Case Study Misconfigured VLSM

Host C in Figure 7.20 cannot communicate across the internetwork, and it cannot even ping the other hosts or routers on the local data link. Hosts A and B have no communications problems with each other or with any other host across the internetwork, but they cannot communicate with C. All hosts are configured to use 172.19.35.1 as the default gateway address.

Figure 7.20. Hosts A and B can communicate across the internetwork, but host C cannot.

Figure 7.20. Hosts A and B can communicate across the internetwork, but host C cannot.

When an attempt is made to ping host C from host A or B, the first ping is successful and subsequent pings fail (Figure 7.21). The fact that at least one ICMP Echo Request packet reached C and at least one Echo Reply packet was returned indicates that the problem is not related to the hardware or data link.

Figure 7.21. When host B pings host C, the first ping is successful and subsequent pings fail.

Figure 7.21. When host B pings host C, the first ping is successful and subsequent pings fail.

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The strange ping behavior leads to the hypothesis that after the first successful packet, subsequent packets—either the Echo Requests from B or the Echo Replies from C—are somehow being misdirected. Because this behavior is happening on the local data link, the Address Resolution Protocol (ARP) caches should be examined.

Figures 7.22 and 7.23 show the ARP caches for C and B, respectively. The suspicions about ARP are confirmed here. C's cache contains the correct MAC address for B (00a0.2470.febd), but B's cache has a

MAC address of 0000.0c0a.2aa9 associated with C. A closer look at both caches shows that 0000.0c0a.2aa9 is the MAC address of San_Felipe's locally attached interface. This information can be deduced from the fact that the same MAC address is mapped to IP address 172.19.35.2 and to the IP addresses reachable via that router.

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