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

Transcription

1 Building the Routing Table Introducing the Routing Table Directly Connected Networks Static Routing Dynamic Routing Routing Table Principles

2 Introducing the 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 C /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/0 Routing table is a data file in RAM that is used to store route information about: Directly connected Remote networks 2

3 Introducing the Routing Table R1# show ip route <output omitted> C C /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/0 Exit Interfaces 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) 3

4 Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <output omitted> C C /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/0 Directly Connected Networks 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. 4

5 Introducing the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <output omitted> C C /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/0 Remote Network 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. 5

6 Static Routes R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <output omitted> Gateway of last resort is not set C C S /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/ /24 [1/0] via c Static Route 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. 6

7 Dynamic Routes 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 /24 is directly connected, FastEthernet0/0 C /24 is directly connected, Serial0/0/0 S /24 [1/0] via R /24 [120/1] via , 00:00:20, Serial0/0/0 R1 has automatically learned about the /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. 7

8 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. 8

9 Examining Router Interfaces 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 R1# show ip route command is used to display the routing table. Initially, the routing table is empty if no interfaces have been configured. Note: Static routes and dynamic routes cannot be added to the routing table until the appropriate local interfaces, also known as the exit interfaces, have been configured on the router. (later) 9

10 Interfaces and their Statuses R1# show interfaces FastEthernet0/0 is administratively down, line protocol is down Hardware is AmdFE, address is 000c (bia 000c ) <output omitted> Serial0/0/0 is administratively down, line protocol is down <output omitted> 10

11 Interfaces and their Statuses R1# show ip interface brief Interface IP-Address OK? Method Status Protocol FastEthernet0/0 unassigned YES manual administratively down down Serial0/0 unassigned YES unset administratively down down FastEthernet0/1 unassigned YES unset administratively down down Serial0/1 unassigned YES unset administratively down down R1# Used to see a portion of the interface information in a condensed format Note: Great command for checking interfaces before starting a lab! 11

12 Interfaces and their Statuses R1# show running-config <output omitted> interface FastEthernet0/0 mac-address 000c no ip address duplex auto speed auto shutdown! interface FastEthernet0/1 mac-address 000c no ip address duplex auto speed auto shutdown <output omitted> show running-config command displays current configuration file another way to verify the configuration of an interface 12

13 Configuring an Ethernet Interface R1(config)# interface fastethernet 0/0 R1(config-if)# ip address R1(config-if)# no shutdown *Mar 1 01:16:08.212: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up *Mar 1 01:16:09.214: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up changed state to up message indicates that, physically, the connection is good. interface is properly connected to a switch or a hub. receiving a carrier signal from another device (switch, hub, PC, or another router). changed state to up message indicates that the data link layer is operational. LAN interfaces, typically no data link parameter changes. WAN interfaces in a lab environment require clocking on one side of the link 13

14 Reading the Routing Table R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <output omitted> Gateway of last resort is not set C /24 is subnetted, 1 subnets is directly connected, FastEthernet0/0 The interface was configured with the /24 IP address, which makes it a member of the /24 network. C = directly connected R1 has an interface that belongs to this network The /24 subnet mask for this route is displayed in the line above the actual route. 14

15 Routers Usually Store Network Addresses R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP <output omitted> Gateway of last resort is not set C /24 is subnetted, 1 subnets is directly connected, FastEthernet0/0 Phone book analogy: Families not individuals with same number Occasionally, a host route is entered in the routing table; the host route represents an individual host IP address. The host route is listed with the device s host IP address and a /32 ( ) subnet mask. The topic of host routes is discussed in another course. 15

16 Pings from R2 to If you convert these addresses to binary and compare them, you will see that the first 24 bits of this route do not match because the twenty-third bit does not match. Packets are dropped. 16

17 Pings from R2 to R2# ping !!!! R2# show ip route /24 is subnetted, 2 subnets C is directly connected, FastEthernet0/0 C is directly connected, Serial0/0/0 C /24 is directly connected, Serial0/0/1 This time the ping succeeds! It is successful because R2 has a route in its routing table that matches , which is the ping packet s destination IP address. 17

18 Static Routes with Next-Hop Addresses ip route command Configuring Static Routes Routing Table Principles Resolving to an Exit Interface with a Recursive Lookup

19 Purpose and Command Syntax of the ip route Command Static routes are commonly used when routing from a network to a stub network. A stub network is a network accessed by a single route. R1 only has one way to reach other destinations via R2. Therefore, network is a stub network and R1 is a stub router. Dynamic routing protocol between R1 and R2 is a waste of resources. 19

