bgp interview questions and answers Archives

BGP Interview Questions and Answers:

May 17, 2023March 25, 2023 by Aakriti

1. What is BGP in CCNA?

Border Gateway Protocol (BGP) is the routing protocol of the Internet. Meaning it chooses the best path for communication to happen between two or more routers. Some of the distinguishing features of BGP include:

It is the biggest routing protocol in the world.
It manages the trusted and untrusted routes.
It enables routing through autonomous systems instead of routers.
It is the slowest routing protocol in the world.
It is primarily used for service providers.
It can also be used for enterprise customers.

Now that you are completely aware of BGP, we can move ahead. Also, this works as the very first interview question.

It is time to graze over some other tricky BGP interview questions and answers.

2. How does BGP work?

BGP connection between two routers is called a peering session. This simply means that the two routers have established a communication channel. This would allow the exchange of information.

Once peering occurs, here is how BGP works step by step:

It will look for routes among all the External BGP routers it has in the neighborhood.
It will also look for the Internal BGP routers it has in its own Autonomous System (AS).
After applying filters, it picks up the best path.
It installs it in the routing table (RIB).
Then, the best route is sent to the external BGP neighbors.

This further leads to several sub-processes. These are:

BGP ‘in’ Process:

It receives path information from peers.
The results of the best BGP path selection are placed in the BGP table.
The best path is then ‘flagged’.

BGP ‘out’ Process:

Share the best path information to peers.

Its very common asked question in BGP interview questions

3. What does BGP mean in networking?

BGP makes the Internet work. The Internet is nothing with BGP.

BGP allows every Autonomous System (AS) to discover routes to other ASs. It is only the responsibility of BGP to filter and change the routes. In short, BGP lets you apply a wide range of policies to control the traffic. It can change the path of traffic.

As we know, the Internet is a network of networks. If we zoom into them, networks work on BGP. That’s how important BGP is!

4. Why is BGP used?

Border Gateway Protocol (BGP) works as a GPS for the packets of information. The BGP comes up with the best possible route for the packets to travel. It takes into consideration various factors, one of them being the situation of the network nodes.

BGP is designed for the following functions:

Exchange routing information
Exchange reachability information between autonomous systems on the Internet.

Each BGP speaker is called a peer, and exchanges routing information with its neighboring peers as network prefix announcements.

An Autonomous System (AS) doesn’t need to be connected to the other AS to know its network prefix.

BGP chooses the most suitable route on the basis of collected information such as an organization’s routing policy, based on cost, speed and reliability, etc.

5. Why is BGP used over OSPF?

Both BGP and OSPF are dynamic routing protocols. However, there are a few reasons why BGP is used over OSPF. These reasons are mentioned below:

OSPF is an intra-domain routing protocol and it uses link routing protocol. BGP, on the other hand, is an inter-domain routing protocol and it uses path vector routing.

OSPF is used to determine the fastest route whereas BGP tells the best path.

Now, why to use BGP over OSPF:

BGP is often used in Wide Area Network (WAN) and IaaS environments.
It is also used for Internet redundancy.
OSPF is used for Local Area Network (LAN) and data center.
BGP is primarily used in large networks.

Through the above-mentioned pointers, it can be concluded that BGP is better than OSPF.

6. Is BGP faster than OSPF?

OSPF has faster convergence time than OSPF. Network convergence is the is the speed at which a router adjusts path used to a destination network in the times of network outrage.

OSPF works better in Local Area Network (LAN) and private data centers.

7. What is the AD value of BGP?

Administrative Distance (AD) is a value that a router uses in order to choose the best path.

It helps the router to select the best path when there are two or more different routes to the same destination from two different routing protocols.
The AD value is used to rank routes from most preferred route to the least preferred route.
The most-preferred route has the lowest AD value while the least-preferred route has the highest AD value.

8. What is OSPF used for?

Open Shortest Path first (OSPF) is also a routing protocol such as BGP. It is an Interior Gateway protocol. OSPF is used for the following:

OSPF is used in large and very complex networks.
OSPF is a link state routing protocol. Thus, you can use it to converge fast.
OSPF converges faster than distance vector protocols such as BGP.
It is a routing protocol that is used to share routes between routers.
It is relatively easy to configure and is easily understood.
OSPF is used in multi-site networks.
With OSPF, you do not need to add a new route to every other site’s router. OSPF can automatically update the routing tables of various routers.

9. Do home routers use BGP?

Yes, BGP is used by routers present at our homes. BGP helps to route your e-mails and web requests across the Internet. In short, Internet routing cannot happen without BGP.

