Border Gateway Protocol (BGP) is an exterior gateway protocol. It communicates using a Transmission Control Protocol (TCP) and sends the updated router table information. The best path is chosen on the basis of cost metric associated with the route. It is used between gateway hosts in a network. Answer option C is incorrect. Open Shortest Path First (OSPF) is a routing protocol that is used in large networks. Internet Engineering Task Force (IETF) designates OSPF as one of the Interior Gateway Protocols. A host uses OSPF to obtain a change in the routing table and to immediately multicast updated information to all the other hosts in the network. Answer option A is incorrect. IGMP stands for Internet Group Management Protocol. IGMP is a communication protocol that is used to manage the membership of Internet protocol multicast groups. It is an integral part of the IP multicast specification. Although it does not actually act as a transport protocol, it operates above the network layer. It is analogous to ICMP for unicast connections. It is susceptible to some attacks, so firewalls commonly allow the user to disable it if not needed. Answer option B is incorrect. ICMP Router Discovery Protocol (IRDP) uses Internet Control Message Protocol (ICMP) router advertisements and router solicitation messages to allow a host to discover the addresses of operational routers on the subnet.It basically consists of 2 message types used for discovering local routers. The message type 9 is sent periodically or on request (using a message of type 10) to the local subnet from the local routers to propagate themselves. On boot, the client may send an ICMP message of type 10 to ask for local routers. When a client receives a message type 9, they add the router to their local routing-table.

Single key encryption uses a single word or phrase as the key. The same key is used by the sender to encrypt and the receiver to decrypt. Sender and receiver initially need to have a secure way of passing the key from one to the other. With TLS or SSL this would not be possible. Symmetric encryption is a type of encryption that uses a single key to encrypt and decrypt data. Symmetric encryption algorithms are faster than public key encryption. Therefore, it is commonly used when a message sender needs to encrypt a large amount of data.
Data Encryption Standard (DES) uses the symmetric encryption key algorithm to encrypt data.