20 ip route Command The command for configuring a static route is ip route. The complete syntax for configuring a static route is: ip route prefix mask {ip-address interface-type interface-number [ip-address]} [dhcp] [distance] [name next-hop-name] [permanent track number] [tag tag] 20

21 Configuring Static Routes R1# debug ip routing R1# conf t R1(config)# ip route :20:15: RT: add /24 via , static metric [1/0] R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, /24 is subnetted, 3 subnets S [1/0] via C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/0 R1 in our chapter topology knows about its directly connected networks. The remote networks that R1 does not know about are as follows: /124: The LAN on R2 (static route above solved this) /24: The serial network between R2 and R /24: The LAN on R3 21

22 Verifying Static Routes R1# show running-config <output omitted>! ip route ip route ip route ! <output omitted> R1# copy running-config startup-config Now is a good time to save the configuration to NVRAM with the copy running-config startup-config command. 22

23 Configure R2 and R3 Static Routes R2(config)# ip route R2(config)# ip route R3(config)# ip route R3(config)# ip route R3(config)# ip route

24 R1# show ip route <output omitted> /24 is subnetted, 3 subnets S [1/0] via C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/0 S /24 [1/0] via S /24 [1/0] via R2# show ip route <output omitted> /24 is subnetted, 3 subnets C is directly connected, FastEthernet0/0 C is directly connected, Serial0/0/0 S [1/0] via C /24 is directly connected, Serial0/0/1 S /24 [1/0] via R3# show ip route <output omitted> /24 is subnetted, 3 subnets S [1/0] via S [1/0] via S [1/0] via C /24 is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 24

25 Verify End-to-End Connectivity R1# ping !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/28/32 ms R1# ping !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/56 ms R1# ping !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms R1# ping !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/56 ms R1# 25

26 Summary and Default Static Routes Summary Static Routes Default Static Routes

27 Summarizing Routes to Reduce the Size of the Routing Table Summary route is a single route that can be used to represent multiple routes. Generally a set of contiguous networks (but do not have to be) Have the same exit interface or next-hop IP address. Example: /16, /16, /16, /16, /16, /16, all the way through /16 Can be represented by a single network address: /8. Creates smaller routing tables More efficient routing table lookup process more efficient. A single static route can be used to represent dozens, hundreds, or even thousands of routes. As of March 2007, there are more than 200,000 routes in the Internet core routers. Most of these are summarized routes. 27

28 Summarizing Routes to Reduce the Size of the Routing Table R3: ip route Serial0/0/1 ip route Serial0/0/1 ip route Serial0/0/1 All three routes are forwarding traffic out the same Serial 0/0/1 interface. Can be summarized to the single

29 Calculating a Summary Route Step 1. Write out the networks that you want to summarize in binary. Step 2. To find the subnet mask for summarization, start with the leftmost bit. Step 3. Work your way to the right, finding all the bits that match consecutively. ip route serial0/0/1 Step 4. When you find a column of bits that do not match, stop. You are at the summary boundary. Step 5. Count the number of leftmost matching bits, which in our example is 22. This number becomes your subnet mask for the summarized route, /22 or Step 6. To find the network address for summarization, copy the matching 22 bits and add all 0 bits to the end to make 32 bits.. 29

30 Configuring a Summary Route R3(config)# no ip route serial0/0/1 R3(config)# no ip route serial0/0/1 R3(config)# no ip route serial0/0/1 R3(config)# ip route serial0/0/1 Delete individual static routes. Configure single summary route. 30

31 Changes in Routing Table R3# show ip route Before <output omitted> /24 is subnetted, 3 subnets S is directly connected, Serial0/0/1 S is directly connected, Serial0/0/1 S is directly connected, Serial0/0/1 C /24 is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 R3# show ip route After <output omitted> /22 is subnetted, 1 subnets S is directly connected, Serial0/0/1 C /24 is directly connected, Serial0/1 C /24 is directly connected, FastEthernet0/0 Any packet with a destination IP address belonging to the /24, /24, or /24 network matches this summarized route. 31

32 Verify the Summary Route R3# ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:!!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms R3# ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:!!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/60 ms R3# ping Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to , timeout is 2 seconds:!!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 56/56/60 ms R3# 32

33 Default Static Route A default route is used to represent all routes with zero or no bits matching. In other words, when there are no routes that have a more specific match, the default route will be a match. (more later throughout the course) A default static route is a route that will match all packets. Default static routes are used To represent destination networks outside the router s own routing domain. A common use is when connecting a company s edge router to the ISP network. When no other routes in the routing table match the packet s destination IP address in other words, when a more specific match does not exist. When a router has only one other router to which it is connected. This condition is known as a stub router. 33

