0.0 194.181.210.1 S0 5 l “Next-Hop” routers must be directly reachable

9. Routing table (cont.) Default Route - a special entry in the routing table: –“Pass all datagrams for unknown networks to this router” –Represented by the entry for network 0.0.0.0 Routing uses network part of the address!

10. Routing Algorithm Extract destination IP address from datagram Extract network address from the IP address If destination network equals my network –Send directly to destination using physical network Else If destination address matches a host- specific route in the routing table: –Send to the router specified in the routing table

11. Routing Algorithm (cont.) Else if destination network matches a network in the routing table –Send to the router specified in the routing entry Else If there is a default route in the routing table: –Send to the router specified in the default route entry Else: –Send a “No route to host” message to the source

12. Step-by-Step: IP Routing Process (book, pp 331-36) The IP routing process is fairly simple and doesn’t change, regardless of the size of your network. For an example, we’ll use Figure 6.2 to describe step-by-step what happens when Host_A wants to communicate with Host_B on a different network 12 / 331

13. Step 1 Internet Control Message Protocol (ICMP) creates an “echo request” payload (which is just the alphabet in the data field). –The echo request is the first part/half of what is commonly called a “Ping”; the second part is the echo reply, from the device being “pinged”. [So, A is going to “ping” B] 13

14. Step 2 ICMP hands that payload to Internet Protocol (IP), which then creates a packet. At a minimum, this packet contains an IP source address, an IP destination address, and a Protocol field with 01h. (Remember that Cisco likes to use 0x in front of hex characters, so this could look like 0x01.) All of that tells the receiving host to whom it should hand the payload when the destination is reached—in this example, ICMP. 14

15. Step 3 Once the packet is created, IP determines whether the destination IP address is on the local network or a remote one. 15

16. Step 4 Since IP determines that this is a remote request, the packet needs to be sent to the default gateway so the packet can be routed to the remote network. The Registry in Windows is “parsed” to find the configured default gateway. 16

17. The default gateway of host 172.16.10.2 (Host_A) is configured to 172.16.10.1. For this packet to be sent to the default gateway, the hardware address of the router’s interface Ethernet 0 (configured with the IP address of 172.16.10.1) must be known. Why? So the packet can be handed down to the Data Link layer, framed, and sent to the router’s interface that’s connected to the 172.16.10.0 network. Because hosts only communicate via hardware addresses on the local LAN, it’s important to recognize that for Host_A to communicate to Host_B, it has to send packets to the Media Access Control (MAC) address of the default gateway. 17 Step 5

18. Next, the Address Resolution Protocol (ARP) cache of the host is checked to see if the IP address of the default gateway has already been resolved to a hardware address. Two possibilities ensue: 1. If it has, the packet is then free to be handed to the Data Link layer for framing. (The hardware destination address is also handed down with that packet.) To view the ARP cache on your host, use the following command: C:\>arp -a Interface: 172.16.10.2 --- 0x3 Internet Address Physical Address Type 172.16.10.1 00-15-05-06-31-b0 dynamic 2. If the hardware address isn’t already in the ARP cache of the host, an ARP broadcast is sent out onto the local network to search for the hardware address of 172.16.10.1. The router responds to the request and provides the hardware address of Ethernet 0, and the host caches this address. 18 Step 6

19. Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). A LAN driver provides communication control between the NOS and NIC (network interface card). A frame is then generated, encapsulating the packet with control info. Within that frame are the hardware destination and source addresses plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP. At the end of the frame is that Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC). The frame would look something like what is detailed in Figure 6.3. It contains Host_A’s hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host’s MAC address—remember that! 19 Step 7 Destination MACSource MAC Ether-Type field Packet FCS (CRC) (routers E0 MAC address)(Host_A MAC address) FIGURE 6. 3 Frame used from Host_A to the Lab_A router when Host_B is pinged

