Users Guide

If you configured Option-82, the DHCP server allocates the IP address based on the options present in Option-82. Otherwise,
the DHCP server allocates the IP address with the on-link subnet.
If you disable Option-82 in the DHCP relay switch, the DHCP packet from the client forward without Option-82 and the DHCP
server allocates the IP address from the on-link subnet value.
Option-82 with the Client and the Server in different VLANs
In this topology, Host1, Host2,and Host3 are the DHCP clients connected to the DHCP-relay-enabled switch. The DHCP clients
and the DHCP server are part of different VLANs, VLAN 100 and VLAN 200, respectively.
In this scenario, the DHCP-relay-enabled switch floods the DHCP packets from the DHCP client in VLAN 100 and also forwards
the DHCP packet with Option-82 set in the DHCP packet header to the DHCP Server in VLAN 200. If you configured
Option-82, the DHCP server allocates the IP address based on Option-82. Otherwise, the DHCP server allocates the IP address
from the subnet based on the giaddr value in the DHCP relay packet.
If you disabled Option-82 in the DHCP relay switch, the DHCP packets from the client forward without Option-82 and the
DHCP server allocates the IP address based on the giaddr value, which is the VLAN IP address.
You can configure Option-82 at the Global level and interface level. When both the global and interface level Option-82
configuration is present, the configuration to disable Option-82 takes precedence. By default, Option-82 is enable both at Global
and Interface levels.
Option-82 is enabled by default.
If you disable Option-82 Globally or at a specific Interface, Option-82 sub-options such as option 1,2,5,11,151,152 are also
disabled.
If Global DHCP snooping is enabled after disabling Option-82 globally, an error message displays. Similarly, if you disable
Option-82 Globally after enabling Global DHCP snooping, an error message displays.
If you enable DHCP snooping at the Interface level, you cannot disable the VLAN interface level Option-82. Similarly, if you
disable Option-82 in the VLAN, you cannot enable DHCP snooping at the VLAN interface level.
DHCP relay agent options
When enabled on VLANs, DHCP Option-82 is inserted by the DHCP relay agent. When a network device, such as a DHCP client,
is connected to the VLAN on an untrusted interface sends a DHCP request, the relay agent inserts information about the client
network location into the packet header (the options field of the DHCP packet is Option-82 circuitid) of that request.
The relay agent then sends the request to the DHCP server. The DHCP server reads the Option-82 information in the packet
and uses it to determine the IP address or another parameter assignments for the client. After receiving the response from the
DHCP server, the relay agent strips the DHCP Option-82 and forwards it to the client.
Use Option-82 to uniquely identify the domain in which the DHCP client is connected. Option-82 carries two sub-options:
circuit-id and remote-id (for snooping-enabled VLANs).
● circuit-id: This sub option contains the VLAN and port information of the DHCP client. VLAN id and Port name are used for
this option. The circuit- id is added in the <VLANID>-<INTERFACE_NAME> format. For example: vlan100-eth1/1/1.
The DHCP relay packet is appended to the circuit id and the circuit id becomes the interface port number.
● remote-id: This sub option contains the system identification. System MAC address is used for this sub-option. For example:
00:04:89:76:62:78, for dhcp snooping.
Additionally, DHCPv4 Option-82 sub-options include:
● Server ID override suboption Sub-option 11(0xb)
● Link selection suboption- Sub-option 5(0x5)
● DHCPv4 virtual subnet selection option - Sub-option 151(0x97)
● DHCPv4 virtual subnet selection control - Sub-option 152(0x98)
● source-interface CLI for relay agents. The gateway address (giaddr) field carries the source interface address.
Server ID override suboption sub-option 11(0xb)
The server ID override suboption allows the DHCP relay agent to specify a new value for the server ID option, which is inserted
by the DHCP server in the reply packet. This suboption allows the DHCP relay agent to act as the proxy for the DHCP server so
that the renew requests comes to the relay agent rather than to the DHCP server directly.
System management
267