Cisco Interview Preparation and Recruitment Process

A firewall is a security device — either hardware, software, or both — that monitors and controls incoming and outgoing network traffic based on predefined security rules.

Its main purpose is to create a barrier between a trusted internal network (like your organization's network) and untrusted external networks (like the Internet) to block malicious traffic and unauthorized access.

Simple Definition:

A firewall is like a security guard for your network — it decides what data is allowed in or out based on rules you set.

Key Functions of a Firewall:

• Packet Filtering: Checks data packets and allows or blocks them based on source/destination IP, port number, and protocol.

• Stateful Inspection: Tracks the state of active connections and makes decisions based on the context of the traffic.

• Proxy Services: Acts as a middleman between users and the internet, hiding internal network details.

• Intrusion Prevention: Some firewalls can detect and prevent attacks in real-time.

Types of Firewalls:

• Network-based Firewalls: Protect entire networks (hardware devices).

• Host-based Firewalls: Installed on individual devices (software, e.g., Windows Firewall).

• Next-Generation Firewalls (NGFW): Include deep packet inspection, intrusion prevention systems (IPS), and advanced threat protection.

A link in networking refers to a connection between two devices that allows them to communicate with each other by transmitting data.

Simple Definition:

A link is the physical or logical connection that carries data between two nodes (like computers, routers, or switches) in a network.

Types of Links:

• Physical Link: A real, tangible connection like a cable (Ethernet cable, fiber optics) connecting two devices.

• Logical Link: A virtual connection over a network, even if devices aren't directly connected physically (e.g., VPN connections, tunnels).

Examples:

• An Ethernet cable connecting a PC to a switch = Physical Link.

• A Wi-Fi connection between your laptop and a router = Wireless Physical Link.

• A VPN tunnel connecting two company offices over the internet = Logical Link.

In Cisco devices, the default TCP session timeout typically depends on the specific Cisco platform and configuration, but generally, it is:

Default TCP Session Timeout: ~1 hour (3600 seconds)

This means if a TCP session stays idle (no traffic) for 1 hour, the Cisco device will close the session automatically.

Example Values on Common Cisco Platforms:

• Cisco ASA (Firewall):

  • Default TCP idle timeout = 1 hour (3600 seconds)

  • You can see it with the command:

    show running-config timeout
    

• Cisco Routers/Switches (with firewall features):

  • TCP session timeout defaults to about 60 minutes unless configured otherwise.

• Cisco FTD (Firepower Threat Defense):

  • TCP idle timeout = 3600 seconds by default.

You Can Manually Change It

Example (Cisco ASA):

timeout tcp 2:00:00

This would set the TCP idle timeout to 2 hours.

A diskless workstation is a computer that does not have its own hard disk (or local storage). Instead, it boots and loads its operating system over a network from a central server.

Simple Definition:

A diskless workstation is a computer that relies entirely on a network server to get its operating system, files, and applications — it has no internal storage like a hard drive or SSD.

How it Works:

• When the workstation powers on, it uses a network boot protocol (like PXE – Preboot Execution Environment).

• It connects to a boot server (like a TFTP server) to download the necessary system files.

• All user data and applications are stored on and accessed from the central server.

Why Use Diskless Workstations?

• Cost Saving: No hard drives needed in every machine.

• Easier Management: Update or fix software in one place (the server), not on each computer.

• Security: Data stays centralized; if a workstation is stolen, no data is lost.

• Energy Efficient: No spinning disks = less power consumption.

Where You Find Them:

• Corporate offices (for basic client computers)

• Schools and universities (computer labs)

• Thin client setups (in Virtual Desktop Infrastructure - VDI environments)

• Kiosks and public terminals

* Example: In a university lab, dozens of diskless PCs might boot Linux from a central server. If the OS needs an update, the admin only updates the server — not each PC individually.

Routing (in networking) is:

The process of selecting a path for traffic in a network, so that data packets can travel from their source to their destination efficiently.

In simple words:

Routing is like choosing the best road for your data to travel across a network (like the Internet) to reach its target device.

How Routing Works:

• Devices called routers examine the destination IP address of each data packet.

