12 Most Important topics in CCNA

1. Address Resolution Protocol (ARP):

ARP is a layer of 2 protocols. It’s full form Address Resolution Protocol itself defines its function. A PC sends an ARP request to know the MAC address of a device. ARP request is broadcast. In response to an ARP request, PC generates an ARP reply that is unicast in nature. 

2. Dynamic Host Configuration Protocol (DHCP):

  • The DHCP server assigns an IP address automatically to the host machine. In a small network, we can configure an IP address manually, but in a larger network, we can’t assign IP addresses individually to each machine. 
  • So, we configure the DHCP server so that all hosts automatically receive an IP address.  
  • To get an IP address, there are 4 DHCP messages that are exchanged between a DHCP server and a host.  
  • Discover: First host sends a discover message to discover the DHCP server. 
  • Offer: In response to the discovery message, DHCP server sends an IP address. 
  • Request: After getting the offer message, the host sends a request message to the DHCP server, I (host) am OK with this IP address, please send full configuration details. 

Acknowledge: In the acknowledge message, DHCP server sends an IP address with all essential configuration.         

All four messages are broadcast. This process of getting an IP address is also known as the DORA process. 

3. Spanning Tree Protocol (STP):

Spanning tree protocol is used to reduce layer 2 loops. In general, to ensure network availability, some extra links are added. These extra links are also known as redundant links. They act as backup links, it means, if any active link fails, then traffic shifts to these backup links.  

We saw, the purpose of adding these links is good, but these links have some cons also, these links also create loops at layer 2, and because of these loops, broadcast storm and MAC instability problems arise. 

Now let’s see how STP reduces layer 2 loops. By default, STP is running in all manageable switches. By putting extra links in block mode, STP creates a single path between each point. 

Spanning Tree Protocol (STP)

4. Secure Shell (SSH) and Telnet:

SSH and Telnet both are used to remotely access the device. SSH is preferred over Telnet, as Telnet is less secure and sends data and password in plain text, so there is a possibility anyone can view the passwords, while SSH sends encrypted data, no one can view the actual data. 

5. Hot Standby Routing Protocol (HSRP):

HSRP is also known as gateway redundancy protocol. It is Cisco proprietary protocol. HSRP is used for load balancing the traffic. In general, to ensure 24/7 network connectivity, enterprise networks have 2 internet connections from 2 different vendors so that if one goes down traffic will shift to another and this is achieved with the help of HSRP. In HSRP, there are 2 routers, one operates in active mode and another in standby mode, if due to some technical issue active router goes down, then traffic will shift to the standby router. 

Hot Standby Routing Protocol (HSRP)

6. Open Shortest Path First (OSPF):

OSPF is the interior gateway dynamic protocol. Dynamic protocol means it learns routes dynamically or whenever a change occurs in the topology it automatically updates the routing table. 

It works on the shortest path first algorithm to calculate the best path from available paths. 

OSPF is a link-state protocol, to advertise routes to neighbours, OSPF uses Link state advertisements (LSAs). AD value of OSPF is 110.1 

TCP and UDP protocol: 

  • TCP stands for Transmission control protocol. UDP stands for user datagram protocol. 
  • Both are transport layer protocols. 

7. TCP Transmission Control Protocol:

TCP is a connection-oriented protocol, first a connection is established between the sender and the receiver and then the actual transmission starts. To establish a connection in TCP, there is a particular procedure that is known as the 3-way handshake process. 

#### 3 way handshake image is inserted 

  • First sender sends a syn message to the receiver. 
  • In response to a syn message, the receiver sends a syn/ack message to the sender. 
  • In response of syn/ack message, the sender sends ack message. 

Here syn stands for synchronize and ack stands for acknowledge. 

Transmission via TCP protocol in CCNA is reliable because there is an acknowledged message that conveys the status of packets (lost or received). It means if a packet is successfully received, it sends the message packet is received and if a packet is lost, then it acknowledges the sender, a packet is lost in the midway, please re-transmit the packet.  

TCP Transmission Control Protocol

8. User Datagram Protocol (UDP):

UDP is a connectionless protocol. Also, there is no acknowledgement, if a packet is lost, it means, it’s lost. It is not reliable. UDP is useful in VoIP applications.  

Note: Difference between UDP and TCP and 3-way handshake are the most commonly asked questions in the interview. 

9. HyperText Transfer Protocol (HTTP):

HTTP is an application layer protocol. HTTP is used to pull the content of web pages from the web browser. 

10. IEEE 802.1Q

IEEE 802.1q is a VLAN tagging protocol. VLAN tagging is done to identify, from which VLAN packet belongs. Many times, IEEE 802.1q is known as dot1Q. 

11. DTP Dynamic Trunking Protocol

DTP stands for dynamic trunking protocol. DTP allows Cisco switches to dynamically learn their interface status (access or trunk). It means when one switch interface is manually configured, another switch interface dynamically learns the status by looking at the manually configured interface status (no need to do manual configuration in both switches). 

It is Cisco proprietary protocol. By default, DTP is enabled in all switches. 

12. SNMP- Simple Network Management Protocol

SNMP stands for Simple Network Management Protocol. SNMP is used to monitor and manage the network devices (routers, switches, printers etc.) 

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