interview L1/L2 Answer

1. What is BPDU Guard?

BPDU (Bridge Protocol Data Unit) Guard is a feature in networking that is used to protect the network from potential loops. It disables a port when a BPDU is received on a port that is configured with PortFast, which is typically used for end devices like computers. By disabling the port, BPDU Guard helps to prevent misconfigurations and potential loops in the network.

2. What is loop-back detect?

Loop-back detection is a network feature that helps to identify and mitigate network loops. Network loops can cause broadcast storms and network congestion. Loop-back detection mechanisms typically send out special frames to detect loops in the network topology and, upon detection, disable the affected ports to prevent the loop from impacting the network.

3. What is the Spanning Tree Protocol (STP)? How are loops avoided in STP?

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for Ethernet networks. STP avoids loops by selectively blocking certain redundant paths in the network. It creates a spanning tree that represents the shortest path without loops, and any redundant paths are put into a blocking state.

4. Explain the election process of Root Bridge in STP

The Root Bridge in STP is elected based on the Bridge ID, which consists of a priority value and the MAC address of the switch. The switch with the lowest Bridge ID is elected as the Root Bridge. The election process is as follows:

  1. Each switch sends BPDU frames containing its Bridge ID.
  2. When a switch receives a BPDU with a lower Bridge ID, it updates its root ID to reflect the lowest Bridge ID it has seen.
  3. This process continues until all switches agree on the switch with the lowest Bridge ID as the Root Bridge.

5. What is the convergence period in RSTP? Why is convergence faster in RSTP?

The convergence period in RSTP (Rapid Spanning Tree Protocol) is the time it takes for the network to reach a stable state after a topology change. RSTP converges faster than STP because it introduces new port roles and states, allowing it to rapidly switch between states without waiting for timers to expire.

6. What is a VLAN? What is the difference between the collision domain and the broadcast domain?

A VLAN (Virtual Local Area Network) is a logical grouping of devices on a network, which allows devices to be grouped together regardless of their physical location. This improves network management and security.

  • Collision Domain: A network segment where data packets can collide with each other when being sent on a shared medium.
  • Broadcast Domain: A logical division of a network where any broadcast sent by a device is received by all other devices in that domain.

7. Explain VTP and DTP.

  • VTP (VLAN Trunking Protocol): A Cisco protocol used to manage VLANs across multiple switches. VTP propagates VLAN information to all switches in a VTP domain, ensuring consistent VLAN configuration.
  • DTP (Dynamic Trunking Protocol): A Cisco protocol that negotiates and establishes trunk links between switches dynamically. It allows switches to automatically form trunk links and carry VLAN traffic.

8. What is VTP pruning? What are the three modes in VTP?

  • VTP Pruning: VTP pruning enhances network bandwidth by reducing unnecessary flooded traffic. It prevents VLAN updates from being sent across trunk links that do not need them.
  • Three Modes in VTP:
    1. Server Mode: Allows full VTP management, including adding, deleting, and modifying VLANs.
    2. Client Mode: Receives VLAN information from VTP servers but cannot make changes.
    3. Transparent Mode: Forwards VTP advertisements but does not apply them. Allows local VLAN configuration without affecting the VTP domain.

9. Explain the working of DNS and DHCP.

  • DNS (Domain Name System): Resolves human-readable domain names (e.g., www.example.com) into IP addresses. It operates through a hierarchy of servers, starting from root servers to top-level domain (TLD) servers to authoritative name servers.
  • DHCP (Dynamic Host Configuration Protocol): Automatically assigns IP addresses to devices on a network. When a device connects, it sends a DHCP Discover message. The DHCP server responds with an offer, and the device requests the offered IP address. Finally, the server acknowledges, completing the IP assignment process.

10. What is APIPA IP? What is the range of APIPA IPs?

APIPA (Automatic Private IP Addressing) is a feature in Windows that automatically assigns an IP address from a specific range if the DHCP server is unavailable. The range of APIPA IPs is 169.254.0.1 to 169.254.255.254.

11. Why is DR elected in OSPF?

In OSPF (Open Shortest Path First), a Designated Router (DR) is elected on multi-access networks to reduce the number of adjacencies and simplify the network topology. The DR minimizes the number of exchanges of OSPF routing information, improving efficiency.

12. How are DR and BDR elected in OSPF?

The DR and Backup Designated Router (BDR) in OSPF are elected based on router priority and, if priorities are equal, the router ID. The election process involves:

  1. Each router announces its priority and router ID in Hello packets.
  2. The router with the highest priority is elected as the DR.
  3. The router with the next highest priority becomes the BDR.
  4. If priorities are equal, the router with the highest router ID is elected as DR/BDR.

13. What is Type-3 and Type-5 LSA?

  • Type-3 LSA (Summary LSA): Generated by Area Border Routers (ABRs) to advertise networks from one area to another. It summarizes and floods routing information between areas.
  • Type-5 LSA (External LSA): Generated by Autonomous System Boundary Routers (ASBRs) to advertise external networks (networks outside the OSPF autonomous system).

14. What are the types of networks in OSPF?

OSPF classifies networks into several types:

  1. Broadcast: Typically Ethernet networks where all routers can communicate directly.
  2. Non-Broadcast Multi-Access (NBMA): Networks like Frame Relay where routers must be manually configured to communicate.
  3. Point-to-Point: Direct link between two routers.
  4. Point-to-Multipoint: One router connected to multiple routers in a hub-and-spoke configuration.

15. What is the difference between Private IP and Public IP?

  • Private IP: Used within a private network. Not routable on the internet. Examples include IP ranges 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.
  • Public IP: Routable on the internet. Unique across the entire internet.

16. What is the range of Class B IP?

The range of Class B IP addresses is 128.0.0.0 to 191.255.255.255. The default subnet mask for Class B is 255.255.0.0.

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