20. Once the packet and destination hardware address are handed to the Data Link layer, the LAN driver is used to provide media access via the type of LAN being used (in this example, Ethernet). A frame is then generated, encapsulating the packet with control information. Within that frame are the hardware destination and source addresses plus, in this case, an Ether-Type field that describes the Network layer protocol that handed the packet to the Data Link layer—in this instance, IP. At the end of the frame is the Frame Check Sequence (FCS) field that houses the result of the cyclic redundancy check (CRC). The frame would look something like what is detailed in Figure 6.3. It contains Host_A’s hardware (MAC) address and the destination hardware address of the default gateway. It does not include the remote host’s MAC address— remember that! 20 Step 7 Destination MACSource MAC Ether-Type field Packet FCS (CRC) (routers E0 MAC address)(Host_A MAC address) FIGURE 6. 3 Frame used from Host_A to the Lab_A router when Host_B is pinged

21. Once the frame is completed, it’s handed down to the Physical layer to be put on the physical medium (in this example, twisted-pair wire) one bit at a time. 21 Step 8

22. Every device in the collision domain receives these bits and builds the frame. They each run a CRC and check the answer in the FCS field. If the answers don’t match, the frame is discarded. If the CRC matches, then the hardware destination address is checked to see if it matches too (which, in this example, is the router’s interface Ethernet 0). If it’s a match, then the Ether-Type field is checked to find the protocol used at the Network layer. 22 Step 9

23. 23 Step 10 The packet is pulled from the frame, and what is left of the frame is discarded. The packet is handed to the protocol listed in the Ether-Type field — i.e., it’s given to IP. –[So now the packet is at the router, having entered at interface E0, the default gateway for the 172.16.10.0 network. –Next, the router will try to send the packet to its destination in the 172.16.20.0 network. –To do so, it will have to find this network in its routing tables.]

24. IP receives the packet and checks the IP destination address. Since the packet’s destination address doesn’t match any of the addresses configured on the receiving router itself, the router will look up the destination IP network address in its routing table. 24 Step 11

25. The routing table must have an entry for the network 172.16.20.0 or the packet will be discarded immediately and an ICMP message will be sent back to the originating device with a “destination network unreachable” message. –[Note that 172.16.x.x is a Class B network..10 and.20 would ordinarily be part of the same network and therefore couldn’t be set up on 2 networks. But this network is subnetted, i.e., the subnet mask is 255.255.255.0. 25 Step 12

26. If the router does find an entry for the destination network in its table, the packet is switched to the exit interface—in this example, interface Ethernet 1. The output below (next slide) displays the Lab_A router’s routing table. The “C” means “directly connected.” No routing protocols are needed in this network since all (both) networks are directly connected. 26 Step 13

27. Lab_A>sh ip route Codes: C – connected, S – static, I - IGRP,R - RIP,M - mobile, – 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 intearea * - candidate default, U - per-user static route, o – ODR P - periodic downloaded static route Gateway of last resort is not set 172.16.0.0/24 is subnetted, 2 subnets C 172.16.10.0 is directly connected, Ethernet0 C 172.16.20.0 is directly connected, Ethernet1 27 Step 13 (continued)

28. The router packet-switches the packet to the Ethernet 1 buffer. –[OK, ready to go out to Host_B, but first …] 28 Step 14

29. The Ethernet 1 buffer needs to know the hardware address of the destination host and first checks the ARP cache. –If the hardware address of Host_B has already been resolved and is in the router’s ARP cache, then the packet and the hardware address are handed down to the Data Link layer to be framed. –Let’s take a look at the ARP cache on the Lab_A router by using the “show ip arp” command: Lab_A#sh ip arp Protocol Address Age(min) Hardware Addr Type Interface Internet 172.16.20.1 - 00d0.58ad.05f4 ARPA Ethernet0 Internet 172.16.20.2 3 0030.9492.a5dd ARPA Ethernet0 Internet 172.16.10.1 - 00d0.58ad.06aa ARPA Ethernet0 Internet 172.16.10.2 12 0030.9492.a4ac ARPA Ethernet0 –The dash (-) means that this is the physical interface on the router. 29 Step 15

30. From the output in the previous slide, we can see that the router knows the 172.16.10.2 (Host_A) and 172.16.20.2 (Host_B) hardware addresses. –Cisco routers will keep an entry in the ARP table for 4 hours. If the hardware address has not already been resolved, the router sends an ARP request out E1 looking for the hardware address of 172.16.20.2. Host_B responds with its hardware address, and the packet and destination hardware address are both sent to the Data Link layer for framing. 30 Step 15 (continued)

