To specify a list of networks for IGRP routing processes, use the network router con-figuration command: RouterAconfig# router igrp 101 RouterAconfig-router# network 192.168.1.0 To remov
Trang 1The autonomous system number is one that identifies the IGRP process It is also used
to tag the routing information
To specify a list of networks for IGRP routing processes, use the network router
con-figuration command:
RouterA(config)# router igrp 101
RouterA(config-router)# network 192.168.1.0
To remove an entry, use the no form of the command:
RouterA(config)# no router igrp 101
RouterA(config-router)# no network 192.168.1.0
Example 16-13 demonstrates how to configure IGRP using AS 101 on RouterA and
RouterB
Verifying the IGRP Configuration
To verify that IGRP has been configured properly, enter the show ip route command
and look for IGRP routes signified by an I
Additional commands for checking IGRP configuration are as follows:
■ show interface interface—Verifies that the Ethernet interface is properly configured.
■ show running-config—Verifies that IGRP is enabled on the router.
■ show running-config interface interface—Verifies that the proper IP address has
been used
■ show running-config | begin interface interface—Verifies that IGRP is running on
the router’s interfaces starting at a specific interface
Example 16-13 Configuring IGRP
RouterA(config)# router igrp 101
RouterA(config-router)# network 192.168.1.0
RouterA(config-router)# network 192.168.2.0
RouterB(config)# router igrp 101
RouterB(config-router)# network 192.168.2.0
RouterB(config-router)# network 192.168.3.0
Lab Activity Configuring IGRP
In this lab, you set up an IP addressing scheme using Class C networks and configure IGRP on all routers
Trang 2■ show running-config | begin igrp—Verifies that IGRP is enabled on the router.
■ show ip protocols—Verifies that IGRP is enabled on the router.
To verify that the Ethernet interface is properly configured, enter the show interface
fa0/0 command Example 16-14 illustrates the output.
To see if IGRP is enabled on the router, use the commands demonstrated in Example 16-15
To check for proper IP addresses use the command demonstrated in Example 16-16
Example 16-14 show interface Command Output
RouterA# show interface fa0/0 FastEthernet0/0 is up, line protocol is up Hardware is AmdFE, address is 0009.7c89.5620 (bia 0009.7c89.5620) Internet address is 192.168.1.1/24
Output Omitted
-Example 16-15 show ip protocols and show running-config Command Output
RouterA# show ip protocols Routing Protocol is "igrp 101"
Output Omitted
-RouterA# show running-config | begin igrp router igrp 101
network 192.168.1.0 network 192.168.2.0
! Output Omitted
-Example 16-16 show running-config interface Command Output
RouterA# show running-config interface fa0/0 Building configuration
Current configuration:
! interface FastEthernet0/0
Trang 3Example 16-17 illustrates the output from the show ip route command and displays
the routes available via this router’s interfaces
Troubleshooting IGRP
Most IGRP configuration errors involve a mistyped network statement, discontiguous
subnets, or an incorrect autonomous system number
The following commands are useful when troubleshooting IGRP:
■ show ip protocols—Used to display IP routing protocol information in summary
form
■ show ip route—Used to show the IP routing table on the router.
■ debug ip igrp events—Diplays information about summary IGRP routing that is
running on the network
■ debug ip igrp transactions—Shows message requests from neighbor routers
ask-ing for an update and the broadcasts sent from the originatask-ing router towards that neighbor router
ip address 192.168.1.1 255.255.255.0
no ip directed-broadcast
end
Example 16-17 show ip route Command Output
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, Loopback0
C 192.168.2.0/24 is directly connected, Serial0/0
I 192.168.3.0/24 [100/80135] via 192.168.2.2, 00:01:00, Serial0/0
Example 16-16 show running-config interface Command Output (Continued)
Trang 4■ ping—A utility used to determine whether a specific IP address is accessible.
■ traceroute—A utility that traces a packet from your computer to an Internet
host, showing how many hops the packet requires to reach the host and how long each hop takes
Example 16-18 illustrates output from debug ip igrp events command
Example 16-19 illustrates output from debug ip igrp transactions command.
