RIP Features 739 The router sends update information out the two RIP interfaces. The output shows the router is using RIP-1 and broadcasts the update (address 255.255.255.255). The num- ber in parentheses represents the source address encapsulated into the IP header of the RIP update. You can look for several problems in the debug ip rip output. A couple of the problems that you can diagnose are discontiguous subnetworks or duplicate networks. A symp- tom of these problems is a routing protocol’s advertising a network route with a metric that is less than the metric received for that network. Example 16-8 shows the output of the debug ip rip command. Additionally, you can use the following commands to troubleshoot RIP: ■ show ip rip database—Used to display the contents of the private database when triggered extensions to RIP are enabled. ■ show ip protocols {summary}—Used to display IP routing protocol information. ■ show ip route—Used to show the IP routing table on the router. ■ debug ip rip {events}—Used to display the RIP information the router is process- ing across the prompt for an administrator to see. ■ show ip interface brief—Lists a summary of an interface’s IP information and status in privileged EXEC mode. The brief parameter is an option that displays a brief summary of IP status and configuration. Example 16-8 debug ip rip Command Output BMH# debug ip rip RIP event debugging is on BHM# 7w2d: RIP: received v1 update from 192.168.13.2 on serial0/0 7w2d: 192.168.14.0 1 hop 7w2d: 172.31.0.0 in 2 hops 7w2d: RIP: sending v1 update to 255.255.255.255 via Serial0/0 (192.168.13.1) 7w2d: network 172.31.0.0 metric 1 7w2d: RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (10.0.0.254) 7w2d: 192.168.13.0 metric 1 7w2d: 192.168.14.0 metric 2 chpt_16.fm Page 739 Tuesday, May 27, 2003 2:16 PM 740 Chapter 16: Distance Vector Routing Protocols All of these commands provide information that can be helpful when troubleshooting a router. Load Balancing with RIP Load balancing is a concept that allows a router to take advantage of multiple best paths to a given destination. These paths are derived either statically or with a dynamic protocol such as RIP. RIP is capable of load balancing over as many as six equal-cost paths. Load balancing over four paths is the default. RIP performs what is referred to as round robin load balancing, which means that RIP takes turns forwarding packets over the parallel paths. Figure 16-16 shows an example of RIP routes with four equal-cost paths. The router starts with an interface pointer to the interface connected to router 1. Then the inter- face pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1 and so on. Because the metric for RIP is hop count, no regard is given to the speed of the links. Therefore, the 56-kbps path handles as much traffic between the two networks as the 155-Mbps path. Figure 16-16 Load Balancing RIP Lab Activity Troubleshooting RIP In this lab, you set up an IP addressing scheme using Class B networks and configure RIP on the routers. You observe routing activity using the debug ip rip command and examine routes using the show ip route command. chpt_16.fm Page 740 Tuesday, May 27, 2003 2:16 PM RIP Features 741 Equal-cost routes can usually be found by using the show ip route command. Exam- ple 16-9 is a display of the output for show ip route to a particular subnet with multiple routes. Notice there are two routing descriptor blocks. Each block is one route. Also, an aster- isk (*) is next to one of the block entries. This asterisk corresponds to the active route that is used for new traffic. Integrating Static Routes with RIP Static routes are user-defined routes that force packets to take a specified path to their destination. Static routes become very important if Cisco IOS Software cannot build a route to a particular destination. They are also useful for specifying a “gateway of last resort,” which all packets without a more specific route are sent through. Example 16-9 Verifying Equal Cost Routes via show ip route Command Output RouterC# show ip route 192.168.2.0 Routing entry for 192.168.2.0/24 Known via "rip", distance 120, metric 1 Redistributing via rip Last update from 192.168.4.2 on FastEthernet0/0, 00:00:18 ago Routing Descriptor Blocks: 192.168.4.1, from 192.168.4.1, 00:02:45 ago, via FastEthernet0/0 Route metric is 1, traffic share count is 1 * 192.168.4.2, from 192.168.4.2, 00:00:18 ago, via FastEthernet0/0 Route metric is 1, traffic share count is 1 