After configuring RIPng on two routers that are connected via a WAN link, a network engineer notices that the two routers cannot exchange routing updates.
What is the reason for this?
A. Either a firewall between the two routers or an ACL on the router is blocking TCP 521.
B. Either a firewall between the two routers or an ACL on the router is blocking UDP 520.
C. Either a firewall between the two routers or an ACL on the router is blocking UDP 521.
D. Either a firewall between the two routers or an ACL on the router is blocking TCP 520.
C
https://www.cisco.com/assets/sol/sb/RV215W_GUI/help/EN_help/networking26.html
RIP Next Generation (RIPng) is a routing protocol based on the distance vector (D-V) algorithm. RIPng uses UDP packets to exchange routing information through port 521.