Refer to the exhibit. After configuring GRE between two routers running OSPF that are connected to each other via a WAN link, a network engineer notices that the two routers cannot establish the GRE tunnel to begin the exchange of routing updates. What is the reason for this?
| A. |
Either a firewall between the two routers or an ACL on the router is blocking IP protocol number 47. | |
| B. |
Either a firewall between the two routers or an ACL on the router is blocking UDP 57. | |
| C. |
Either a firewall between the two routers or an ACL on the router is blocking TCP 47. | |
| D. |
Either a firewall between the two routers or an ACL on the router is blocking IP protocol number 57. |