Example 16-18 debug ip igrp events Command Output
RouterA# debug ip igrp events IGRP event debugging is on
00:21:38: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1 .1)
00:21:38: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes 00:21:38: IGRP: Total routes in update: 2
00:21:38: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:21:38: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes 00:21:38: IGRP: Total routes in update: 1
Example 16-19 debug ip igrp transactions Command Output
RouterA# debug ip igrp transactions IGRP protocol debugging is on
00:22:17: IGRP: received update from 192.168.2.2 on Serial0/0 00:22:17: network 192.168.3.0, metric 80135 (neighbor 110) 00:23:07: IGRP: sending update to 255.255.255.255 via FastEthernet0/0 (192.168.1 .1)
00:23:07: network 192.168.2.0, metric=80125 00:23:07: network 192.168.3.0, metric=80135 00:23:07: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1) 00:23:07: network 192.168.1.0, metric=110
Trang 5If it is discovered that the wrong AS number is being used, correcting it as in Example
16-20 results in the corrected output
Example 16-20 debug ip igrp transactions
RouterA(config)# no router igrp 102
RouterA(config)# router igrp 101
RouterA(config-router)# network 192.168.1.0
RouterA(config-router)# network 192.168.2.0
00:27:50: IGRP: broadcasting request on FastEthernet0/0
00:27:50: IGRP: sending update to 255.255.255.255 via FastEthernet0/0
(192.168.1.1)
00:27:51: IGRP: Update contains 0 interior, 0 system, and 0 exterior routes.
00:27:51: IGRP: Total routes in update: 0 - suppressing null
00:28:01: IGRP: sending update to 255.255.255.255 via FastEthernet0/0
(192.168.1.1)
00:28:01: network 192.168.2.0, metric=80125
00:28:01: network 192.168.3.0, metric=80135
00:28:01: IGRP: Update contains 0 interior, 2 system, and 0 exterior routes.
00:28:01: IGRP: Total routes in update: 2
00:28:01: IGRP: sending update to 255.255.255.255 via Serial0/0 (192.168.2.1)
00:28:01: network 192.168.1.0, metric=110
00:28:01: IGRP: Update contains 0 interior, 1 system, and 0 exterior routes.
00:28:01: IGRP: Total routes in update: 1
Lab Activity Default Routing with RIP and IGRP
In this lab, you configure a default route and use RIP to propagate this default information to other routers When you have this configuration working, you must migrate the network from RIP to IGRP and configure default routing to work with that protocol as well
Lab Activity Unequal Cost Load Balancing with IGRP
In this lab, you configure and tune IGRP for unequal-cost load balancing and observe load balancing using debug commands
Trang 6In this chapter, you learned the following:
■ Routing information is maintained through updates as the topology changes in a network
■ Routing loops occur in a network from alternate routes, slow convergence, and inconsistent routing updates
■ Defining a maximum can be achieved to prevent count to infinity
■ Three methods of preventing routing loops: split horizon, triggered updated, and hold-down timers
■ Route poisoning is used by various distance vector protocols to overcome large routing loops and offer information about when a subnet or network is not accessible
■ To configure routing protocols: RIP and IGRP
■ To use the ip classless command
■ To troubleshoot routing protocols: RIP and IGRP
■ To verify routing protocols: RIP and IGRP
■ To configure default routes
To supplement all that you’ve learned in this chapter, refer to the chapter-specific Videos, PhotoZooms, and e-Lab Activities on the CD-ROM accompanying this book
Trang 7Key Terms
adjacent neighbor Two directly connected routers that participate in the exchange of
routing information are said to be adjacent.
convergence The speed and capability of a group of internetworking devices running
a specific routing protocol to agree on the topology of an internetwork after a change
in that topology
count to infinity A problem that can occur in routing algorithms that are slow to
converge in which routers continuously increment the hop count to particular
net-works Typically, some arbitrary hop-count limit is imposed to prevent this problem
exterior routes Routes to networks outside the autonomous system that are
consid-ered when identifying a gateway of last resort
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
holddowns Prevent regular update messages from inappropriately reinstating a route
that might not be up
hold-time timer Specifies the amount of time for which information about better
routes is ignored The IGRP default for this variable is 3 times the update timer period
plus 10 seconds
IGRP (Interior Gateway Routing Protocol) An IGP developed by Cisco to address
the problems associated with routing in large, heterogeneous networks Compare with
EIGRP See also IGP, OSPF, and RIP.