• They decide the best next hop (the next router or device) based on routing tables and routing protocols.

• Routers then forward the packet along the selected path until it reaches its final destination.

Key Elements of Routing:

• Routing Table: A database in a router listing paths to various network destinations.

• Routing Protocols: Special rules routers use to communicate and share information (like OSPF, BGP, RIP).

• Static Routing: Manually configured paths.

• Dynamic Routing: Paths automatically learned and updated using routing protocols.

Example:

When you send a message from your phone in India to a server in the USA:

• Your phone sends the data to a local router.

• The router checks its routing table and forwards the packet toward a bigger router (ISP router).

• Step-by-step, each router decides where to send the packet next.

• After multiple hops, the data finally reaches the server.

* Summary: Routing is the heart of network communication — it ensures your data finds the right way through a complex web of networks.

Types of Routing Protocols

Routing protocols help routers learn routes dynamically and share information about networks. They are mainly divided into two broad categories:

1. Interior Gateway Protocols (IGP)

(Used within a single organization or autonomous system)

2. Exterior Gateway Protocols (EGP)

(Used to route between different organizations or autonomous systems)

Short explanation of important protocols:

• RIP (Routing Information Protocol):

  • Distance vector protocol.

  • Uses hop count as the metric.

  • Max hop count = 15; 16 = unreachable.

  • Simple but slow and not ideal for large networks.

• IGRP (Interior Gateway Routing Protocol):

  • Cisco proprietary (replaced by EIGRP).

  • Improves over RIP by considering bandwidth, delay, reliability, and load.

• EIGRP (Enhanced IGRP):

  • Hybrid protocol (both distance vector and link state features).

  • Cisco proprietary (now partly open).

  • Fast convergence and scalable.

• OSPF (Open Shortest Path First):

  • Link-state protocol.

  • Calculates the best path using Dijkstra’s algorithm.

  • Suitable for large, complex enterprise networks.

  • Divides networks into areas to optimize traffic.

• IS-IS (Intermediate System to Intermediate System):

  • Another link-state protocol (used mostly by ISPs).

  • Similar to OSPF but more flexible for very large networks.

• BGP (Border Gateway Protocol):

  • "Protocol of the Internet."

  • Connects different companies, ISPs, or data centers.

  • Focuses on path attributes like AS-PATH.

  • Very scalable but complex to configure.

Quick Table to Remember

Different Types of Memory in a Cisco Router

A Cisco router typically uses four types of memory, each serving a different purpose:

Quick Explanation of Each:

1. RAM

• Think of it like the router’s active workspace.

• Contents include:

  • Running-config (current configuration)

  • Routing tables

  • ARP cache

• Wipes clean when you turn off the router.

2. ROM

• Acts like a backup brain.

• Contains:

  • Minimal IOS version (ROM Monitor/ROMMON)

  • Bootstrap program (to load IOS from flash)

• Cannot be easily changed.

3. NVRAM

• Stores the startup-config file.

• When the router boots, it loads startup-config from NVRAM into RAM.

4. Flash

• Think of this like the hard drive for the router.

• Stores:

  • Main IOS image

  • Backup IOS images

• Flash is electrically erasable (you can upgrade IOS versions easily).

Summary in one line:

RAM = running workspace,
ROM = basic startup instructions,
NVRAM = saved startup settings,
Flash = storage for the router’s operating system.

What is a Gateway?

A gateway is a network device that connects two different networks and translates communication between them, allowing data to flow from one network to another.

Simple Explanation:

• A gateway acts like a bridge between two networks that might use different protocols, architectures, or formats.

• It receives data from one network, translates it if necessary, and forwards it to another network.

Example:

• Your home router serves as a gateway:
  It connects your private home network (192.168.x.x addresses) to the public Internet (global IP addresses).

• In a corporate setup, a gateway might connect a company's internal network to a partner’s network using different communication rules.

Important Points:

• A default gateway in networking is the router IP address that devices send traffic to when they need to reach outside their own local network.

• In complex enterprise networks, specialized gateways might also do:

  • Protocol conversion (e.g., TCP/IP to AppleTalk)

  • Data format translation

  • Security filtering (firewalls often act as gateways too)

Summary:

A gateway connects different networks, translates communication between them, and routes data to the correct destination.