31. The Data Link layer creates a frame with the destination and source hardware address, Ether-Type field, and FCS field at the end. –[Still a small packet – just four fields] The frame is handed to the Physical layer to be sent out on the physical medium one bit at a time. –[Now we see packets actually going to Host_B] 31 Step 16

32. Host_B receives the frame and immediately runs a CRC. [finally!!] If the result matches what’s in the FCS field, the “hardware destination address” is then checked. If the host finds a match, the Ether-Type field is then checked to determine the protocol that the packet should be handed to at the Network layer — IP in this example. –[IP is by far the most common Layer 3 protocol.] –[Moving up the OSI model. Data Link to Network] m as s Step 17

33. At the Network layer, IP receives the packet and checks the IP destination address. Since there’s finally a match made, the Protocol field is checked to find out to whom the payload should be given. 33 Step 18

34. The payload is handed to ICMP, which understands that this is an echo request. ICMP responds to this by immediately discarding the packet and generating a new payload as an echo reply. 34 Step 19

35. A packet is then created, including the –source and destination addresses, –Protocol field, and –payload. The destination device is now Host_A 35 Step 20

36. IP then checks to see whether the destination IP address is a device on the local LAN or on a remote network. Since the destination device is on a remote network, the packet needs to be sent to the default gateway. 36 Step 21

37. The default gateway IP address is found in the Registry of the Windows device, and the ARP cache is checked to see if the hardware address has already been resolved from an IP address. –You can search the Registry by going into the Registry Editor (start/Run/regedit), then searching for “DefaultGateway” (F3 – enter search parameters). –See “Default” / “DHCP Default Gateway” next slide 37 Step 22

38. 38 Step 22 (continued) Above is a view of my home computer’s Registry settings: HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\longkey\Parameters\Tcpip

39. Once the hardware address of the default gateway is found, the packet and destination hardware addresses are handed down to the Data Link layer for framing. 39 Step 23

40. The Data Link layer frames the packet of information and includes the following in the header: 1. The destination & source hardware addresses 2. The Ether-Type field [with 0x0800 (IP) in it] 3. The FCS field with the CRC result in tow 40 Step 24

41. The frame is now handed down to the Physical layer to be sent out over the network medium one bit at a time. 41 Step 25

42. The router’s Ethernet 1 interface receives the bits and builds a frame. The CRC is run, and the FCS field is checked to make sure the answers match. 42 Step 26

43. Once the CRC is found to be okay, the hardware destination address is checked. Since the router’s interface is a match, the packet is pulled from the frame and the Ether- Type field is checked to see to what protocol at the Network layer the packet should be delivered. 43 Step 27

44. The protocol is determined to be IP, so it gets the packet. IP runs a CRC check on the IP header first and then checks the destination IP address. –IP does not run a complete CRC as the Data Link layer does—it only checks the header for errors. 44 Step 28

45. Since the IP destination address doesn’t match any of the router’s interfaces, the routing table is checked to see whether it has a route to 172.16.10.0. If it doesn’t have a route over to the destination network the packet will be discarded immediately This is the source-point of confusion for a lot of administrators—when a ping fails, most people think the packet never reached the destination host. But as we see here, that’s not always the case! All it takes is for just one of the remote routers to be lacking a route back to the originating host’s network and — poof ! — the packet is dropped on the return trip, not on its way to the host. 45 Informational note: Between 29 and 30

46. Just a quick note to mention that if the packet is lost on the way back to the originating host, you will typically see a “request timed out” message, because it is an unknown error. If the error occurs because of a known issue, (such as if a route is not in the routing table) on the way to the destination device, you will see a “destination unreachable” message. This should help you determine if the problem occurred on the way to the destination or on the way back. 46 Troubleshooting note: Between 29 and 30

47. In this case, the router does know how to get to network 172.16.10.0 — the exit interface is Ethernet 0 — so the packet is switched to interface Ethernet 0. 47 Step 29

48. The router checks the ARP cache to determine whether the hardware address for 172.16.10.2 has already been resolved. 48 Step 30

49. Since the hardware address to 172.16.10.2 is already cached from the originating trip to Host_B, the hardware address and packet are handed to the Data Link layer. 49 Step 31

