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Chapter 6
Troubleshooting TCP/IP Connectivity
Router_C>show arp
Protocol Address Age (min) Hardware Addr Type Interface
Internet 172.16.60.1 - 0010.7bd9.2881 ARPA
Ethernet0/1
Internet 172.16.50.2 - 0010.7bd9.2880 ARPA
Ethernet0/0
Internet 172.16.50.1 108 0000.0c09.99cc ARPA
Ethernet0/0
Router_C>
Notice the Age field in the ARP table. ARP entries are stored or cached for
future use. This allows a router to look up the MAC address, instead of hav-
ing to send a broadcast to learn it again. However, the ARP entry does not
stay in the table indefinitely.
Several problems could occur if a MAC address were permanently
mapped to an IP address. You learned that DHCP can assign a given IP
address to any requesting host, if it is available. In this scenario, the IP
address could be assigned to different MAC addresses. If this were to hap-
pen, any existing entry in an ARP table would be invalidated. If a NIC is
replaced on a host, the MAC address is changed as well. If the ARP cache
wasn't cleared and updated, the IP address would still be mapped to the old
MAC address. You get the picture. These mappings are not permanent, so
the cache entries cannot be permanent either.
Sometimes problems occur within a network because of ARP problems.
The best way to troubleshoot these issues is by looking at the ARP table on
the router with the show arp command and (if necessary) using the debug
arp
tool. Problems may be fixed by simply clearing the ARP cache and
allowing the router to rebuild the table.
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