34 Configuring a Default Static Route Router(config)# ip route [exit-interface ip-address ] network address and mask is called a quad-zero route. 34

35 Configuring a Default Static Route ip route serial 0/0/0 ip route serial 0/0/0 ip route serial 0/0/0 R1 is a stub router and is connected only to R2. R1 doesn t need specific routing information to reach R3 networks. Currently R1 has three static routes. All three static routes have: the exit interface Serial 0/0/0 next-hop Router R2 35

36 Configuring a Default Static Route R1(config)# no ip route serial 0/0/0 R1(config)# no ip route serial 0/0/0 R1(config)# no ip route serial 0/0/0 R1(config)# ip route serial 0/0/0 Delete individual static routes. Configure single default static route. 36

37 Verifying a Default Static Route R1# show ip route Before S C C S S /24 is subnetted, 3 subnets is directly connected, Serial0/0/ is directly connected, Serial0/0/ is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/ /24 is directly connected, Serial0/0/0 R1# show ip route After <some codes omitted> * - candidate default, U - per-user static route, o - ODR Gateway of last resort is to network /24 is subnetted, 2 subnets C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/0 S* /0 is directly connected, Serial0/0/0 37

38 Verifying a Default Static Route R1# show ip route <some codes omitted> * - candidate default, U - per-user static route, o - ODR Gateway of last resort is to network /24 is subnetted, 2 subnets C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/0 S* /0 is directly connected, Serial0/0/0 A /0 mask indicates that zero or no bits are needed to match. As long as a more specific match doesn t exist, the default static route will match all packets. Default routes are very common on routers. Instead of routers having to store routes for all the networks in the Internet, they can store a single default route to represent any network that is not in the routing table. You will see in later chapters that a default route does not always have to be a static route. 38

39 Classifying Dynamic Routing Protocols IGP and EGP Distance Vector and Link-State Classful and Classless Convergence

40 Classifying Routing Protocols Routing Protocols can be classified by: IGP or EGP Distance vector or link-state Classful or classless 40

41 IGP and EGP An autonomous system (AS) otherwise known as a routing domain is a collection of routers under a common administration. Company s internal network An ISP s network. Because the Internet is based on the autonomous system concept, two types of routing protocols are required: Interior routing protocols Exterior routing protocols 41

42 IGP and EGP Interior gateway protocols (IGP): Used for intra-autonomous system routing Routing inside an autonomous system Exterior gateway protocols (EGP): Used for inter-autonomous system routing Routing between autonomous systems 42

43 Distance Vector and Link-State Routing Protocols Interior gateway protocols (IGP) can be classified as two types: Distance vector routing protocols Link-state routing protocols 43

44 Distance Vector Routing Protocol Operation Distance vector Routes are advertised as vectors of distance and direction. Distance is defined in terms of a metric Such as hop count, Direction is simply the: nexthop router or exit interface. Typically use the Bellman-Ford algorithm for the best-path route determination 44

45 Distance Vector Routing Protocol Operation Routing protocol Does not know the topology of an internetwork. Only knows the routing information received from its neighbors. Like signposts along the path to the final destination. 45

46 Distance Vector Routing Protocol Operation Distance vector protocols work best in situations where: The network is simple and flat and does not require a hierarchical design. The administrators do not have enough knowledge to configure and troubleshoot link-state protocols. Specific types of networks, such as hub-and-spoke networks, are being implemented. Worst-case convergence times in a network are not a concern. More in Chapter 4. 46

47 Link-State Protocol Operation Link-state routing protocol can create a complete view, or topology, of the network. Like having a complete map of the network topology Link-state protocols are associated with Shortest Path First (SPF) calculations. A link-state router uses the linkstate information to: Create a topology map Select the best path to all destination networks in the topology. 47

48 Link-State Protocol Operation Link-state protocols work best in situations where The network design is hierarchical, usually occurring in large networks. The administrators have a good knowledge of the implemented link-state routing protocol. Fast convergence of the network is crucial. More in later chapters. 48

49 Classful and Classless Routing Protocols All routing protocols can also be classified as either Classful routing protocols Classless routing protocols IPv6 routing protocols are classless 49

50 Classful Routing Protocols Classful routing protocols do not send subnet mask information in routing updates. The first routing protocols, such as RIP When network addresses were allocated based on classes. Class A, B, or C. Routing protocol did not need to include the subnet mask in the routing update. Network mask determined based on value of first octet of the network address. 50