Cut-Through LAN Switching

Cut-through switching is a method used in Ethernet LAN switches where the switch begins forwarding a frame as soon as it reads the destination MAC address, without waiting for the entire frame to be received. This reduces latency compared to other switching methods like store-and-forward.

How Cut-Through Switching Works

1. Reads Destination MAC Address:

   • The switch examines only the first 6 bytes (destination MAC) of the incoming Ethernet frame.

2. Immediate Forwarding:

   • The switch starts forwarding the frame before the entire packet is received, reducing delay.

3. No Full Error Checking:

   • Unlike store-and-forward, it does not check the frame for errors (CRC) before forwarding.

Advantages

✔ Lower Latency – Faster forwarding since the switch doesn’t wait for the full frame.
✔ Better for Real-Time Apps – Ideal for VoIP, gaming, and high-frequency trading where delay matters.

Disadvantages

✖ No Error Checking – Forwards corrupted or runt frames, increasing network errors.
✖ Less Reliable – Not suitable for noisy or unreliable networks.

Cut-Through vs. Store-and-Forward Switching

Variations of Cut-Through Switching

• Fast Forward Switching – Minimal latency; forwards immediately after reading MAC.

• Fragment-Free Switching – Checks first 64 bytes (to avoid collision fragments) before forwarding.

When to Use Cut-Through?

• In low-latency, high-speed networks (e.g., data centers).

• When error rates are low (e.g., fiber-optic networks).

// C Program to print all permutations of a given string including duplicates

#include <stdio.h>
#include <string.h>
// Function for swapping values at two pointers
void swap(char *a, char *b)  
{
   char temp;
   temp = *a;
   *a = *b;
   *b = temp;
}

/* Function for printing permutations of a string. This function takes
three parameters: String, Starting index of the string, last index of
the string. */
void permute(char *a, int beg, int end)
{
   int i;
   if (beg == end)
       printf("%s\n", a);
   else
   {
       for (i = beg; i <= end; i++)
       {
           swap((a+beg), (a+i));
           permute(a, beg+1, end);
           //backtracking method
           swap((a+beg), (a+i));
       }
   }
}

// Driver program for testing above defined functions
int main()
{
   char string[] = "XYZ";
   int n = strlen(string);
   permute(string, 0, n-1);
   return 0;
}​

XYZ
XZY
YXZ
YZX
ZYX
ZXY​

// Java program
class MaxDiffrence
{
   /* The function will assume that there will be at least two elements
   in an array. The function will return a negative value if the array
   is in decreasing order of sorting. This function will return 0
   if elements are equal. */
   
   int maximumDiff(int x[], int size)
   {
       int res = x[1] - x[0];
       int i, j;
       for (i = 0; i < size; i++)
       {
           for (j = i + 1; j < size; j++)
           {
               if (x[j] - x[i] > res)
                   res = x[j] - x[i];
           }
       }
       return res;
   }

   // Driver program for testing above function
   public static void main(String[] args)
   {
       MaxDifference md = new MaxDifference();
       int array[] = {2, 3, 90, 10, 120};
       System.out.println("Maximum difference between two elements of an array is " + md.maximumDiff(array, 5));
   }
}​

Maximum difference between two elements of an array is 118​

// C++ program
#include <bits/stdc++.h>
using namespace std;

void findFirstAndLastFunc(int a[], int n, int x)
{
   int firstPos = -1, lastPos = -1;
   for (int i = 0; i < n; i++)
   {
       if (x != a[i])
           continue;
       if (firstPos == -1)
           firstPos = i;
       lastPos = i;
   }
   if (firstPos != -1)
       cout << "First Occurrence = " << firstPos<< "\n Last Occurrence = " << lastPos;
   else
       cout << "Element not Found";
}

int main()
{
   int a[] = { 1, 2, 2, 2, 3, 3, 4, 5, 7, 7};
   int n = sizeof(a) / sizeof(int);
   int x = 7;
   findFirstAndLastFunc(a, n, x);
   return 0;
}​

First Occurrence = 8
Last Occurrence = 9​