Troubleshooting Routing and Routing Protocols
Cisco would like all its certification exams—CCNA included—to prove that the test taker can build and troubleshoot live networks. Some people work with Cisco routers daily. Others' job function does not allow frequent access to routers. If the latter description applies to you, you might be trying to pass this certification so that you can move into jobs that involve routers and switches. Regardless, this section gives you some final insights into some tricky problems with routing protocols.
The show ip route command has a myriad of options that are helpful when you're troubleshooting a large network. The show ip protocol command also can provide some useful information when you're troubleshooting a routing problem. With a small network, most of the options with the show ip route command are unnecessary. However, knowing the options and what each can do is useful for your work with larger networks.
Example 6-21 lists the options of the show ip route command and gives examples of several of them. Figure 7-13 shows the network; it should look familiar from previous examples. In this case, EIGRP is used between Albuquerque and Seville, and RIP-2 is used between Albuquerque and Yosemite. There is no PVC between Yosemite and Seville. The configurations of the three routers are listed in Examples 7-18, 7-19, and 7-20. Example 7-21 lists the show ip route options.
Figure 7-13 Network Environment for Use with the show ip route Options
172.16.2.251
Figure 7-13 Network Environment for Use with the show ip route Options
172.16.2.251
Example 7-18 Albuquerque Configuration for the show ip route Options in Example 7-21
Albuquerque#show running-config
Current configuration : 964 bytes version 12.2
Example 7-18 Albuquerque Configuration for the show ip route Options in Example 7-21 (Continued)
service timestamps debug uptime service timestamps log uptime no service password-encryption !
hostname Albuquerque !
enable secret 5 $1$J3Fz$QaEYNIiI2aMu.3Ar.q0Xm.
enable password fred !
ip subnet-zero no ip domain-lookup !
interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay IETF clockrate 56000 frame-relay lmi-type cisco
interface Serial0.1 point-to-point ip address 172.16.3.251 255.255.255.0 no ip directed-broadcast frame-relay interface-dlci 902
interface Serial0.2 point-to-point ip address 172.16.1.251 255.255.255.0 no ip directed-broadcast frame-relay interface-dlci 903
interface Serial1 no ip address no ip directed-broadcast shutdown
interface Ethernet0 ip address 172.16.2.251 255.255.255.0 no ip directed-broadcast
router eigrp 9 passive-interface Serial0.1 network 172.16.0.0 no auto-summary
passive-interface Serial0.2 network 172.16.0.0 no auto-summary
ip classless no ip http server !
access-list 1 permit 10.0.0.0 0.255.255.255
Example 7-19 Yosemite Configuration for the show ip route Options in Example 7-21
Yosemite#show running-config
Current configuration : 968 bytes !
version 12.2
service timestamps debug uptime service timestamps log uptime no service password-encryption !
hostname Yosemite !
enable secret 5 $1$J3Fz$QaEYNIiI2aMu.3Ar.q0Xm. !
ip subnet-zero no ip domain-lookup !
