20 Layer 3 VPN Interview Questions and Answers

Layer 3 VPNs are a type of virtual private network that uses IPsec to secure communications between two or more devices. When interviewing for a position that involves Layer 3 VPNs, it is important to be prepared to answer questions about your experience and technical knowledge. This article reviews some common questions that you may be asked during a job interview, as well as tips on how to answer them.

Layer 3 VPN Interview Questions and Answers

Here are 20 commonly asked Layer 3 VPN interview questions and answers to prepare you for your interview:

1. What is a Layer 3 VPN?

A Layer 3 VPN is a type of VPN that uses IP routing to connect two or more locations. This type of VPN is often used by businesses to connect their different locations together, or to connect their network to a partner’s network.

2. How does it differ from a VPLS and an MPLS VPN?

A Layer 3 VPN is a type of VPN that uses IP routing to connect sites. A VPLS is a virtual private LAN service that uses Ethernet to connect sites. An MPLS VPN is a multi-protocol label switching VPN that uses MPLS to connect sites.

3. Can you explain the benefits of using a Layer 3 VPN over a traditional LAN connection?

Layer 3 VPNs offer a number of advantages over traditional LAN connections, including increased security, flexibility, and scalability. With a Layer 3 VPN, each site in the VPN has its own unique IP address, which makes it much more difficult for hackers to gain access to the network. Additionally, Layer 3 VPNs can be easily expanded as needed, without having to reconfigure the entire network.

4. What’s the difference between a site-to-site and point-to-point L3VPN?

A site-to-site L3VPN is a VPN that connects an entire site to another site. A point-to-point L3VPN is a VPN that connects a single device to another device.

5. Which protocol is used to establish communication between PE nodes in a L3VPN network?

The Border Gateway Protocol (BGP) is used to establish communication between PE nodes in a L3VPN network.

6. What are some important L3VPN routing protocols?

Some important L3VPN routing protocols include Border Gateway Protocol (BGP), Routing Information Protocol (RIP), and Open Shortest Path First (OSPF). These protocols are responsible for exchanging routing information between VPN sites and ensuring that data is routed properly through the network.

7. How do you configure a L3VPN on Cisco IOS routers?

The configuration of a L3VPN on Cisco IOS routers is a bit more involved than configuring a traditional VPN, as it requires the use of MP-BGP (Multiprotocol Border Gateway Protocol) in order to exchange VPNv4 routes. In addition, you will need to configure appropriate route targets and route distinguishers, as well as setting up the VPNv4 address family on your router interfaces.

8. How do you troubleshoot a L3VPN connection issue on Cisco IOS routers?

There are a few different things that you can do in order to troubleshoot a L3VPN connection issue on Cisco IOS routers. One thing that you can do is to use the “debug vpdn events” command in order to see what is happening with the L3VPN connection. Another thing that you can do is to use the “show vpdn session” command in order to see the status of the L3VPN connection. Finally, you can use the “show vpdn l2tp” command in order to see the L2TP information for the L3VPN connection.

9. Why is it important to use route reflectors when configuring a L3VPN?

Route reflectors are important when configuring a L3VPN because they help to keep the VPN network topology hidden from the underlying IP network. By using route reflectors, you can keep the VPN network topology separate from the IP network, which makes it more secure and scalable.

10. What are some L3VPN security best practices?

Some L3VPN security best practices include ensuring that your VPN is properly encrypted, using strong authentication methods, and restricting access to authorized users only. Additionally, it is important to keep your VPN software up to date and to monitor your VPN traffic for any suspicious activity.

11. What is VRF lite? When would you use it instead of a full blown L3VPN?

VRF lite is a simpler version of L3VPN that uses only one routing table per customer. It is typically used in smaller networks where there is not a lot of traffic between different customers.

12. How does a BGP/MPLS IP VPN work?

A BGP/MPLS IP VPN uses a combination of the Border Gateway Protocol (BGP) and Multi-Protocol Label Switching (MPLS) to create a virtual private network (VPN) over an IP network. BGP is used for routing between different VPN sites, while MPLS is used to forward traffic within each site.

13. What are the different types of BGP routes?

There are four different types of BGP routes:

1. Internal BGP (IBGP) routes are those that are learned from other BGP speakers within the same AS.
2. External BGP (EBGP) routes are those that are learned from BGP speakers in other ASes.
3. Local BGP (LBGP) routes are those that are learned from other sources within the same AS, such as IGP routing protocols.
4. Static BGP (SBGP) routes are those that are manually configured and not learned from any other BGP speaker.

14. Is it possible to deploy a L3VPN with overlapping CIDR blocks? If yes, then how?

Yes, it is possible to deploy a L3VPN with overlapping CIDR blocks. This can be accomplished by using a technique called route reflection. Route reflection is a method of distributing routing information between VPN sites without the need for full mesh connectivity. This allows for the creation of VPNs with overlapping CIDR blocks, as long as the route reflectors are configured correctly.

15. What is the process of establishing a BGP session between two devices?

The process of establishing a BGP session between two devices is known as peering. In order to peer, the two devices must first exchange BGP routing information. This is done by exchanging BGP UPDATE messages. Once the two devices have exchanged routing information, they can then establish a BGP session.

16. Can you give me examples of real-world applications that make use of L3VPNs?

L3VPNs are commonly used in enterprise networks to connect different locations together. This can be done either through the public internet or through a private network. L3VPNs can also be used to connect different service providers together, which is known as inter-provider VPN (IPVPN).

17. What are some common causes for intermittent connectivity issues when using L3VPNs?

There are a few common causes for intermittent connectivity issues when using L3VPNs. One is if the route between the customer and the provider network changes. This can happen if the provider network changes, or if the customer network changes. Another common cause is if the VPN tunnel goes down for some reason. This can happen if there is a problem with the VPN equipment, or if there is a problem with the network connection.

18. How is QoS implemented in a L3VPN?

QoS can be implemented in a L3VPN in a few different ways. One common way is to use MPLS Traffic Engineering (MPLS TE) to create tunnels between customer sites. These tunnels can then be used to route traffic in a way that meets the customer’s QoS requirements. Another way to implement QoS in a L3VPN is to use DiffServ. This allows for different types of traffic to be given different priorities, ensuring that time-sensitive traffic is not delayed by less important traffic.

19. Are there any alternatives to using L3VPNs?

There are a few alternatives to using L3VPNs, such as MPLS VPNs or VPLS. However, L3VPNs are generally the most popular option due to their flexibility and scalability.

20. What are the disadvantages of using L3VPNs?

One disadvantage of using L3VPNs is that they can be more complex to configure than other types of VPNs. This is because L3VPNs require the use of multiple routing protocols in order to function properly. Additionally, L3VPNs can be more expensive to set up and maintain than other types of VPNs because of the need for specialized equipment.