Figure 722 Host Cs ARP cache shows the correct MAC address associated with all addresses

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Figure 7.23. Host B's ARP cache shows that C's IP address is mapped to the MAC address of San_Felipe's interface 172.19.35.2.

Figure 7.23. Host B's ARP cache shows that C's IP address is mapped to the MAC address of San_Felipe's interface 172.19.35.2.

[CKopyrioht Microsoft '^rji 1981-1995.

[CKopyrioht Microsoft '^rji 1981-1995.

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The ping results begin to make sense. B broadcasts an ARP Request for 172.19.35.72. C sends an ARP Reply, and B sends its first ping correctly. In the meantime, San_Felipe has received the ARP Request and apparently believes that it has a route to 172.19.35.72. It responds with a proxy ARP (later than C because it has to perform a route lookup first), which causes B to overwrite C's MAC address. Subsequent Echo Request packets are sent to San_Felipe, where they are routed off the local data link and lost. A protocol analyzer attached to the Ethernet proves the point (Figure 7.24).

Figure 7.24. A protocol analyzer, filtering for ARP packets, shows B's ARP request to C and replies from both host C (00a0.24a8.a1a5) and router San_Felipe (0000.0c0a.2aa9).

Figure 7.24. A protocol analyzer, filtering for ARP packets, shows B's ARP request to C and replies from both host C (00a0.24a8.a1a5) and router San_Felipe (0000.0c0a.2aa9).

If you know that the trouble is a routing problem, it remains only to find the cause. First, the subnet addresses for each data link should be determined (Figure 7.25). Next, C's IP address should be compared with all the subnets reachable via San_Felipe, in binary, to find any conflicts. Figure 7.26 shows the addresses with the subnet bits of the last octet in bold.

Figure 7.25. When analyzing any addressing scheme, and espeecially a VLSM design, the subnets for every data link should be determined so that conficts and overlaps may be discovered.

Figure 7.25. When analyzing any addressing scheme, and espeecially a VLSM design, the subnets for every data link should be determined so that conficts and overlaps may be discovered.

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