51 Classful Routing Protocols /16 Major Classful Network All /24 subnets Classful routing protocols do not include the subnet mask Therefore do not support VLSM and CIDR. All subnets within the same major classful network address must have the same mask. Other limitations to classful routing protocols, including: Inability to support discontiguous networks (later) More later! 51

52 Classless routing Protocols /16 Major Classful Network /27 and /30 subnets / / /30 Classless routing protocols include the subnet mask with the network address in routing updates. Today s networks are no longer allocated based on classes Subnet mask cannot be determined by the value of the first octet. Classless routing protocols are required in most networks today because of their support for: VLSM CIDR Discontiguous networks. 52

53 Dynamic Routing Protocols and Convergence R2 s Routing Table R1 s Routing Table R3 s Routing Table An important characteristic of a routing protocol: How quickly it converges when there is a change in the topology. Convergence is when the routing tables of all routers are at a state of consistency. The network has converged when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to: share information calculate best paths update their routing tables. A network is not completely operable until the network has converged; therefore, most networks require short convergence times. 53

54 Dynamic Routing Protocols and Convergence R2 s Routing Table R1 s Routing Table R3 s Routing Table Generally, convergence time: Slow: RIP and IGRP Faster: EIGRP, OSPF, and IS-IS 54

55 Metrics Purpose of the Metric Metrics and Routing Protocols Load Balancing

56 Purpose of a Metric? Metrics are a way to measure or compare. Determine which route is the best path. Assign costs to reach remote networks. Routing protocol learns multiple routes to the same destination. Metric is used to determine which path is most preferable 56

57 Purpose of a Metric Routing protocol metrics: RIP: Hop count IGRP and EIGRP: Bandwidth, delay, reliability and load OSPF (Cisco s version): Bandwidth IS-IS: Four values (Cisco uses default ) Covered in CCNP BGP: Attributes Covered in CCNP More later 57

58 Metric Parameters 56 Kbps R1 to reach the /24 network. RIP: Fewest number of hops via R2. OSPF: Path with the highest cumulative bandwidth through R3. This results in faster packet delivery. 58

59 Metric Field in the Routing Table The routing table displays the metric for each dynamic and static route. Static routes always have a metric of 0 with RIP. Routing protocols install route in routing table with the lowest metric. 59

60 R2# show ip route <output omitted> Gateway of last resort is not set R /24 [120/1] via , 00:00:24, Serial0/0/0 C /24 is directly connected, Serial0/0/0 C /24 is directly connected, FastEthernet0/0 C /24 is directly connected, Serial0/0/1 R /24 [120/1] via , 00:00:26, Serial0/0/1 R /24 [120/1] via , 00:00:24, Serial0/0/0 [120/1] via , 00:00:26, Serial0/0/1 R /24 [120/1] via , 00:00:26, Serial0/0/1 R /24 [120/2] via , 00:00:26, Serial0/0/1 All routers running RIP R2 has a route to the /24 network that is 2 hops away. The 2 in the command output is where the routing metric is displayed. 120 is the Administrative Distance (later) 60

61 Load Balancing What happens when two or more routes to the same destination have identical metric values? The router load balances between these equal-cost paths. The packets are forwarded using all equal-cost paths. 61

62 Load Balancing R2# show ip route <output omitted> R /24 [120/1] via , 00:00:24, Serial0/0/0 [120/1] via , 00:00:26, Serial0/0/1 All the routing protocols discussed in this course are capable of automatically load balancing traffic for up to four equal-cost routes by default. EIGRP is also capable of load balancing across unequal-cost paths. This feature of EIGRP is discussed in the CCNP courses. 62

63 RIPv1: A Distance Vector, Classful Routing Protocol Background and Perspective RIPv1 Characteristics and Message Format RIP Operation

64 RIPv1: Distance Vector, Classful Routing Protocol The first protocol used was Routing Information Protocol (RIP). RIP still popular: simple and widespread support. Why learn RIP? Still in use today (there is even a RIPng for IPv6). Help understand fundamental concepts and comparisons of protocols such as classful (RIPv1) and classless (RIPv2) 64

65 RIPv1 Characteristics and Message Format Note: The first version of RIP is often called RIPv1 to distinguish it from RIP version 2 (RIPv2). However, both versions share many of the same features. When discussing features common to both versions, we will refer to RIP. When discussing features unique to each version, we will use RIPv1 and RIPv2. RIPv2 is discussed in Chapter 7. RIP characteristics: Distance vector routing protocol. Metric: hop count Advertised routes with hop counts greater than 15 are considered unreachable. Response messages (routing table updates) are broadcast every 30 seconds. (RIPv2 uses multicasts) 65

