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Interaction Between the DHCP Relay Agent, DHCP Client, and
DHCP Servers
In a typical carrier edge network configuration,
the DHCP client is on the subscriber’s computer, and the DHCP
relay agent is configured on the router between the DHCP client and
one or more DHCP servers. The following steps describe, at a high
level, how the DHCP client, DHCP relay agent, and DHCP server interact
in a configuration that includes two DHCP servers.
- The DHCP client sends a discover packet to find a DHCP
server in the network from which to obtain configuration parameters
for the subscriber, including an IP address.
- The DHCP relay agent receives the discover packet and
forwards copies to each of the two DHCP servers. The DHCP relay agent
then creates an entry in its internal client table to keep track of
the client’s state.
- In response to receiving the discover packet, each DHCP
server sends an offer packet to the client. The DHCP relay agent receives
the offer packets and forwards them to the DHCP client.
- On receipt of the offer packets, the DHCP client selects
the DHCP server from which to obtain configuration information. Typically,
the client selects the server that offers the longest lease time on
the IP address.
- The DHCP client sends a request packet that specifies
the DHCP server from which to obtain configuration information.
- The DHCP relay agent receives the request packet and forwards
copies to each of the two DHCP servers.
- The DHCP server requested by the client sends an acknowledgement
(ACK) packet that contains the client’s configuration parameters.
- The DHCP relay agent receives the ACK packet and forwards
it to the client.
- The DHCP client receives the ACK packet and stores the
configuration information.
- If configured to do so, the DHCP relay agent installs
a host route and Address Resolution Protocol (ARP) entry for this
client.
- After establishing the initial lease on the IP address,
the DHCP client and the DHCP server use unicast transmission to negotiate
lease renewal or release. The DHCP relay agent “snoops”
on all of the packets unicast between the client and the server that
pass through the router to determine when the lease for this client
has expired or been released. This process is referred to as lease
shadowing or passive snooping.
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