The %IP-3-LOOPPAK: LOOPING PACKET DETECTED AND DROPPED error message indicates that a looping packet is detected. A common cause is the incorrect configuration of the ip helper-address command. The helper address must be the same address as that of the server of the intended service.
In some scenarios, the presence of the address of the router in the helper address creates a routing loop. The IP helper address is the IP address of the server that provides the Dynamic Host Control Protocol (DHCP) services, not the IP address of the PC or router that requests that service. If the IP helper address is configured as the latter, it creates a routing loop. This is also true for virtual LANs (VLANs) whose IP address is configured with the IP helper address.
These messages also appear in the logs after you configure Cisco IOS Server Load Balancing (SLB), which defines a virtual server that represents a group of real servers in a cluster of network servers known as a server farm. In this environment, the clients are configured to connect to the IP address of the virtual server. The virtual server IP address is configured as a loopback address, or secondary IP address, on each of the real servers. When a client initiates a connection to the virtual server, the Cisco IOS SLB function chooses a real server for the connection based on a configured load-balancing algorithm.
In order to resolve this issue, analyze the source and destination address of the looped packets.
Verify that the configuration of the IP helper addresses in the switch correctly point to the right device, for example, the DHCP server, the Domain Name System (DNS) server, or the Microsoft Windows Internet Name Service (WINS), and do not point to the switch itself.