When a router running Routing Information Protocol (RIP) receives an update from a neighboring router, it checks whether the source of the update belongs to the same network or sub-network as the receiving interface.
If they are the same, the routes are accepted for installing into the routing table. Otherwise, the update is dropped.
But we can change this behavior:
R1
interface Serial2/0
ip address 10.1.1.1 255.255.255.0
encapsulation ppp
interface Loopback0
ip address 1.1.1.1 255.0.0.0
R2
interface Serial2/0
ip address 10.2.2.2 255.255.255.0
encapsulation ppp
R2#PING 10.1.1.1
!!!!!
R2#sh ip route
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 10.1.1.1/32 is directly connected, Serial2/0
C 10.2.2.0/24 is directly connected, Serial2/0
R1
router rip
ver 2
no auto
network 10.1.1.0
network 1.0.0.0
R2
router rip
ver 2
no auto
network 10.2.2.0
R2#sh ip route
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 10.1.1.1/32 is directly connected, Serial2/0
C 10.2.2.0/24 is directly connected, Serial2/0
R2
router rip
no validate-update-source
R2#sh ip route
R 1.0.0.0/8 [120/1] via 10.1.1.1, 00:00:01
10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks
C 10.1.1.1/32 is directly connected, Serial2/0
C 10.2.2.0/24 is directly connected, Serial2/0
L 10.2.2.2/32 is directly connected, Serial2/0
R2#ping 1.1.1.1
!!!!!
Yasser Auda Lab 