10. Why is BGP not a routing protocol?

BGP is an inter-domain routing protocol. It uses path vector routing, with the routing operations performed between two or more autonomous systems. BGP works on finding the best path possible.

11. What are LSA types?

OSPF communicates by using Link State Advertisement (LSA) to communicate for the Internet Protocol (IP). OSPF uses a Link State Database (LSDB) and it uses LSA to fill it up.

OSPF consists of many types of LSAs. There are given below:

LSA Type 1: Router LSA

In this LSA, you can find the list of all the directly connected links of this router. These types of routers always stay within the area.

LSA Type 2: Network LSA

This type of LSA is constructed for multi-access networks. Network LSAs are made by DR.

LSA Type 3: Summary LSA

The summary LSA is developed by the ABR. This is why ABR will generate a summary ASBR LSA. It will consist of the router ID of the ASBR.

LSA Type 4: Summary ASBR LSA

The summary ASBR LSA consists of the router ID of the ASBR in the link-state routing field. It makes it easy for other routers to find ASBR.

LSA Type 5: Autonomous System External LSA

These are generated by the external ASBR.

LSA Type 6: Multicast OSPF LSA

It is not used and is not supported.

LSA Type 7: Not-so-stubby area LSA

It is also referred to as not-so-stubby (NSSA) LSA. NSSAs do not allow external LSAs which are Type 5 LSA. With LSA Type 7, you can allow them.

LSA Type 8: External Attribute LSA for BGP

12. What is RIP OSPF EIGRP BGP?

RIP:

Routing Information Protocol (RIP) is a distance vector routing protocol.
It can be used to configure the hosts as a part of a RIP network.

OSPF:

OSPF is a link-state routing protocol.
It is developed for IP networks.
It is based on the Shortest Path First (SPF) algorithm.
It comes under the group of interior gateway protocols.

EIGRP:

Enhanced Interior Gateway Routing Protocol (EIGRP) is a network protocol.
It allows routers to exchange information in a better way than the network protocols that were present earlier.
It is a dynamic routing protocol used for routing and configuration decisions.

BGP:

BGP is the biggest routing protocol in the world.
It manages the trusted and untrusted routes.
It enables routing through autonomous systems instead of routers.
It is the slowest routing protocol in the world.
It is primarily used for service providers.
It can also be used for enterprise customers.

13. When should BGP not be used?

If you only want to connect to an external domain and if there is only one connection then you should not use BGP.

It’s a Wrap!

We have summed up the Cisco BGP interview questions and answers altogether. Also, the questions and answers consist of scenario-based BGP questions, BGP troubleshooting questions and BGP attribute questions and answers.

Go over all these questions and you are all set to ace your exam!

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1. What is MPLS?

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks. It is a data forwarding technology that increases the flow and speed of the network traffic. It provides a way to process packets based on their labels.

It uses labels instead of routing table lookups to allow high-end network communications from one network node to the other. MPLS uses the LFIB/forwarding table to transfer labels from one node to the other.

2. What are the benefits of using MPLS?

The benefits of using MPLS are as follows:

Multiple degrees of QoS: Multiple degrees of QoS is supported. They check latency, jitter, and packet loss for various types of traffic such as voice, video, email, and bulk file transfers, etc.
Label-based switching: Convergence is fast due to label-based switching. It cuts the need for routing tables.
IP VPNs: IP VPNs are expandable.
MPLS TE: Network congestion is minimum by using MPLS TE.
Reliable: MPLS is reliable, safe, and trustworthy.

3. What are the MPLS router types?

The following are the MPLS router types:

C – Customer Router
CE – Customer Edge Router
PR – Provider Router
PE – Provider Edge Router

4. What is the difference between a P and a PE router?

P router does not contain customer network routes. These routes are, however, available on the PE router. Also, P routers do not need MP-iBGP. For PE routers, MP-iBGP is a must.