50. The Data Link layer builds a frame with the destination hardware address and source hardware address and then puts IP in the Ether-Type field. A CRC is run on the frame and the result is placed in the FCS field. 50 Step 32

51. The frame is then handed to the Physical layer to be sent out onto the local network one bit at a time. 51 Step 33

52. The destination host receives the frame, runs a CRC, checks the destination hardware address, and looks in the Ether-Type field to find out to whom to hand the packet. 52 Step 34

53. IP is the designated receiver, and after the packet is handed to IP at the Network layer, it checks the protocol field for further direction. IP finds instructions to give the payload to ICMP, and ICMP determines the packet to be an ICMP echo reply. 53 Step 35

54. ICMP acknowledges that it has received the reply by sending an exclamation point (!) to the user interface. ICMP then attempts to send four more echo requests to the destination host. The End 54 Step 36

55. These steps are the basic routing process, no matter how large the network. –There would just be more hops in a big internetwork. Point to recap: –Moving from router to router in a big internetwork, at each hop the hardware address changes; from one router’s Mac address to the next’s. –But from hop to hop, the IP address remains the same! –This reflects the fact that hardware addresses (Mac) are always local, while logical addresses (IP, for example), are always remote. I.e., in a local LAN, you always use a Mac addrss, not IP. 55 Post Script

56. Example 1: pp 336-37 – Here, the point is that if you have multiple hosts communicating to the server using HTTP, they must all use a different source port number. That is how the server keeps the data separated at the Transport layer. Example 2: p 337ff – Switches have nothing to do with routing! Example 3: p 338 – ICMP error messages are sent by the router with the problem device, such as an interface which is down. 56 Exercises: Test IP Routing Understanding Key Points: pp 336 - 362

57. Look at the output of a corporate router’s routing table: Corp#sh ip route[output cut]R 192.168.215.0 [120/2] via 192.168.20.2, 00:00:23, Serial0/0R 192.168.115.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0R 192.168.30.0 [120/1] via 192.168.20.2, 00:00:23, Serial0/0C 192.168.20.0 is directly connected, Serial0/0C 192.168.214.0 is directly connected, FastEthernet0/0 The corporate router received an IP packet with a source IP address of 192.168.214.20 and a destination address of 192.168.22.3, what do you think the Corp router will do with this packet? Since the routing table doesn’t show a route to network 192.168.22.0 (or a default route), the router will discard the packet and send an ICMP “destination unreachable” message back out interface FastEthernet 0/0 Normally, a router will have a default route set up, AKA a “gateway of last resort”. 57 Exercises: Test IP Routing Understanding Key Points: pp 338-39

58. This is a project that runs from pp 336 to 362. Setup: 5 Routers and an wireless Access Point Neither of our network simulators has these routers, so all we can do is read over the configurations. Notes: –P.345: With an ISR router, no need to use the “clock rate” command; they automatically detect it. –P346: See the interface “serial 0/0/1”. The book explains the way interfaces are labeled in a couple of places: Pg 184 and 195: “x/y/z Slot/Subslot/Port” (brief) 58 Configuring IP Routing

59. Configuring IP Routing (continued) Notes: (continued) –Page 205: Better explanation here: –Some modular routers use three numbers instead of two. –The first 0 is the router itself, and then you choose the slot, and then the port. Here’s an example of a serial interface on a 2811: Todd(config)#interface serial ? Serial interface number Todd(config)#interface serial 0/0/? Serial interface number Todd(config)#interface serial 0/0/0 Todd(config-if)# 59

60. Configuring IP Routing (continued) Notes: (continued) –You should always view a running-config output first so you know what interfaces you have to deal with. Here’s a 2801 output: –Todd(config-if)#do show run –Building configuration... –[output cut] –! –interface FastEthernet0/0 –no ip address –Shutdown –duplex auto –speed auto –! –interface FastEthernet0/1[continued on next slide] 60

61. Configuring IP Routing (continued) –no ip address –shutdown –duplex auto –speed auto –! –interface Serial0/0/0 –no ip address –shutdown –no fair-queue –! –interface Serial0/0/1 –no ip address –shutdown –! –interface Serial0/1/0 –[continued in next column] 61 –no ip address –shutdown –! –interface Serial0/2/0 –no ip address –shutdown –clock rate 2000000 –! –[output cut]

62. Configuring IP Routing (continued) 62 At other times you may see a x/x/x config for modular units (like WICs) where you have a slot, a subslot, and a port. From Cisco.com: –“The slot/subslot/port format only applies to WIC interfaces. Interfaces that are native to the network modules still use only the slot/port format. That is: slot/port is used whenever the interfaces are native on the network module. slot/subslot/port is used whenever the interfaces are on the WIC slot of a network module (NM).” There are still more examples where the interface is a 3-part config.

