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.16.11.1, 00:00:53, Serial0C 192.168.3.0/24 is directly connected, Serial1FortWorth#<<<Figure 7-53 FortWorth IP Routing Table After IGRP Configuration>>>lAustin#show ip route[text omitted]I 172.16.0/16 [100/41162] via 192.168.2.1, 00:00:27, Serial1C 192.168.1.0/24 is directly connected, Ethernet0C 192.168.2.0/24 is directly connected, Serial1C 192.168.3.0/24 is directly connected, Serial0Austin#<<<Figure 7-54 Austin IP Routing Table After IGRP Configuration>>>Just in all the other displays of the IP routing table, the numbers in brackets are the administrative distance followed by the metric.For the IGRP-learned routes indicated by the letter I in the left column, the distance is 100, and the metric is the composite metric.We cannot get a real perspective on how far away a network is from a router except that a very large metric generally indicates the presence of a low-bandwidth WAN.In Figure 7-54 (Line 3), the summary route of 172.16.0/16 is evidence that IGRP does auto-summarization just like RIP does.We cannot turn off auto-summarization for IGRP; therefore, we have the same issues with noncontiguous subnets and static network masking as we did with RIPv1.When the routers were running RIP, the IP routing table showed two equal-cost paths to the summary network address; however, with IGRP, there is only one path.The metric is the reason for this.IGRP’s composite metric takes in account the bandwidth of a network in determining best path.The link from Austin to Dallas has a bandwidth of 256 kbps, and the link from Austin to FortWorth has a bandwidth of 56 kbps.The higher bandwidth link is preferred, but we had to configure the bandwidth of each interface with the bandwidth command (Figure 7-12, Lines 19 and 24).Verifying Connectivity With TraceTrace is a utility that we use to check connectivity to a destination by finding the route that packets take to get to the destination.We saw in Figure 7-54 that Austin’s path to the 172.16.0 network is via Dallas.The Ethernet LAN on FortWorth is part of the 172.16.0 network, and we can use trace to verify that packets to FortWorth actually go through Dallas.Figure 7-55 shows the output of a trace to FortWorth’s Ethernet0 interface from Austin.Austin#trace 172.16.20.1Type escape sequence to abort.Tracing the route to 172.16.20.11 192.168.2.1 4 msec 4 msec 8 msec2 172.16.11.2 12 msec * 8 msecAustin#<<<Figure 7-55 Trace from Austin to FortWorth>>>We can specify either an IP address or a host name in the trace command.We have issued a trace to the IP address 172.16.20.1 (Line 1), which is the address of FortWorth’s Ethernet0.The trace output tells us the routers (hops) that our traffic to the IP address is going through.The first router in the path to the address has the IP address 192.168.2.1 (Line 6), which is Dallas.Trace tries each router in the path three times and returns a round-trip time in milliseconds for each try.The second, and last, router in the path has the IP address 172.16.11.2 (Line 7), which is FortWorth.If Austin had a host table with entries for the IP addresses returned by the trace, IOS would return the name of the router after it resolved the address to a host name.The same would be true if we were using DNS, and the IP addresses had reverse lookup entries in the DNS database.IGRP Configuration SummaryIGRP is configured by entering the router igrp command with an ASN in global configuration mode and then the appropriate network commands for enabling IGRP on the router’s interfaces.All of the routers that are to share information with IGRP must use the same ASN.To stop updates on an interface, issue the passive-interface command, with the interface name, in router configuration mode for the IGRP process.The IGRP-specific configuration commands entered in this chapter for Dallas, FortWorth, and Austin are shown in Figure 7-56, Figure 7-57, and Figure 7-58, respectively.Dallas#show running-config[text omitted]!router igrp 100network 172.16.0network 192.168.2
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