66 RIP Operation Startup 1. Each RIP-configured interface sends out a Request message Asking for their complete routing tables. 2. A Response message is sent back by RIP-enabled neighbors. If new route: Installs in routing table. If existing route: Replace if better hop count. Startup router then sends a triggered update out all RIP-enabled interfaces containing its own routing table so that RIP neighbors can be informed of any new routes. 66

67 IP Address Classes and Classful Routing No subnet mask RIPv1: Classful routing protocol. Does not send subnet mask in update. A router either uses the subnet mask: (discussed later) configured on a local interface or applies the default classful subnet mask Because of this limitation, RIPv1 networks cannot be discontiguous, nor can they implement VLSM. 67

68 Basic RIPv1 Configuration RIPv1 Scenario A Enable RIP: router rip Command Specifying Networks

69 RIPv1 Scenario A Notice that this topology uses five Class C network addresses. Remember, RIPv1 is a classful routing protocol We will see that the class of the network is used by RIPv1 to determine the subnet mask. 69

70 Enabling RIP: router rip Command R1# conf t Enter configuration commands, one per line. End with CNTL/Z. R1(config)# router? bgp Border Gateway Protocol (BGP) egp Exterior Gateway Protocol (EGP) eigrp Enhanced Interior Gateway Routing Protocol (EIGRP) igrp Interior Gateway Routing Protocol (IGRP) isis ISO IS-IS iso-igrp IGRP for OSI networks mobile Mobile routes odr On Demand stub Routes ospf Open Shortest Path First (OSPF) rip Routing Information Protocol (RIP) R1(config)# router rip R1(config-router)# Enter router configuration mode for RIP, enter router rip at the global configuration prompt. Notice that the prompt changes. 70

71 Enabling RIP: router rip Command R1# conf t R1(config)# router rip R1(config-router)# router rip Does not directly start the RIP process. Provides access to configure routing protocol settings. No routing updates are sent until additional commands are configured. no router rip To remove the RIP routing process from a device Stops the RIP process Erases all existing RIP configuration commands. 71

72 Specifying Networks Router(config-router)# network directly-connected-classfulnetwork-address To enable RIP routing for a network, use the network command in router configuration mode Enter the classful network address for each directly connected network. 72

73 Specifying Networks Router(config-router)# network directly-connected-classfulnetwork-address The network command performs the following functions: Enables RIP on all interfaces that belong to a specific network. Associated interfaces will now both send and receive RIP updates. Advertises the specified network in RIP routing updates sent to other routers every 30 seconds. 73

74 Specifying Networks Only directly connected classful network addresses! R1(config)# router rip R1(config-router)# network R1(config-router)# network R2(config)# router rip R2(config-router)# network R2(config-router)# network R2(config-router)# network R3(config)# router rip R3(config-router)# network R3(config-router)# network If you enter a subnet or host IP address, IOS automatically converts it to a classful network address. For example, if you enter the command network , the router will convert it to network

75 Verification and Troubleshooting Verifying RIP: show ip route Verifying RIP: show ip protocols Verifying RIP: debu ip rip Passive Interfaces

76 Verifying RIP: show ip route Command R1# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, <output omitted> Gateway of last resort is not set R R C C R /24 [120/1] via , 00:00:02, Serial0/0/ /24 [120/2] via , 00:00:02, Serial0/0/ /24 is directly connected, FastEthernet0/ /24 is directly connected, Serial0/0/ /24 [120/1] via , 00:00:02, Serial0/0/0 An R in the output indicates RIP routes. Because this command displays the entire routing table, including directly connected and static routes, it is normally the first command used to check for convergence. Routes might not immediately appear when you execute the command because networks take some time to converge.. 76

77 Verifying RIP: show ip route Command R1# show ip route <output omitted> R /24 [120/2] via , 00:00:23, Serial0/0/0 77

78 Verifying RIP: show ip protocols Command Verifies that RIP routing is configured and running on Router R2 At least one active interface with an associated network command is needed before RIP routing will start. 78

79 Passive Interfaces Got router? Some routers can have interfaces that do not connect to another router. You can use the passive-interface command with RIP to configure an interface not to send those updates. Bandwidth is wasted transporting unnecessary updates. All devices on the LAN must process the RIPv1 update up to the transport layer. Security risk 79

