Last Updated on August 1, 2021 by Admin 1
Examine the following output of the show ip route command and the partial output of the show run command from the router R63:
What will the router do with a packet with a source address of 192.168.5.5/24 and a destination address of 10.11.11.20/ 24 that arrives on the Serial0/0 interface?
- forward it out the Ethernet0/0 interface
- forward it out the Tunnel0 interface
- drop the packet
- forward it out the Ethernet0/1 interface
It will drop the packet. The partial output of the show run command shows that the ip verify unicast source reachable via rx command has been executed on the Serial 0/0 interface. This enables the Unicast Reverse Path Forwarding (Unicast RPF) feature. This feature prevents IP spoofing by verifying from the routing table that there is a valid return path to the source IP address. If there is not valid return path, you can assume the IP address has been spoofed. When the command ends in the keyword rx, it means that there must be a return path through the interface where the command was executed. This is called strict mode.
The packet arrived on the Serial0/0 interface. The routing table shows that there is no routing entry for the 192.168.5.0/24 network that leads back through the entry interface of Serial0/0. In fact, in this instance there is no routing table entry for that network leading to any interface. When this occurs, the router will drop the packet.
The router will not send the packet to either the Ethernet0/0 or the Tunnel0 interfaces because the destination network, 10.11.11.0/24, is not a reachable destination on those interfaces. Even if it were reachable, the Unicast Reverse Path Forwarding (Unicast RPF) feature will drop the packet because it has been spoofed.
It will not send the packet to the Ethernet0/1 interface. The Unicast Reverse Path Forwarding (Unicast RPF) feature will drop the packet because it has been spoofed. If the packet were not spoofed, it would be sent to the Ethernet0/1 interface because that is the interface used by the default route. Because there is no route in the table to the 10.11.11.0/24 network, it would be sent to the default route.
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