63. Configuring IP Routing (continued) Notes: (continued) –Pg 346-47: Just a command idiosyncrasy: –With ISR routers you can’t use “erase start”, you must enter “erase startup-config” –This is so even though no other command begins with “S”: Eg: Router#erase s? startup-config So under the normal rules of the Cisco IOS, “erase s” should work exactly like “erase startup-config”, but it doesn’t. –This is probably just an oversight that will be corrected in the next IOS version. Just be aware that you will sometimes find anomalies like this. 63

64. Configuring IP Routing (continued) Notes: (continued) –Pg 351 ff: Wireless interfaces: 2 things unique to them: SSID #: “The Service Set Identifier that creates a wireless network that hosts can connect to.” DHCP Pool for wireless clients: Actually just like DHCP with wired clients. More on this in Chapter 12. –Pg 352 ff: Author uses the SDM here – “Security Device Manager” to configure interface R3 in the example. The book goes through a series of steps using the SDM’s wizard – through page 359. 64

65. Even after the previous pages/slides, we still we need to do some things to get our network up to speed. 3 things to do: 1.Static Routing 2.Default Routing 3.Dynamic Routing 65/362 Configuring IP Routing in Our Network

66. 172.16.3.2 SO Static Routes 172.16.1.0 B 172.16.3.1 A B Stub Network 172.16.2.0 SO A Routes must be unidirectional 66 /364

67. You can optionally add a distance if you want to change the metric of the route; for example, you may want to prefer any dynamic route Router(config)#172.16.1.22 255.255.0.0 192.168.5.45 This means: to get here (ip address and mask) go here next (address only) Router(config)#ip route remote_network mask next_hop ip route remote network [mask] { address|interface} [distance] - all static routes have a distance of “1”; very trustworthy [permanent] - to keep the route in the table no matter what; even if the interface goes down. Static Route Configuration

68. ip route 172.16.1.0 255.255.255.0 172.16.3.2. or ip route 172.16.1.0 255.255.255.0 s0 Static Route Example 68 172.16.3.2 SO 172.16.1.0 B 172.16.3.1 A B Stub Network 172.16.2.0 SO

69. Default Routes 69 / 374 172.16.3.2 SO 172.16.1.0 B 172.16.3.1 A B Stub Network 172.16.2.0 SO To send packets with a remote destination network not in the routing table to the next-hop router, only used for stub networks. ip route 0.0.0.0 0.0.0.0 172.16.3.1 ip classless [Note: This configuration sends every packet out Router A’s 3.1 interface] creates a wireless network that hosts can connect to.

70. Static Route Considerations When configuring static routes, consider the following: –By default, a static route will take precedence over a dynamic route because of its lower administrative distance. –Without additional configuration, a dynamic route to a network will be ignored if a static route is present in the routing table for the same network. –To reduce the number of static route entries, define a summarized or default static route

71. Static Route Considerations 71 The benefit of using static routes is that they do not require the router to spend CPU cycles and memory space to determine the best route to a destination. The route has already been placed in the routing table manually. This can work against the network, however, if a device in the static route’s path goes down. In this case, the packets may still attempt to use the path (especially if the “permanent” option is chosen), and in any event, no other route will be chosen, as in a dynamic routing network, because the static route has limited the choices.

