LAN Connectivity Problems
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Lease information is one of the enhancements of DHCP. It allows an IP
address to be assigned for a preconfigured amount of time. When the lease
expires, the IP address is added back to the available address pool. Each host
tries to renew the lease when the time is half-expired.
BootP
The BootP process is much simpler. When a host tries to obtain an IP
address, it sends a bootrequest, which contains the client's MAC address.
When the BootP server receives the request, it checks its database for the
MAC address. If it finds an entry, then a bootreply, which contains the IP
address and other configuration settings, is sent. If the BootP server does not
find the client's MAC address in its database, it does not respond.
Troubleshooting DHCP and BootP
Because these protocols are dynamic, there may be times when they fail or
when an end user is unable to connect to the network. If you have a protocol
analyzer, you could capture the DHCP and BootP sequences to make sure
that the clients and servers are talking.
You can also use the show commands available to aid in troubleshooting
DHCP on Cisco routers.
Router_C#show dhcp server
DHCP Proxy Client Status:
DHCP server: ANY (255.255.255.255)
Leases: 0
Offers: 0 Requests: 0 Acks: 0 Naks: 0
Declines: 0 Releases: 0 Bad: 0
Router_C#
If the router is configured to use DHCP, you can also get information
regarding the lease by issuing the show dhcp lease command.
ARP
Address Resolution Protocol maps Layer 2 MAC addresses to Layer 3 IP
addresses. An ARP table is built on the router through the exchange of ARP
requests and replies. Here is a sample ARP table:
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