DHCP option 82 is relay-agent information that carries client or circuit context. A relay agent can add it while forwarding client messages to a remote DHCP server, helping inform address allocation. section 13.2.2
DHCP Snooping-enabled Cisco switches insert option 82 into DHCP client messages received on untrusted ports by default. They also reject messages that already contain option 82 on untrusted ports, which can break DHCP across Layer 2 switches. section 13.2.2 section 13.3
Control and troubleshoot option 82 insertion with these commands:
- Disable insertion globally with no ip dhcp snooping information option. section 13.2.2
- Verify the setting with show ip dhcp snooping; the output reports whether insertion is enabled and displays the default circuit-ID and remote-ID formats. section 13.2.2
- Recognize drops involving nonzero giaddr or option 82 on an untrusted port from the DHCP Snooping syslog message. section 13.2.2
Layer 2 switch DHCP failure
A client’s DHCPDISCOVER reaches SW2, which inserts option 82 and forwards it. When SW1 receives the message on an untrusted port, SW1 discards it. Disabling insertion on the affected switches removes this default incompatibility when the switches are not acting as DHCP relay agents. section 13.2.2
SW2# show ip dhcp snooping . . . Insertion of option 82 is enabled #1 circuit-id default format: vlan-mod-port remote-id: 5254.000c.8b85 (MAC) Option 82 on untrusted port is not allowed #2 . . . #1 Switches insert option 82 into DHCP requests by default. #2 Switches do not accept messages with option 82 on untrusted ports.
Inspect the Cisco IOS output showing default insertion, circuit-ID formatting, remote-ID, and rejection on untrusted ports. section 13.2.2