72. Routing Protocols (Dynamic) Routing protocols are used between routers to: –Determine the path of a packet through a network –Maintain routing tables –Two types: Interior gateway protocols (IGPs) exterior gateway protocols (EGPs) Examples: –IGP: RIP, IGRP, OSPF, IS-IS, EIGRP –EGP: Border Gateway Protocol (BGP) [Note: This is only one way to distinguish between routing protocols; others include: distance vector v. link state, and we’ve already begun to distinguish static v. dynamic] 72 / 377

73. Autonomous System 1Autonomous System 2 IGPs: RIP, IGRP EGPs: BGP Routing Protocols 73 An autonomous system is a collection of networks under a “common administrative domain”, i.e., all routers sharing the same routing table are in the same AS. IGPs operate within an autonomous system. EGPs connect different autonomous systems.

74. Classful Routing Overview “Classful” routing protocols do not include the subnet mask with the route advertisement. –Within the same network, consistency of the subnet masks is assumed. –Summary routes are exchanged between foreign networks. –Examples of classful routing protocols: RIP Version 1 (RIPv1) IGRP [The problem with classful routes is that they don’t 74

75. Classless Routing Overview Classless routing protocols include the subnet mask with the route advertisement. –Classless routing protocols support variable-length subnet masking (VLSM). –Summary routes can be manually controlled within the network. –Examples of classless routing protocols: RIP Version 2 (RIPv2) EIGRP OSPF IS-IS 75

76. Classful Versus Classless Routing Protocols –A classful routing protocol always considers the IP network class Address summarization is automatic by major network number and discontiguous subnets are not visible to each other –Classless protocols transmit prefix-length or subnet mask information with IP network addresses. The IP address can be mapped so that discontinuous subnets and VLSM are supported 76

77. IGRP Administrative Distance=100 Router D Router B Router A Router C RIP Administrative Distance=120 Default Administrative Distance Directly Connected: 0 Static Route: 1 RIP: 120 IGRP: 100 EIGRP: 90 OSPF: 110 Administrative Distance 77 The administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. An administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. If a router receives two updates listing the same remote network, the first thing the router checks is the AD. If one of the advertised routes has a lower AD than the other, then the route with the lowest AD will be placed in the routing table. If both advertised routes to the same network have the same AD, then routing protocol will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table. If it’s a tie, load balancing is used. 77

78. CB A D Routing Table Routing Table Routing Table Routing Table Routing Table Routing Table Routing Table Routing Table Distance—How far Vector—In which direction All routers just broadcast their entire routing table out all active interfaces on periodic time intervals Distance vector algorithms do not allow a router to know the exact topology of an internetwork. Distance Vector 78 / 379

79. Discovering Routes 79

80. Discovering Routes: Converged Routing Tables By “converged” we mean that each of the routers above has the same view of the internetwork, i.e., each router sees the same number of links from one router to any other router.

81. Meaning of Distance Vector (1/2) A router using a distance vector routing protocol does not have the knowledge of the entire path to a destination network. The router only knows –The direction or interface in which packets should be forwarded and –The distance or how far it is to the destination network

82. Meaning of Distance Vector (2/2)

83. Operation of distance vector (1/4) Some distance vector routing protocols call for the router to periodically broadcast the entire routing table to each of its neighbors. This method is inefficient because the updates not only consume bandwidth but also consume router CPU resources to process the updates.

84. Operation of distance vector (2/4) Periodic Updates are sent at regular intervals (30 seconds for RIP and 90 seconds for IGRP). –Even if the topology has not changed in several days, periodic updates continue to be sent to all neighbors. –Neighbors are routers that (1) share a link and are configured to (2) use the same routing protocol. –The router is only aware of the network addresses of its own interfaces and the remote network addresses it can reach through its neighbors

85. Operation of distance vector (3/4) Broadcast Updates are sent to 255.255.255.255 –Neighboring routers that are configured with the same routing protocol will process the updates. –All other devices will also process the update up to Layer 3 before discarding it. –Some distance vector routing protocols use multicast addresses instead of broadcast addresses.

86. Operation of distance vector (4/4) Entire Routing Table Updates are sent, periodically to all neighbors. –Neighbors receiving these updates must process the entire update to find pertinent information and discard the rest. –Some distance vector routing protocols like EIGRP do not send periodic routing table updates.

87. Routing Algorithm The algorithm used for the routing protocols defines the following processes: –Mechanism for sending and receiving routing information. –Mechanism for calculating the best paths and installing routes in the routing table. –Mechanism for detecting and reacting to topology changes.