Lab Activity Preventing Routing Updates Through an Interface In this lab, you prevent routing updates through an interface to regulate adver- tised routes and observe the results. You use the Passive-interface command and add a default route. Lab Activity Load Balancing Across Multiple Paths In this lab, you configure load balancing across multiple paths with RIP and then observe the load balancing process. chpt_16.fm Page 741 Tuesday, May 27, 2003 2:16 PM 742 Chapter 16: Distance Vector Routing Protocols A router running RIP can receive a default network address through an update from another router running RIP. Another option is for the router to generate the default network itself. The static routes can be removed using the no ip route global configuration command. The administrator can override a dynamic route with static routing information using administrative distance values. Each dynamic routing protocol has a default adminis- trative distance, which allows the static route to act as a backup for the dynamic route in the event that it fails. Static routes that point to an interface are advertised via RIP, because static routes that point to an interface are considered to be connected in the routing table and thus lose their static nature. If a static route is assigned to an interface that is not one of the net- works defined in a network command, no dynamic routing protocols advertise the route unless a redistribute static command is specified for these protocols. When an interface goes down, all static routes through that interface are removed from the IP routing table. Additionally, when the software can no longer find a valid next hop for the address specified as the forwarding address for a router in a static route, the static route is removed from the IP routing table. A static route is one that is specifically entered by an administrator so that the router specifically knows the route to a destination. A dynamic route is one that is learned by the router using the various routing protocol standards. In this case, it is not guaran- teed that the router knows the route to the intended destination. To configure a static route, use the following command in global configuration mode: ip route prefix mask {address | interface} [distance] [tag tag] [permanent] In Figure 16-17, a static route is configured in the GAD router to take the place of the RIP route in the event that the RIP route fails. Example 16-10 shows the static route being added with an administrative distance of 130,which is referred to as a floating static route. The floating static route is configured by declaring an administrative dis- tance (130), which is greater than the administrative distance of RIP (120). The BHM router needs to be configured with a default route. chpt_16.fm Page 742 Tuesday, May 27, 2003 2:16 PM RIP Features 743 Figure 16-17 RIP with Floating Static Routes Example 16-10 shows the configuration for the static route in Figure 16-17. Note that the static route is treated like a dynamic route because the next-hop interface is specified. It starts with an R instead of an S. Example 16-10 Floating Static Route GAD# configure terminal GAD(config)# ip route 172.16.0.0 255.255.0.0 192.168.14.2 130 GAD(config)# ^z GAD# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 192.168.113.0/24 is directly connected, Serial0/0 C 192.168.14.0/24 is directly connected, BRI0/1 R 172.16.0.0/16 [120/1] via 192.168.13.2, 00:00:24, Serial0/0 Ethernet BHM Host B GAD S0/0 .1 1.544 Mbps 192.168.13.0/24 .2 192.168.14.0/24 .2 BRI0/1 .1 Dialup 1 7 2 . 1 6 . 0 . 0 chpt_16.fm Page 743 Tuesday, May 27, 2003 2:16 PM 744 Chapter 16: Distance Vector Routing Protocols IGRP Like RIP, Interior Gateway Routing Protocol (IGRP) is a distance vector routing pro- tocol. Unlike RIP, IGRP is a Cisco-proprietary protocol rather than a standards-based protocol. While remaining very simple to implement, IGRP is a more complex routing protocol than RIP, and it is able to use a number of factors to determine the best route to a destination network. This section introduces IGRP configuration and troubleshoot- ing as well as the following IGRP topics: ■ IGRP features ■ IGRP metrics ■ IGRP routes ■ IGRP stability features ■ Configuring IGRP ■ Migrating RIP to IGRP ■ Verifying IGRP configuration ■ Troubleshooting IGRP IGRP Features IGRP is a distance vector IGP. Distance vector routing protocols mathematically com- pare routes by measuring distances. This measurement is known as