ICND1 – Protocols & Services
File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another host over TCP-based network, such as the Internet.
Cisco Discovery Protocol (CDP) is a proprietary protocol of Cisco so if you can see the VoIP phone via the “show cdp neighbors” command on a Cisco switch then that phone is from Cisco.
User Datagram Protocol (UDP) provides a connectionless datagram service that offers best-effort delivery, which means that UDP does not guarantee delivery or verify sequencing for any datagrams. UDP is typically used by programs that transmit small amounts of data at one time or have real-time requirements (voice, for example).
CDP is a device discovery protocol that runs over Layer 2. We can view the CDP information with the show cdp neighbors command (thus the provided information is at layer 2), notice this command only shows information about directly connected devices. The output of the show cdp neighbors command is shown below:
There are 3 columns you must pay attention to:
* Local interface: type & ID of the local interface on which CDP information of the neighbor were received.
* Device platform: the neighboring device model.
* Port ID: the connected interface of the neighbor.
After resolving a browser URL to an IP address (via DNS server), the workstation must learn the MAC address of the server so that it can create a complete packet (a complete packet requires destination MAC and IP address, source MAC and IP address). Therefore the workstation must use ARP to find out the MAC address from the IP address.
Before two computers can communicate over TCP, they must synchronize their initial sequence numbers (ISN) -> B is correct.
TCP uses a sequence number to identify each byte of data. The sequence number identifies the order of the bytes sent from each computer so that the data can be reconstructed in order, regardless of any fragmentation, disordering, or packet loss that may occur during transmission -> D is correct.
By default CDP is enabled on Cisco routers -> A is not correct.
CDP runs at Layer 2 in the OSI model and it does not need an IP address to run -> C & D are not correct.
Tracert (or traceroute) is used to trace the path between the sender and the destination host. Traceroute works by sending packets with gradually increasing Time-to-Live (TTL) value, starting with TTL value = 1. The first router receives the packet, decrements the TTL value and drops the packet because it then has TTL value zero. The router sends an ICMP Time Exceeded message back to the source. The next set of packets are given a TTL value of 2, so the first router forwards the packets, but the second router drops them and replies with ICMP Time Exceeded. Proceeding in this way, traceroute uses the returned ICMP Time Exceeded messages to build a list of routers that packets traverse, until the destination is reached and returns an ICMP Echo Reply message -> C is correct.
ICMP is encapsulated in an IP packet. In particular, the ICMP message is encapsulated in the IP payload part of an IP datagram -> D is correct.
Note: The TRACERT command on Windows Operating System uses ICMP while MAC OS X and Linux TRACEROUTE use UDP.
CDP runs at Layer 2 so it can recognize a switch (if that switch also runs CDP).