88. Routing protocol characteristics (1/3) Time to Convergence - Time to convergence defines how quickly the routers in the network topology share routing information and reach a state of consistent knowledge. –The faster the convergence, the more preferable the protocol. –Routing loops can occur when inconsistent routing tables are not updated due to slow convergence in a changing network.

89. Routing protocol characteristics (2/3) Scalability - Scalability defines how large a network can become based on the routing protocol that is deployed. –The larger the network is, the more scalable the routing protocol needs to be. Classless (Use of VLSM) or Classful - Classless routing protocols include the subnet mask in the updates. –This feature supports the use of Variable Length Subnet Masking (VLSM) and better route summarization. –Classful routing protocols do not include the subnet mask and cannot support VLSM.

90. Routing protocol characteristics (3/3) Resource Usage - Resource usage includes the requirements of a routing protocol such as memory space, CPU utilization, and link bandwidth utilization –Higher resource requirements necessitate more powerful hardware to support the routing protocol operation in addition to the packet forwarding processes. Implementation and Maintenance - Implementation and maintenance describes the level of knowledge that is required for a network administrator to implement and maintain the network based on the routing protocol deployed.

91. Distance Vector Routing Protocols

92. Comparison of Routing Protocol

93. Routing Loops (1/6) A routing loop is a condition in which a packet is continuously transmitted within a series of routers without ever reaching its intended destination network. A routing loop can occur when two or more routers have routing information that incorrectly indicates that a valid path to an unreachable destination exists.

94. Routing Loop (2/6) The loop may be a result of: –Incorrectly configured static routes –Incorrectly configured route redistribution (redistribution is a process of handing the routing information from one routing protocol to another routing protocol) –Inconsistent routing tables not being updated due to slow convergence in a changing network –Incorrectly configured or installed “discard routes”

95. Routing Loop (3/6)

96. Routing Loop (4/6)

97. Routing Loop (5/6)

98. Routing Loop (6/6)

99. Routing Loops & Ways to Stop Them 99 / 380 Maximum hop count, AKA, Counting to Infinity: RIP permits a hop count of up to 15. At 16 hops, a route is considered to be an infinite distance away. This is called counting to infinity, and it’s caused by gossip (broadcasts) and wrong information being communicated and propagated throughout the internetwork. Without some form of intervention, the hop count increases indefinitely each time a packet passes through a router.

100. Count to infinity (1/5) Count to infinity is a condition that exists when inaccurate routing updates increase the metric value to "infinity" for a network that is no longer reachable.

101. Count to infinity (2/5)

102. Count to infinity (3/5)

103. Count to infinity (4/5)

104. Count to infinity (5/5)

105. Routing Loops 105 / 380 Split Horizon: Routing information cannot be sent back in the direction from which it was received.

106. Split Horizon Rules (1/5) The split horizon rule says that a router should not advertise a network through the interface from which the update came.

107. Split Horizon Rules (2/5)

108. Split Horizon Rules (3/5)

109. Split Horizon Rules (4/5)

110. Split Horizon Rules (5/5)

111. Routing Loops 111 / 380 Route poisoning: Advertising the downed network as unreachable. When one router receives a route poisoning from another, it sends an update, called a poison reverse, back to the other router. This ensures that all routes on the segment have received the poisoned route information

112. Route Poisoning (1/4) Route poisoning is yet another method employed by distance vector routing protocols to prevent routing loops. Route poisoning is used to mark the route as unreachable in a routing update that is sent to other routers. Unreachable is interpreted as a metric that is set to the maximum. –For RIP, a poisoned route has a metric of 16.

113. Route Poisoning (2/4)

114. Route Poisoning (3/4)

115. Route Poisoning (4/4)

116. Split Horizon with Poison reverse (1/5) Now we can put Split Horizon together with Route Poisoning / Poison Reverse. The concept of split horizon with poison reverse is that explicitly telling a router to ignore a route is better than not telling it about the route in the first place.

117. Split Horizon with Poison reverse (2/5) The following process occurs: Network 10.4.0.0 becomes unavailable due to a link failure. R3 poisons the metric with a value of 16 and then sends out a triggered update stating that 10.4.0.0 is unavailable. R2 processes that update, invalidates the routing entry in its routing table, and immediately sends a poison reverse back to R3.

