21. 
Refer to the exhibit. R1 is connected to the Internet through its serial 0/0/0 interface. Hosts on the 192.168.100.0/24 LAN on R1 cannot communicate with hosts on the Internet. What two NAT configuration issues might explain this failure? (Choose two.)
• The ip nat pool command has not been applied correctly.
• The inside interface has not been defined on R1.
• The access list does not include the group of IP addresses that are supported by the inside network.
• The ip address that is assigned to the serial 0/0/0 interface is incorrect
• The outside interface has not been defined on R1.
22. 
Refer to the exhibit. R1 and R2 are connected via serial interfaces. Both interfaces show that there is Layer 2 connectivity between them. The administrator verifies that CDP is operational; however, pings between the two interfaces are unsuccessful. What is the cause of this connectivity problem?
• no set loopback
• incorrect subnet mask on R2
• incompatible bandwidth
• incorrect IP address on R1
• incompatible encapsulation
23. Which two statements describe how the information contained in a BPDU is used by a switch? (Choose two.)
• to set the duplex mode of a redundant link
• to activate looped paths throughout the network
• to determine the root bridge
• to prevent loops by sharing routing tables between connected switches
• to determine which ports are placed in forwarding mode
24. When MD5 authentication is used for OSPF routing protocol authentication, what two facts are known about the key? (Choose two.)
• The key passes between routers in plain text.
• The key is used to help generate an encrypted number for authentication.
• The key passes between routers in encrypted form.
• The key is never transmitted.
• The key can be captured by using a packet sniffer.
25. 
Refer to the exhibit. If router RTA fails, which statement is true?
• Hosts A and B can reach each other, but cannot reach hosts C and D or the server.
• No host can reach any other host.
• Hosts A, B, C, and D can reach each other, but cannot reach the server.
• All hosts can reach each other.
