R1 and R2 belong to the RIP routing domain that includes the networks 10.20.0.0/16 and
10.21.0.0/16. R3 and R4 are performing two-way route redistribution between OSPF and
RIP. A network administrator has discovered that R2 is receiving OSPF routes for the
networks 10.20.0.0/16 and 10.21.0.0/16 and a routing loop has occurred.
Which action will correct this problem?

A.
Apply an inbound ACL to the R2 serial interface.

B.
Change the RIP administrative distance on R3 to 110.

C.
Configure distribute-lists on R3 and R4.

D.
Set the OSPF default metric to 20.

E.
Change the OSPF administrative distance on R3 to 110.

Explanation:
Distribute List is Like an access-list, use to deny or permit the routing update to pass through
a router/interface. Distribute List allow you apply an access list to a routing updates.
It can be apply on in or out bond of an interface under a routing process. e.g in fig. R1 want
to send a routing update to it neighbor, this update will go through from interface S0/0, router
will check, is there some Distribute List apply to this interface. If there is a Distribute List
which would contain the allow route to pass through this interface.