Network Topology
Networking AC card (chassis) —— Relay SW —— Client
Problem Description
The AC functions as a normal DHCP server without relay, and terminals obtain addresses normally. However, when a relay is present, terminals fail to acquire IP address.
Process Analysis
Packet capture was performed on the interlink port between the AC and the chassis, while debugging dhcp server event on the AC. It was observed that the AC correctly received the relayed discover request and responded with an offer message to assign an address.
Subsequently, packet capture was also performed on the interlink port between the AC and the chassis. The relayed DHCP discover request was captured normally, but no offer message replied by the AC was captured. Meanwhile, complete CAPWAP interaction could be seen in the packets, indicating normal packet capture operation.
Solution
Configuration check found that DHCP snooping was configured on the AC. After deletion, it returned to normal.
The reason this command prevents terminals from obtaining an address is that DHCP Snooping is a security feature of DHCP. The DHCP Snooping function works properly only when the device is located between the DHCP client and the DHCP server or between the DHCP client and the DHCP relay. It does not work properly if the device is located between the DHCP server and the DHCP relay.
Refer to the following link: DHCP snooping