interior routes 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
invalid timer Specifies how long a router waits in the absence of routing-update
mes-sages about a specific route before declaring that route invalid The IGRP default for
this variable is three times the update period
poison reverse updates Updates that are necessary to defeat larger routing loops
Generally speaking, increases in routing metrics indicate routing loops Poison reverse
updates are then sent to remove the route and place it in holddown
RIP (Routing Information Protocol) An IGP supplied with UNIX BSD systems The
most common IGP in the Internet RIP uses hop count as a routing metric
Trang 8routing metric A method by which a routing algorithm determines that one route is better than another This information is stored in routing tables and sent in routing updates Metrics include bandwidth, communication cost, delay, hop count, load, MTU, path cost, and reliability Sometimes referred to simply as a metric
routing protocol A protocol that accomplishes routing through the implementation
of a specific routing algorithm Examples of routing protocols are IGRP, OSPF, and RIP
routing table A table stored in a router or some other internetworking device that keeps track of routes to particular network destinations and, in some cases, metrics associated with those routes
routing update A message sent from a router to indicate network reachability and associated cost information Routing updates are typically sent at regular intervals and after a change in network topology Compare with flash update
split horizon A routing technique in which information about routes is prevented from exiting the router interface through which that information was received Split-horizon updates are useful in preventing routing loops
system routes 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
triggered update A triggered update is an update that is sent without waiting for the update timer to expire
update timer Specifies how frequently routing update messages are sent The IGRP default for this variable is 90 seconds
Check Your Understanding
Complete all the review questions to test your understanding of the topics and con-cepts in this chapter Answers are listed in Appendix C, “Check Your Understanding Answer Key.”
1. What kind of entries does a router initially refer to?
A. Entries about networks or subnets that are directly connected
B. Entries that it has learned about from the Cisco IOS Software
C. Entries whose IP address and mask information are known
D. Entries that it has learned about from other routers
Trang 92. Which of the following best describes a static route?
A. A routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table
B. A route that is explicitly configured and entered into the routing table and that takes precedence over routes chosen by dynamic routing protocols
C. A route that adjusts automatically to network topology or traffic changes
D. A route that adjusts involuntarily to direct frames within a network topology
3. Which of the following best describes a default route?
A. A routing table entry that is used to direct frames for which a next hop is not explicitly listed in the routing table
B. A route that is explicitly configured and entered into the routing table
C. A route that adjusts automatically to network topology or traffic changes
D. A route that adjusts involuntarily to direct frames within a network topology
4. What are exterior routing protocols used for?
A. To transmit between nodes on a network
B. To deliver information within a single autonomous system
C. To communicate between autonomous systems
D. To set up a compatibility infrastructure between networks
5. What are interior routing protocols used for?
A. They are used to set up a compatibility infrastructure between networks
B. They are used to communicate between autonomous systems
C. They are used to transmit between nodes on a network
D. They are used within a single autonomous system
6. Which of the following is a global task?
A. Addressing IP network numbers by specifying subnet values
B. Enabling a routing protocol such as RIP or IGRP
C. Assigning network/subnet addresses and the appropriate subnet mask
D. Setting up a routing metric to find the best path to each network
Trang 107. What metric does RIP use to determine the best path for a message to travel on?
A. Bandwidth
B. Hop count
C. Varies with each message
D. Administrative distance
8. You suspect that one of the routers connected to your network is sending bad routing information What command can you use to check?
A. router(config)# show ip route
B. router# show ip route
C. router> show ip protocol
D. router(config-router)# show ip protocol
9. Why would you display the IP routing table?
A. To set the router update schedule
B. To identify destination network addresses and next-hop pairs
C. To trace where datagrams are coming from
D. To set the parameters and filters for the router
10. If you want to learn which routing protocol a router was configured with, what command structure should you use?
A. router> show router protocol
B. router(config)> show ip protocol
C. router(config)# show router protocol
D. router> show ip protocol
11. In the following command, what does the last number stand for?
Router (config)# ip route 2.0.0.0 255.0.0.0 1.0.0.2 5
A. The number of hops
B. The number of routes to the destination
C. The administrative distance
D. The destination’s reference number in the routing table