118. Split Horizon with Poison reverse (3/5)

119. Split Horizon with Poison reverse (4/5)

120. Split Horizon with Poison reverse (5/5)

121. Ways to Stop Router Loops Holddowns : Prevents regular update messages from reinstating a route that is going up and down (called flapping). Typically, this happens on a serial link that’s losing connectivity and then coming back up. Holddown timers introduce a certain amount of skepticism to reduce the acceptance of bad routing information. If the distance to a destination increases (for example, the hop count increases from 2 to 4), the router sets a holddown timer for that route. Until the timer expires, the router will not accept any new updates for the route. This is only one type of timer used with RIP – see next 3 slides:

122. RIP Timers (1/3) In addition to the update timer, the IOS implements three additional timers for RIP: Invalid Timer. If an update has not been received to refresh an existing route after 180 seconds (the default), the route is marked as invalid by setting the metric to 16. –The route is retained in the routing table until the “flush timer” expires. Flush Timer. By default, the flush timer is set for 240 seconds, which is 60 seconds longer than the invalid timer. When the flush timer expires, the route is removed from the routing table.

123. RIP Timers (2/3) Holddown Timer: This timer stabilizes routing information and helps prevent routing loops during periods when the topology is converging on new information. –Once a route is marked as unreachable, it must stay in holddown long enough for all routers in the topology to learn about the unreachable network. –By default, the holddown timer is set for 180 seconds.

124. RIP Timers (3/3)

125. 64kbps T1 RIP Overview –Hop count metric selects the path, 16 is unreachable –Full route table broadcast every 30 seconds –Load balance maximum of 6 equal cost paths (default = 4) –RIPv2 supports VLSM and Discontiguous networks

126. Router(config)#router rip Router(config-router)#network network-number* network 172.16.0.0 network 192.168.10.0 router RIP network 172.16.0.0 network 10.0.0.0 router RIP *Network is a classful network address. Every device on network uses the same subnet mask 172.16.10.0 192.168.10.0 10.3.5.0 RIP Routing Configuration 126

127. RIP Version 2 Allows the use of variable length subnet masks (VLSM) by sending subnet mask information with each route update Distance Vector – same AD, and timers. Easy configuration, just add the command “version 2” under the router rip configuration 127 router rip network 10.0.0.0 version 2

128. 128 RIPv1 vs. RIPv2 RIPv1RIPv2 Distance vector Maximum hop count 15 ClassfulClassless Broadcast basedMulticast 224.0.0.9 No support for VLSMSupports VLSM No authenticationMD5 authentication No support for discontiguous networks Supports discontiguous networks

129. 129 Interior Gateway Routing Protocol (IGRP) Maximum hop count: 255 for larger network, default 100 Composite metric: bandwidth and delay of the line. –Those are the defaults –Also: Load and Reliability are optionally configurable instead –MTU (Maximum Transmission Unit) is a “tiebreaker” Config t router igrp 10

130. 130 IGRP vs. RIP Large networkSmall network Uses AS number for activation Uses network address, with all subnet and host bits off Full route table update per 90 sec Full route table update per 30 sec AD 100AD 120 Uses bandwidth and delay of the line as metric, maximum hop count 255 Uses only hop count to determine the best path to a remote network, max 15

131. Discontiguous Addressing Two networks of the same classful networks are separated by a different network address 131 192.168.10.0/24 10.1.1.0/24 192.168.10.0/24 –RIPv1 and IGRP do not advertise subnet masks, and therefore cannot support discontiguous subnets. –OSPF, EIGRP, and RIPv2 can advertise subnet masks, and therefore can support discontiguous subnets.

132. Passive Interface Maybe you don’t want to send RIP updates out your router interface connected to the Internet. Use the passive-interface command: Router(config)#router rip Router(config-router)#passive-interface serial0 132 This allows a router to receive route updates on an interface, but not send updates via that interface S0 Gateway Internet Updates X X

133. Verifying RIP Router#show ip protocols Router#show ip route Router#debug ip rip Router#undebug all (un all) 133

134. Summary –Open your books and go through all the written labs and the review questions. –Review the answers in class. 134