652 CHAPTER 44: URPF CONFIGURATION■ In loose approach, URPF does a reverse lookup for the outgoing interfaces ofthe packet. As long as an outgoing interface exists (no matter whether theoutgoing interface is consistent with the incoming interface), the packet passesthe check. Otherwise, the packet is dropped.2 If the source address is not found in the FIB table, URPF makes a decision based onthe configuration of default route (the one configured on the router that receivedthe packet) and the allow-default-route keyword.■ If a default route is available but the allow-default-route keyword is notconfigured, the packet is rejected no matter which check approach is taken.■ If both a default route and the allow-default-route argument are configured,URPF’s decision depends on check approach. In strict approach, URPF lets thepacket pass and be forwarded normally if the outgoing interface of the defaultroute is the interface where the packet is received, and otherwise rejects it. Inloose approach, URPF lets the packet pass and be forwarded directly.3 The packet will come to ACL check if and only if it is rejected. If the packet passesACL check, it is forwarded as normal; otherwise, it is discarded.Configuring URPF Follow these steps to configure URPF:To do... Use the command... RemarksEnter system view system-view --Enter interface view interface interface-typeinterface-number--Enable URPF check ip urpf { loose | strict }[ allow-default-route ] [ aclacl-number ]RequiredDisabled by default.