5. Name the types of labels.

The types of labels are:

Explicit Null
Implicit Null
Aggregate Label

6. What are the types of MPLS available?

There are three types of MPLS available:

Layer 2 point to point
Layer 3 IP VPN
Layer 2 VPLS

a. MPLS Layer 2 Point to Point:

The layer 2 point to point MPLS is the best suited for companies that need high bandwidth between a small number of sites.
It is economical.
It is an excellent alternative to high bandwidth leased lines.
Many network operators depend on Layer 2 and Ethernet for their core network infrastructure.
This protocol allows anything running over the LAN to be sent to the WAN without the need for routers to convert packets to Layer 3 (Network Layer).

b. MPLS Layer 3 IP/VPN:

The Layer 3 IP/VPN is best suited for large multi-site enterprises such as retail chains.
They deploy a large number of low bandwidth sites or large corporates

It is the best fit for companies that are:

- In the process of merging: IP/VPNs are scalable for fast deployment.
- Need ‘any to any’ connectivity: a shorter hop count between two local sites is more efficient than -’tromboning’ back into a central point. It is best suited for global networks where latency is increased.
- Preparing for voice and data convergence: to implement a blanket ‘class of service’. It is made simple across multi-site networks.
- Migrating from traditional ATM to IP: ATM has very high maintenance charges.
- Low bandwidth needs at small branch offices.
- Need of only a secure dial-up capability in smaller locations

c. Layer 2 Virtual Private LAN Services (VPLS):

- The VPLS services are popular for delivering Ethernet services.
- They combine both MPLS and Ethernet for customer and carrier benefits.
- IP backbones have been used to provide Internet access as well as IP VPN access.
- VPLS is also known as transparent Ethernet services.
- It works over MPLS and gives benefits of two network types:
  - Ability to operate a multipoint network
  - Pass all traffic at Layer 2 over the WAN
- VPLS is popular among TV broadcasters, the financial sector and media houses.

7. What is the difference between VPN and MPLS?

VPN:

VPN is referred to as Virtual Private Network.
It could be configured using GRE tunnels.
If you want a full mesh then the administrator needs to set u n*n-1 tunnels.

MPLS:

In the case of MPLS VPN, CPE works in the full mesh by default.
It works in full mesh form because of the route-target.

8. Can you make your PE router a P?

You need to remove the Border Gateway Protocol (BGP) configurations to make your PE a P. After you do that, it will not participate with the customer network.

9. If your LDP router ID, OSPF router ID and BGO router ID are different, will it work to forward the traffic of customers or not?

The BGP router ID and the LDP router ID should be the same if SP is using labels only for loopbacks. If labels are generated for each and every route then there is no problem at all.

10. What protocol is used by MPLS?

MPLS uses the Tag Distribution Protocol (TDP) or Label Distribution Protocol (LDP) protocols.

Tag Distribution Protocol (TDP):

Tag Distribution Protocol (TDP) is a two-party protocol. It runs over a connection-oriented transport layer with guaranteed sequential delivery.

Label Distribution Protocol (LDP):

Label Distribution Protocol (LDP) is a protocol used to establish MPLS transport LSPs when there is no need for traffic engineering. It establishes LSPs that follow the existing IP routing table. It is best suited for establishing a full mesh of LSPs between all of the routers on the network.

11. What is penultimate hop popping?

Penultimate hop popping is a method of reducing label lookups on the egress router. It is done by the one-hop before the egress router.

12. What are the functions done by MPLS?

The following are the functions done by MPLS:

PUSH (Adding the Label)
POP (Removing the Label)
SWAP (Changing the Label)

13. What is downstream on demand?

The downstream router is responsible to advertise the label first to the upstream router when the downstream on-demand method is selected.

The upstream router is the router that advertises the labels to its downstream router after receiving label bindings from it.

14. What is the difference between VPNv4 and IPv4 address families?

We use the IPv4 address family to always accept and forward IP packets to customers.

When the customers’ packets are being received by PE, they become labeled and forward packets to different PE/RR. For this, an address family VPNv4 is needed.

In other words, we can say that IPv4 address-family is being used for customers. VPNv4 address-family is used by SP core.

15. What is SYSVOL?

The SysVOL folder has the server’s copy of the domain’s public files. The contents such as users, group policy, etc. of the SysVOL folders are replicated to all domain controllers in the domain.

16. MPLS works on which layer?

It works between layer 2 and layer 3.

17. What is the difference between RD and RT?

RT is an extended community. RD is not an extended community.

18. How to filter MPLS labels?

MPLS filters can be labeled by using ACLs.

19. Two routers are having 4 equal-cost links, how many LDP sessions will be established?

Only one session will be established between the two routers having 4 equal-cost links.

FAQs

What is MPLS in Networking?

Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks. It is a data forwarding technology that increases the flow and speed of the network traffic.

What does MPLS use to transfer from one node to another?

MPLS uses the LFIB/forwarding table to transfer labels from one node to the other. It uses labels instead of routing table lookups to allow high-end network communications from one network node to the other.

What are the benefits of using MPLS?