CLI Guide

Layer 3 Routing Commands 1871
When allow-default is set in loose mode (any), if the source IP address is not
found but a default route is present in the table, the uRPF check will pass.
When allow-default is set in strict mode (rx), it will prevent the incoming
packet's source IP address to have a route out of a different interface than
received. The strict mode option with the default route is used typically on
the upstream interface.
Default Configuration
By default uRPF checking is disabled on interfaces.
Command Mode
Interface (Ethernet) Configuration mode
User Guidelines
In any mode, a check is performed to see if the source address is reachable in
the routing table and, if found, the packet is forwarded.
In rx mode, a check is performed to see if the source address is reachable in
the routing table via the same interface on which the packet was received. If
both conditions are met, the packet is forwarded.
The allow-default option can be set in conjunction with strict and loose
modes. It will include IP addresses not specifically contained in the routing
table in the uRPF check. In loose mode, if the source ip address is not found
but a default route is present in the table, the uRPF check will allow the
packet to be routed. In strict mode, it will drop incoming packets where the
route to the packets source IP address is on an interface other than the
interface on which the packet was received. The strict mode option with the
allow-default option is used typically on upstream interfaces.
Command History
Command introduced in version 6.6 firmware.
Example
console#configure
console(config)#system urpf enable
console(config)#interface gi1/0/7
console(config-Gi1/0/7)#routing
console(config-Gi10/7)#ip verify unicast source reachable-via rx