Concept Guide

Option Number and Description
Species the amount of time after the IP address is granted that the client attempts to renew its lease with the
original server.
Rebinding Time Option 59
Species the amount of time after the IP address is granted that the client attempts to renew its lease with any
server, if the original server does not respond.
End Option 255
Signals the last option in the DHCP packet.
Option 82
RFC 3046 (the relay agent information option, or Option 82) is used for class-based IP address assignment.
The code for the relay agent information option is 82, and is comprised of two sub-options, circuit ID and remote ID.
Circuit ID This is the interface on which the client-originated message is received.
Remote ID This identies the host from which the message is received. The value of this sub-option is the MAC address of
the relay agent that adds Option 82.
The DHCP relay agent inserts Option 82 before forwarding DHCP packets to the server. The server can use this information to:
• track the number of address requests per relay agent. Restricting the number of addresses available per relay agent can harden a
server against address exhaustion attacks.
• associate client MAC addresses with a relay agent to prevent oering an IP address to a client spoong the same MAC address on a
dierent relay agent.
• assign IP addresses according to the relay agent. This prevents generating DHCP oers in response to requests from an unauthorized
relay agent.
The server echoes the option back to the relay agent in its response, and the relay agent can use the information in the option to forward a
reply out the interface on which the request was received rather than ooding it on the entire VLAN.
The relay agent strips Option 82 from DHCP responses before forwarding them to the client.
To insert Option 82 into DHCP packets, follow this step.
• Insert Option 82 into DHCP packets.
CONFIGURATION mode
int ma 0/0
ip add dhcp relay information-option remote-id
For routers between the relay agent and the DHCP server, enter the trust-downstream option.
DHCPv6 relay agent options
By default, the DHCPv6 relay agent inserts Options 18 and 37 before forwarding DHCPv6 packets to the server.
Interface ID (Option
18)
This is the interface on which the client-originated message is received.
Default format: The default format of this option is VLAN ID:LAG ID:slot ID:port ID.
Remote ID (Option
37)
This identies the host from which the message is received.
284 Dynamic Host Conguration Protocol (DHCP)