80 Passive Interfaces Router(config-router)# passive-interface interface-type interfacenumber What about using on R2: R2(Config-router)# no network But then R2 would not advertise this LAN as a route in updates sent to R1 and R3. Correct solution is to use the passive-interface command 80

81 Passive Interfaces R2(config)# router rip R2(config-router)# passive-interface FastEthernet 0/0 X 81

82 Passive Interfaces R2# show ip protocols <output omitted> Interface Send Recv Triggered RIP Key-chain Serial0/0/ Serial0/0/ FastEthernet 0/0 no longer included Automatic network summarization is in effect Routing for Networks: LAN network still included in RIP updates that are sent Passive Interface(s): FastEthernet0/0 FastEthernet 0/0 is a passive interface Routing Information Sources: Gateway Distance Last Update :00: :00:23 Distance: (default is 120) 82

83 Automatic Summarization Modified Topology B Boundary Routers and Automatic Summarization Processing RIP Updates Sending RIP Updates Advantages and Disadvantages of Automatic Summarization

84 Modified Topology: Scenario B / / /24 Fewer routes in a routing table means that the routing table process can more quickly locate the route needed to forward the packet. Summarizing several routes into a single route is known as route summarization or route aggregation. Some routing protocols, such as RIP, automatically summarize routes on certain routers. 84

85 Modified Topology: Scenario B / / /24 Three classful networks are used: / / /24 The /16 network is subnetted into three subnets: / / /24 The /24 network is subnetted as a single subnet /30. 85

86 Configuration Changes for R1 R1(config)# interface fa0/0 R1(config-if)# ip address R1(config-if)# interface S0/0/0 R1(config-if)# ip address R1(config-if)# no router rip R1(config)# router rip R1(config-router)# network R1(config-router)# network network address R1(config-router)# passive-interface FastEthernet 0/0 R1(config-router)# end R1# show run <output omitted>! router rip passive-interface FastEthernet0/0 network ! <output omitted> IOS automatically corrects subnet entries to classful The no shutdown and clock rate commands are not needed because these commands are still configured from Scenario A. 86

87 Configuration Changes for R2 R2(config)# interface S0/0/0 R2(config-if)# ip address R2(config-if)# interface fa0/0 R2(config-if)# ip address R2(config-if)# interface S0/0/1 R2(config-if)# ip address R2(config-if)# no router rip R2(config)# router rip R2(config-router)# network R2(config-router)# network R2(config-router)# passive-interface FastEthernet 0/0 R2(config-router)# end R2# show run <output omitted>! router rip passive-interface FastEthernet0/0 network network ! <output omitted> IOS automatically corrects subnet entries to classful network address 87

88 Configuration Changes for R3 R3(config)# interface fa0/0 R3(config-if)# ip address R3(config-if)# interface S0/0/1 R3(config-if)# ip address R3(config-if)# no router rip R3(config)# router rip R3(config-router)# network R3(config-router)# network R3(config-router)# passive-interface FastEthernet 0/0 R3(config-router)# end R3# show run <output omitted>! router rip passive-interface FastEthernet0/0 network network ! <output omitted> 88

89 Boundary Routers and Automatic Summarization RIP is a classful routing protocol that automatically summarizes classful networks across major network boundaries. 89

90 Boundary Routers and Automatic Summarization R2 has interfaces in more than one major classful network. This makes R2 a boundary router in RIP. Both Serial 0/0/0 and FastEthernet 0/0 interfaces on R2 are inside the boundary. The Serial 0/0/1 interface is inside the boundary. Boundary routers summarize RIP subnets from one major network to the other, updates for the , , and networks will automatically be summarized into when sent out R2 s Serial 0/0/1 interface. 90

91 Processing RIP Updates R2# show ip route /24 is subnetted, 3 subnets R [120/1] via , 00:00:18, Serial0/0/0 C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/ /30 is subnetted, 1 subnets C is directly connected, Serial0/0/1 R /24 [120/1] via , 00:00:16, Serial0/0/1 Classful routing protocols such as RIPv1 do not include the subnet mask in the routing update. However, the routing table includes RIPv1 routes with both the network address and the subnet mask. So how does a router running RIPv1 determine what subnet mask it should apply to a route when adding it to the routing table? 91

92 Rules for Processing RIPv1 Updates Routing Update and Interface Same Classful Major Network Different Classful Major Network Routing Update Subnet Mask Use mask of interface Use default classful mask The following two rules govern RIPv1 updates: If a routing update and the interface on which it is received belong to the same major network, the subnet mask of the interface is applied to the network in the routing update. If a routing update and the interface on which it is received belong to different major networks, the classful subnet mask of the network is applied to the network in the routing update.. 92