interface Serial0 no ip address no ip directed-broadcast encapsulation frame-relay IETF no fair-queue frame-relay lmi-type cisco
interface Serial0.1 point-to-point ip address 172.16.3.252 255.255.255.0 no ip directed-broadcast frame-relay interface-dlci 901
interface Serial1 no ip address no ip directed-broadcast shutdown
interface Ethernet0 ip address 10.1.8.253 255.255.255.0
interface Ethernet1 ip address 10.1.9.253 255.255.255.0
interface Ethernet2 ip address 10.1.10.253 255.255.255.0
interface Ethernet3 ip address 10.1.11.253 255.255.255.0
router rip version 2 network 10.0.0.0 network 172.16.0.0 no auto-summary
ip classless no ip http server
Example 7-20 Seville Configuration for the show ip route Options in Example 7-21
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Seville#show running-config |
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Current configuration : 960 bytes |
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version 12.2 |
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service timestamps debug uptime |
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service timestamps log uptime |
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no service password-encryption |
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hostname Seville |
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enable secret 5 $1$J3Fz$QaEYNIiI2aMu |
3Ar |
q0Xm. |
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ip subnet-zero |
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no ip domain-lookup |
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interface Serial0 |
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no ip address |
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no ip directed-broadcast |
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encapsulation frame-relay IETF |
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no fair-queue |
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frame-relay lmi-type cisco |
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interface Serial0.1 multipoint |
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ip address 172.16.1.253 255.255.255 |
0 |
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no ip directed-broadcast |
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ip summary-address eigrp 9 10.1.4.0 |
255 |
255.252.0 |
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frame-relay interface-dlci 901 |
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interface Serial1 |
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no ip address |
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no ip directed-broadcast |
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shutdown |
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interface Ethernet0 |
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ip address 10.1.4.253 255.255.255.0 |
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interface Ethernet1 |
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ip address 10.1.5.253 255.255.255.0 |
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interface Ethernet2 |
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ip address 10.1.6.253 255.255.255.0 |
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interface Ethernet3 |
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ip address 10.1.7.253 255.255.255.0 |
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router eigrp 9 |
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network 10.0.0.0 |
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network 172.16.0.0 |
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no auto-summary |
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ip classless |
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no ip http server |
Example 7-21 show ip route: Albuquerque
Albuquerque#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
172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, Serial0.2
C 172.16.2.0 is directly connected, Ethernet0
C 172.16.3.0 is directly connected, Serial0.1
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks R 10.1.11.0/24 [120/1] via 172.16.3.252, 00:00:17, Serial0.1
R 10.1.10.0/24 [120/1] via 172.16.3.252, 00:00:17, Serial0.1
R 10.1.9.0/24 [120/1] via 172.16.3.252, 00:00:17, Serial0.1
R 10.1.8.0/24 [120/1] via 172.16.3.252, 00:00:17, Serial0.1
D 10.1.4.0/22 [90/2185984] via 172.16.1.253, 00:28:01, Serial0.2
Albuquerque#show ip route ?
Hostname or A.B.C.D Network to display information about or hostname
Albuquerque#show ip route ?
Hostname or A.B.C.D Network to display information about or hostname
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bgp |
Border Gateway Protocol (BGP) |
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connected |
Connected |
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egp |
Exterior Gateway Protocol (EGP) |
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eigrp |
Enhanced Interior Gateway Routing Protocol (EIGRP) |
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igrp |
Interior Gateway Routing Protocol (IGRP) |
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isis |
ISO IS-IS |
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list |
IP Access list |
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mobile |
Mobile routes |
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odr |
On Demand stub Routes |
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ospf |
Open Shortest Path First (OSPF) |
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profile |
IP routing table profile |
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rip |
Routing Information Protocol (RIP) |
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static |
Static routes |
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summary |
Summary of all routes |
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supernets-only |
Show supernet entries only |
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vrf |
Display routes from a VPN Routing/Forwarding instance Output modifiers |
Albuquerque#show ip route 10.1.5.8
Routing entry for 10.1.4.0/22
Known via "eigrp 9", distance 90, metric 2185984, type internal Redistributing via eigrp 9
Last update from 172.16.1.253 on Serial0.2, 00:28:36 ago Routing Descriptor Blocks:
* 172.16.1.253, from 172.16.1.253, 00:28:36 ago, via Serial0.2 Route metric is 2185984, traffic share count is 1 Total delay is 20630 microseconds, minimum bandwidth is 1544 Kbit
Example 7-21 show ip route: Albuquerque (Continued)
Reliability 255/255, minimum MTU 1500 bytes Loading 1/255, Hops 1
Albuquerque#show ip route rip
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks R 10.1.11.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.10.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.9.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.8.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
Albuquerque#show ip route igrp
Albuquerque#show ip route eigrp
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks D 10.1.4.0/22 [90/2185984] via 172.16.1.253, 00:29:42, Serial0.2
Albuquerque#show ip route connected
172.16.0.0/24 is subnetted, 3 subnets C 172.16.1.0 is directly connected, Serial0.2
C 172.16.2.0 is directly connected, Ethernet0
C 172.16.3.0 is directly connected, Serial0.1
Albuquerque#show ip route list 1
10.0.0.0/8 is variably subnetted, 5 subnets, 2 masks R 10.1.11.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.10.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.9.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
R 10.1.8.0/24 [120/1] via 172.16.3.252, 00:00:22, Serial0.1
D 10.1.4.0/22 [90/2185984] via 172.16.1.253, 00:29:58, Serial0.2
Albuquerque#show ip route summary
Route Source Networks Subnets Overhead Memory (bytes)
connected 0 3 156 420
static 0 0 0 0
rip 0 4 208 560
eigrp 9 0 1 52 140
internal 2 2320
Total 2 8 416 3440
Albuquerque#show ip route supernet
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, * - candidate default U - per-user static route, o - ODR
Gateway of last resort is not set
The show ip route command with no options has been seen many times in this book. A review of some of the more important bits of the output is in order; most comments refer to a highlighted portion. First, the legend at the beginning of Example 7-21 defines the letter codes that identify the source of the routing information—for example, C for connected routes, R for RIP, and I for IGRP. Each of the Class A, B, and C networks is listed, along with each of the subnets of that network. If a static mask is used within that network, the mask is shown only in the line referring to the network (as is the case in Example 7-21, network 172.16.0.0). If the network uses VLSM, as network 10.0.0.0 appears to do because of the route summarization done by Seville, the mask information is listed on the lines referring to each of the individual subnets.
Each routing entry lists the subnet number and the outgoing interface. In most cases, the next-hop router's IP address is also listed. The outgoing interface is needed so that the router can choose the type of data link header to use to encapsulate the packet before transmission on that interface. The next-hop router's IP address is needed on interfaces for which the router needs the IP address so that it can find the associated data-link address to put in the newly built data link header. For instance, knowing the next-hop IP address of 172.16.3.252, Yosemite's IP address on the Frame Relay VC allows Albuquerque to find the corresponding DLCI in the Frame Relay map.
The numbers in brackets in the show ip route output for each route are interesting. The second number in brackets represents the metric value for this route. The first number defines the administrative distance.
Administrative distance is important only if multiple IP routing protocols are in use in a single router. When this is true, both routing protocols can learn routes to the same subnets. Because their metric values are different (for example, hop count or a function of bandwidth and delay), there is no way to know which routing protocol's routes are better. Therefore, Cisco supplies a method of defining which routing protocol's routes are better. The Cisco IOS Software implements this concept using something called administrative distance.
Administrative distance is an integer value; a value is assigned to each source of routing information. The lower the administrative distance, the better the source of routing information. IGRP's default is 100, OSPF's is 110, RIP's is 120, and EIGRP's is 90. The value 100 in brackets in the show ip route output signifies that the administrative distance used for IGRP routes is 100. In other words, the default value is in use. So, if RIP and IGRP are both used, and if both learn routes to the same subnets, only IGRP's routing information for those subnets is added to the routing table. If RIP learns about a subnet that IGRP does not know about, that route is added to the routing table.
Moving down Example 7-21, the show ip route ? command lists several options, many of which are shown in the ensuing commands in the example. You can limit the show ip route output to the routes learned by a particular routing protocol by referring to that routing protocol. Likewise, the output can be limited to show just connected routes.
One of the more important options for the show ip route command is to simply pass an IP address as the last parameter. This tells the router to perform routing table lookup, just as it would for a packet destined for that address. In Example 7-21, show ip route 10.1.5.8 returns a set of messages, the first of which identifies the route to 10.1.4.0/22 as the route matched in the routing table. The route that is matched is listed so that you can always know the route that would be used by this router to reach a particular IP address.
Finally, another feature of show ip route that is useful in large networks is filtering the command's output based on an access list. Notice the command show ip route list 1 in Example 7-21. Access list 1 is configured so that any route with information about network 10.0.0.0 is matched (permitted by the access list) and all others are denied. By referring to the access list, the show ip route output is filtered, showing only a portion of the routes. This is particularly useful when there are many routes in the routing table.
The many options of the show ip route command can be particularly useful for troubleshooting larger networks.
Continue reading here: Standard IP Access List Configuration
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