When authentication is used, a password must be configured on both sides.
This password is called the authentication key. If this key does not match the
key on the other side, the RIP v2 updates will be ignored on both sides.
This example shows two routers running RIP between each other.

Debugs
and Verification
Figure
shows
the configurations of routers R1 and R2. In this configuration, a different RIP
authentication key is configured on R1 and R2. The R2 Ethernet interface is
configured with the key cisco1, whereas R1 is configured with the key cisco.
These two keys do not match, so they ignore each other’s updates, and the
routes will not be installed in the routing table.
Figure
shows the output
from the debug ip rip command on router R2 that indicates
that R2 is receiving a RIP packet that has invalid authentication. This means
that the authentication key between sender and receiver does not match.
Solution
When using authentication in RIP, make sure that the
sender and receiver are configured with the same authentication key. Sometimes,
adding a space at the end of the key can cause the invalid authentication
problems because a space will be taken as a literal key entry. As a result,
this causes a problem that cannot be corrected just by looking at the
configurations.
Debugs will show that there is a problem with the
authentication key. To solve this problem, configure the same keys on both
sender and receiver, or retype the authentication key, making sure that no
space is being added at the end.
Figure
shows the new
configuration to correct this problem. The authentication key is reconfigured
on router R2 to match the key on router R1.
Figure
shows the
routing table of R2 after the configuration change.