A branch router's WAN-facing interface is configured with 'ip address dhcp' to obtain an address from the ISP. After a reload, the interface remains up/up but never obtains an IP address. Debugging with 'debug dhcp detail' shows the router sending DHCPDISCOVER messages repeatedly but never receiving a DHCPOFFER. The ISP confirms their DHCP server is operational and that other customers on the same segment obtain leases normally. A packet capture on the router's interface shows the outbound DISCOVER packets but no inbound traffic from the server at all. What is the MOST likely cause?