93 Example of RIPv1 Processing Updates R2# debug ip rip (selected output) RIP: received v1 update from on Serial0/0/ in 1 hops R2# show ip route (selected output) /24 is subnetted, 3 subnets R [120/1] via , 00:00:18, Serial0/0/0 Same classful network as the incoming update. Update: in 1 hops Interface received: Serial 0/0/ /24 Same classful network address ( ) Applies subnet mask of its S0/0/0 interface, /24. The /24 subnet was added to the routing table /

94 Sending RIP Updates R2# debug ip rip RIP protocol debugging is on RIP: sending v1 update to via Serial0/0/0 ( ) RIP: build update entries network metric 1 network metric 1 network metric 2 RIP: sending v1 update to via Serial0/0/1 ( ) RIP: build update entries network metric 1 94

95 Sending RIP Updates R2# debug ip rip RIP protocol debugging is on RIP: sending v1 update to via Serial0/0/0 ( ) RIP: build update entries network metric 1 network metric 1 network metric 2 RIP: sending v1 update to via Serial0/0/1 ( ) RIP: build update entries network metric 1 95

96 Sending RIP Updates R2# debug ip rip RIP protocol debugging is on RIP: sending v1 update to via Serial0/0/0 ( ) RIP: build update entries network metric 1 network metric 1 network metric 2 RIP: sending v1 update to via Serial0/0/1 ( ) RIP: build update entries network metric 1 96

97 Sending RIP Updates

98 Determining the mask and network address Receiving an Update: Determining subnet mask for routing table What is the major classful network address of the receiving interface? What is the major classful network address of the network in the routing update? Are they the same major classful network address? Yes: Apply subnet mask of the receiving interface for this network address in the routing table. No: Apply classful subnet mask for this network address in the routing table. Sending an Update: Determining whether or not to summarize route sent What is the major classful network address of the sending interface? What is the major classful network address of the network in the routing update? Are they the same major classful network address? Yes: Send subnet network address No: Send summary address the classful network address 98

99 Verifying Routing Updates R1# show ip route <output omitted> Gateway of last resort is not set /24 is subnetted, 3 subnets C is directly connected, FastEthernet0/0 C is directly connected, Serial0/0/0 R [120/1] via , 00:00:17, Serial0/0/0 R /24 [120/1] via , 00:00:17, Serial0/0/0 R /24 [120/2] via , 00:00:17, Serial0/0/0 R3# show ip route <output omitted> Gateway of last resort is not set R /16 [120/1] via , 00:00:15, Serial0/0/ /30 is subnetted, 1 subnets C is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 99

100 Classful routing protocols do not support VLSM Routers running RIPv1 are limited to using the same subnet mask for all subnets with the same classful network. As you will learn in later chapters, classless routing protocols such as RIPv2 allow the same major (classful) network to use different subnet masks on different subnets, better known as variable-length subnet masking (VLSM).. 100

101 Advantages of Automatic Summarization R3# show ip route <output omitted> Gateway of last resort is not set R /16 [120/1] via , 00:00:15, Serial0/0/ /30 is subnetted, 1 subnets C is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 Smaller routing updates are sent and received, which uses less bandwidth for routing updates between R2 and R3. R3 has a single route for the /16 network, regardless of how many subnets there are or how it is subnetted. Using a single route results in a faster lookup process in the routing table for R3. 101

102 Disadvantage of Automatic Summarization / /16 Discontiguous network, two or more subnets separated by at least one other major network /16 is a discontiguous network. 102

103 Discontiguous Networks Do Not Converge with RIPv1 R1(config)# router rip R1(config-router)# network R1(config-router)# network R2(config)# router rip R2(config-router)# network R2(config-router)# network R3(config)# router rip R3(config-router)# network R3(config-router)# network RIPv1 configuration is correct, but it is unable to determine all the networks in this discontiguous topology. 103

104 Discontiguous Networks Do Not Converge with RIPv / /16 Both routers, however, will advertise the major network address, a summary route to R2. 104

105 Discontiguous Networks Do Not Converge with RIPv1 R2# show ip route R /16 [120/1] via , 00:00:14, Serial0/0/1 [120/1] via , 00:00:19, Serial0/0/0 R2 has two equal-cost paths to the network. R2 will load-balance traffic destined for any subnet of This means that R1 will get half of the traffic and R3 will get the other half of the traffic, whether or not the destination of the traffic is for one of their LANs. 105