the distance vector. Routers using distance vector protocols must send all or a portion of their routing table in a routing update message at regular intervals to each of their neighboring routers. As routing information proliferates through the network, routers can perform the fol- lowing functions, among others: ■ Identify new destinations ■ Learn of failures IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90-second intervals, advertising networks for a particular autonomous system. Key design characteristics of IGRP are as follows: ■ The versatility to automatically handle indefinite, complex topologies ■ The flexibility needed to segment with different bandwidth and delay characteristics ■ Scalability for functioning in very large networks chpt_16.fm Page 744 Tuesday, May 27, 2003 2:16 PM IGRP 745 By default, the IGRP routing protocol uses bandwidth and delay as metrics. Addition- ally, IGRP can be configured to use a combination of variables to determine a composite metric. Those variables include ■ Bandwidth ■ Delay ■ Load ■ Reliability IGRP Metrics The show ip protocols command displays parameters, filters, and network information concerning the routing protocols in use on the router. You need this information to define the value of the K1–K5 metrics and provide information concerning the maxi- mum hop count to calculate the composite metric for IGRP, which is figured as fol- lows: Metric = [K1 × Bandwidth + (K2 × Bandwidth)/(256 – Load) + K3 × Delay] × [K5/(Reliability + K4)] The metric K1 represents bandwidth, and the metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1, while K2, K4, and K5 are set to 0. The default constant values are K1 = K3 = 1 and K2 = K4 = K5 = 0. If K5 = 0, the [K5/(reliability + K4)] term is not used. So, given the default values for K1 through K5, the composite metric calculation used by IGRP reduces to Metric = Bandwidth + Delay. The K values in these formulas are constants that can be defined using the following router configuration command: metric weights tos k1 k2 k3 k4 k5 To find the bandwidth, find the smallest of all the bandwidths from outgoing interfaces and divide 10,000,000 by that number. (The bandwidth is scaled by 10,000,000 in kilobits per second.) To find the delay, add all the delays from the outgoing interfaces and divide this number by 10. (The delay is in tens of microseconds.) Remember, the path with the smallest metric is the best path. This composite metric is more accurate than RIP’s hop-count metric when choosing a path to a destination. The path that has the smallest metric value is the best route. IGRP’s metric includes the following components: ■ Bandwidth—The lowest bandwidth value in the path chpt_16.fm Page 745 Tuesday, May 27, 2003 2:16 PM 746 Chapter 16: Distance Vector Routing Protocols ■ Delay—The cumulative interface delay along the path ■ Reliability—The reliability on the link toward the destination as determined by the exchange of keepalives ■ Load—The load on a link toward the destination based on bits per second ■ MTU—The maximum transmission unit value of the path IGRP uses a composite metric, which is calculated as a function of bandwidth, delay, load, and reliability. By default, only the bandwidth and delay characteristics are con- sidered; the other parameters are considered only if enabled via configuration. Delay and bandwidth are not measured values, but are set via the delay and bandwidth inter- face commands. The show ip route command in Example 16-11 shows the IGRP met- ric values in brackets. The first number represents the administrative distance, and the second number is the calculated metric value. A link with a higher bandwidth has a lower metric, and a route with a lower cumulative delay has a lower metric. Interior, System, and Exterior IGRP Routes IGRP advertises three types of routes: ■ Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes. Example 16-11 show ip route Command Output Reveals IGRP Metric Values RouterA# show ip route Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR P - periodic downloaded static route Gateway of last resort is not set C 192.168.1.0/24 is directly connected, FastEthernet0/0 C 192.168.2.0/24 is directly connected, Serial0/0 I 192.168.3.0/24 [100/80135] via 192.168.2.2, 00:00:30, Serial0/0 chpt_16.fm Page 746 Tuesday, May 27, 2003 2:16 PM IGRP 747 ■ System routes are routes to networks within an autonomous system. Cisco IOS Software derives system routes from directly connected network