106 Discontiguous Networks Do Not Converge with RIPv1 R2# show ip route R /16 [120/1] via , 00:00:14, Serial0/0/1 [120/1] via , 00:00:19, Serial0/0/0 Classful routing protocols do not support discontiguous networks because they do not include the subnet mask in the routing update. Classless routing protocols (RIPv2, EIGRP, OSPF, IS-IS, BGP) do support discontiguous networks. 106

107 Default Route and RIPv1 Modified Topology C Propagating the Default Route in RIPv1

108 Modified Topology: Scenario C Default routes are used by routers to represent all routes that are not specifically in the routing table. A default route is commonly used to represent routes that are not in the locally administered network, such as the Internet.. 108

109 Default Routes In today s networks, customers do not necessarily have to exchange routing updates with their ISP. Customer routers that connect to an ISP do not need a listing for every route on the Internet. Instead, these routers have a default route that sends all traffic to the ISP router when the customer router does not have a route to a destination. The ISP configures a static route pointing to the customer router for addresses inside the customer s network. 109

110 Configuration Changes for R2 and R3 R2(config)# router rip R2(config-router)# no network R2(config-router)# exit R2(config)# ip route serial 0/0/1 R3(config)# no router rip R3(config)# ip route serial 0/0/1 110

111 Routing Table R1# show ip route <output omitted> Gateway of last resort is not set /24 is subnetted, 3 subnets C is directly connected, FastEthernet0/0 C is directly connected, Serial0/0/0 R [120/1] via , 00:00:05, Serial0/0/0 R1 has all /24 subnets, but will drop packets for all other networks. No default route (coming) 111

112 Routing Table R2# show ip route <output omitted> Gateway of last resort is to network /24 is subnetted, 3 subnets R [120/1] via , 00:00:03, Serial0/0/0 C is directly connected, Serial0/0/0 C is directly connected, FastEthernet0/ /30 is subnetted, 1 subnets C is directly connected, Serial0/0/1 S* /0 is directly connected, Serial0/0/1 R2 has routes for /16 subnets. R2 has static default route for all other networks 112

113 Routing Table R3# show ip route <output omitted> Gateway of last resort is not set /22 is subnetted, 1 subnets S is directly connected, Serial0/0/ /30 is subnetted, 1 subnets C is directly connected, Serial0/0/1 C /24 is directly connected, FastEthernet0/0 R3 has static route for /16 network. Doesn t matter if or how /16 is subnetted, R3 will forward packets to R2. 113

114 Propagating the Default Route in RIPv1 R1# show ip route <output omitted> Gateway of last resort is not set /24 is subnetted, 3 subnets C is directly connected, FastEthernet0/0 C is directly connected, Serial0/0/0 R [120/1] via , 00:00:05, Serial0/0/0 Can configure static default route on every router but: inefficient does not react to topology changes In many routing protocols, including RIP, you can use the defaultinformation originate command in router configuration mode to specify that this router is to originate default information, by propagating the static default route in RIP updates. 114

115 Propagating the Default Route in RIPv1 R2(config)# router rip R2(config-router)# default-information originate R2(config-router)# end R2# debug ip rip RIP: sending v1 update to via Serial0/0/0 ( ) RIP: build update entries subnet metric 1 subnet metric 1 115

116 Propagating the Default Route in RIPv1 R1# show ip route <output omitted> * - candidate default, U - per-user static route, o - ODR Gateway of last resort is to network /24 is subnetted, 3 subnets C is directly connected, Serial0/0/0 R [120/1] via , 00:00:16, Serial0/0/0 C is directly connected, FastEthernet0/0 R* /0 [120/1] via , 00:00:16, Serial0/0/0 The static default route on R2 has been propagated to R1 in a RIP update. R1 has connectivity to the LAN on R3 and any destination on the Internet. 116

117 RIPv1 Limitations / /16 In a discontiguous network, a classful major network address, such as /16, is separated by one or more other major networks /16 is divided by the networks: / /30 Classful routing protocols do not include enough routing information to route properly for discontiguous networks. 117

118 RIPv1 and RIPv2 RIPv2 is actually an enhancement of RIPv1 s features and extensions rather than an entirely new protocol. Next-hop addresses included in the routing updates Use of multicast addresses in sending updates Authentication option available Both versions of RIP share the following features and limitations: Use of hold-down and other timers to help prevent routing loops Use of split horizon and split horizon with poison reverse to also help prevent routing loops Use of triggered updates when there is a change in the topology for faster convergence Maximum hop count of 15 hops, with the hop count of 16 signifying an unreachable network 118