interfaces and system route information provided by other IGRP-speaking routers or access servers. System routes do not include subnet information. ■ Exterior routes are routes to networks outside the autonomous system that are considered when identifying a gateway of last resort. Cisco IOS Software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway (router) of last resort if a better route is not found and the destination is not a connected network. If the autonomous system has more than one connection to an external network, different routers can choose differ- ent exterior routers as the gateway of last resort. IGRP Stability Features IGRP has a number of features that are designed to enhance its stability, such as ■ Holddowns ■ Split horizon ■ Poison reverse updates Holddowns are used to prevent regular update messages from inappropriately reinstat- ing a route that might not be up. When a router goes down, neighboring routers detect this status via the lack of regularly scheduled update messages. Split horizon is derived from the premise that it is usually not useful to send information about a route back in the direction from which it came. The split-horizon rule helps prevent routing loops. Split horizon prevents routing loops between adjacent routers, but poison reverse updates are necessary to defeat larger routing loops. Generally speaking, increases in routing metrics indicate routing loops. Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1. IGRP also maintains a number of timers and variables containing time intervals that include the following: ■ Update timer—Specifies how frequently routing update messages are sent. The IGRP default for this variable is 90 seconds. ■ Invalid timer—Specifies how long a router waits in the absence of routing- update messages about a specific route before declaring that route invalid. The IGRP default for this variable is three times the update period. NOTE Today, IGRP is show- ing its age; it lacks support for variable- length subnet masks (VLSMs). Rather than develop an IGRP ver- sion 2 to correct this problem, Cisco has built upon IGRP’s legacy of success with Enhanced IGRP (EIGRP). chpt_16.fm Page 747 Tuesday, May 27, 2003 2:16 PM 748 Chapter 16: Distance Vector Routing Protocols ■ Hold-time timer—Specifies the amount of time for which information about poorer routes is ignored. The IGRP default for this variable is three times the update timer period plus 10 seconds. ■ Flush timer—Indicates how much time passes before a route is flushed from the routing table. The IGRP default is seven times the routing update timer. Example 16-12 shows the output from the show ip protocols command. Notice the line that indicates the IGRP is running and its metric values. Configuring IGRP To configure the IGRP routing process, use the router igrp global configuration command: RouterA(config)# router igrp as-number To shut down an IGRP routing process, use the no form of this command. RouterA(config)# no router igrp as-number Example 16-12 IGRP Routing Statistics RouterB# show ip protocols Routing Protocol is "igrp 101" Sending updates every 90 seconds, next due in 51 seconds Invalid after 270 seconds, hold down 280, flushed after 630 Outgoing update filter list for all interfaces is Incoming update filter list for all interfaces is Default networks flagged in outgoing updates Default networks accepted from incoming updates IGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0 IGRP maximum hopcount 100 IGRP maximum metric variance 1 Redistributing: igrp 101 Routing for Networks: 192.168.2.0 192.168.3.0 Routing Information Sources: Gateway Distance Last Update 192.168.2.1 100 00:00:54 Distance: (default is 100) chpt_16.fm Page 748 Tuesday, May 27, 2003 2:16 PM . 19 2 .16 8 .14 .0 /24 is directly connected, BRI0 /1 R 17 2 .16 .0.0 /16 [ 12 0 /1] via 19 2 .16 8 .13 .2, 00:00 :24 , Serial0/0 Ethernet BHM Host B GAD S0/0 .1 1.544 Mbps 19 2 .16 8 .13 .0 /24 .2 19 2 .16 8 .14 .0 /24 .2 BRI0 /1 .1 Dialup 1 7 2 . 1 6 . 0 . 0 chpt _16 .fm. on BHM# 7w2d: RIP: received v1 update from 19 2 .16 8 .13 .2 on serial0/0 7w2d: 19 2 .16 8 .14 .0 1 hop 7w2d: 17 2. 31. 0.0 in 2 hops 7w2d: RIP: sending v1 update to 25 5 .25 5 .25 5 .25 5 via Serial0/0 (19 2 .16 8 .13 .1) 7w2d:. (19 2 .16 8 .13 .1) 7w2d: network 17 2. 31. 0.0 metric 1 7w2d: RIP: sending v1 update to 25 5 .25 5 .25 5 .25 5 via FastEthernet0/0 (10 .0.0 .25 4) 7w2d: 19 2 .16 8 .13 .0 metric 1 7w2d: 19 2 .16 8 .14 .0 metric 2 chpt _16 .fm Page 739