BGP interview questions and answers aim to provide customers with the most up-to-date technological knowledge, ensuring that the software works as intended and satisfies expectations.

It is a popular internet routing mechanism; it helps routers choose the optimum data transmission channel by exchanging routing information across independent systems, we provide BGP protocol interview questions.

1. What is BGP?

BGP, or the Board of Gateway Protocol, is a protocol that runs the Internet and interconnects through routers running in Internet service providers; the whole Internet routing table now contains over 900,000 routes, approaching 1 million.

2. What is BGP synchronisation, and how does it work?

BGP synchronisation is an older feature that might not be encountered in real-world scenarios but may be encountered on the certification exam; this rule can be configured and shown before and after synchronisation.

3. What is the purpose of the deep dive session on BGP?

The deep-dive session is designed to provide a comprehensive understanding of BGP, its features and how to improve the overall performance of network infrastructure.

4. What is an IGP?

An IGP, or interior gateway protocol, runs within an autonomous network under a single administrative control, for example, ISP1 and ISP2 have autonomous systems, and within their networks, they can run IGPs such as RIP, OSPF, EIGRP, and ISIS.

5. What is an EGP?

Each autonomous system number has its own A S (Autonomous System Number) and an EGP (Exterior Gateway Protocol) that talks between autonomous systems; BGP currently lists one EGP, which is called BGP.

6. What is BGP used for?

BGP is a crucial protocol for managing the Internet and connecting to it is used within an autonomous system, where each system has a unique number of routes, however, there is also an existing exterior gateway protocol called EGP, which is no longer used.

7. What is an autonomous system?

An autonomous system is a network under a single administrative control; for example, ISP1 and ISP2 have autonomous systems, and within their networks, they can run IGPs such as RIP, OSPF, EIGRP, and ISIS.

8. What is the difference between EGP and BGP?

EGP is an external BGP that runs between autonomous systems, while I BGP can run within an independent system, EGP has an administrative distance of 20 and is very believable, beating out other routing protocols like RIP, EAGER, P&O SPF, and ISIS.

9. What is the use case for EGP and I BGP?

EGP connects autonomous systems, such as between a company and different internet service providers, I BGP is used when an interior gateway protocol, like OSPF or EAGERP, carries the same route as I BGP.

10. What is the difference between BGP and RIP?

BGP is an exterior gateway protocol that interconnects autonomous systems, while RIP is a multicast routing protocol that multicasts out or broadcasts routing tables.

11. What is the difference between BGP and OSPF?

BGP sets up a TCP (Trouble-Call Protocol) session between two routers, educating each other about available routes, no longer available routes, errors, and parameters.

OSPF is more strategic about forming neighborships by sending hello messages to multicast addresses.

12. What is the main difference between traditional routing protocols and BGP regarding route advertisements?

Traditional routing protocols typically use a single metric to determine the best path, while BGP uses several different path attributes called PAs to make this determination.

13. How does OSPF determine the best path using route advertisements?

OSPF uses a function of cost, with the reference being divided by length, speed, and cost, the lowest price is chosen using the ads, or P, which is a more complex formula that comes down to the bane within the delay.

14. What are path attributes in BGP, and how are they used to determine the best path?

Path attributes in BGP are used sequentially to determine the best path, with the first being called a weight, if the weights of two paths are equal, the local preference is used.

15. How does BGP manipulate path attributes to influence which path is taken to the internet and the organisation?

BGP can manipulate these path attributes strategically to influence which path is taken out to the internet and into the organisation without reconfiguring ISPs’ routers.

16. What are link-state routing protocols, and what is one example of a link-state routing protocol?

Link-state routing protocols distribute the entire routing table to all routers in the network, OSPF is one example of a link-state routing protocol.

17. What is a path vector routing protocol, and what is one example of a path vector routing protocol?

A path vector routing protocol provides a variety of path attributes that tell us the distance and direction of the path. One example of a path vector routing protocol is BGP.

18. What is a route reflector, and how does it work?

A route reflector is a designated router that receives an advertisement from an NLRI information from R2 and reflects it to all other routers in the autonomous system, this creates a more efficient way to maintain a full mesh of connectivity within an independent system.

19. What is a confederation in routing protocols, and how does it work?

A confederation in routing protocols involves creating a subset of an autonomous system with a specific number of routers that can be connected to any other sub-autonomous system.

This allows for a full mesh of connectivity within the independent system, but it doesn’t scale well, as the number of neighbours required is not large enough for a full mesh of I BGP neighbours.

20. What is the difference between a confederation and a route reflector in routing protocols?

The difference between a confederation and a route reflector in routing protocols is that a confederation involves creating a subset of an autonomous system with a specific number of routers that can be connected to any other Sub-autonomous system.

21. What happens during the three-way handshake process?

During the three-way handshake process, the routers send each other a synchronisation message and SYN message, acknowledging the request with an ACK and then sending a synchronisation or SIN message from one side to the other.

22. What is the keepalive timer?

The keep-alive timer sends a message to the neighbour every minute to ensure the session is active, if the neighbour fails to respond within 180 seconds, the neighborship is destroyed.

23. What happens if a router does not respond within 180 seconds?

The neighbour is destroyed if a router does not respond within 180 seconds.

24. What path attributes are exchanged in an update message?

An update message exchanges path attributes such as information about the router’s version of BGP, autonomous system number, and other details like its router ID.

25. What is the role of the OPEN message in the BGP session?

The open message is used during the setup process, containing information about the router’s version of BGP, autonomous system number and other details like their router ID.

26. What timers are set by the router and can be optionally configured?

The timers set by the router and optionally configured include the keepalive and hold timers.

27. How many path attributes are there in BGP?

There are 13 path attributes to consider in BGP.

28. What is the weight attribute in BGP?

The weight attribute is a Cisco proprietary path attribute determining which route to use for outbound path selection decisions.

29. How can we determine which way to go to the network based on the highest weight?

By assigning a weight of 100 to route advertisements coming in over slower links and 200 for weight advertisements coming in over faster links.

30. What is the importance of weight and local preference in determining outbound path selection in an autonomous system?

Weight and local preference are essential in determining outbound path selection as they help the network decide which route to use based on the highest weight.

31. How can the AS path influence inbound path selection?

AS paths can influence inbound path selection by making them more attractive to internet users trying to enter the network based on their distance from the internet backbone.

32. What is autonomous system path prepending?

Autonomous system path prepending involves adding multiple instances of a router’s independent system path to make it more attractive to internet users trying to access the network.

33. What is the difference between inbound and outbound path selection?

Inbound path selection refers to determining the path internet users take to enter an autonomous system; in contrast, outbound path selection refers to choosing the independent system’s path to exit the internet.

34. What is the local preference in determining outbound path selection?

Local preference is a concept used in determining outbound path selection in an autonomous system, and it involves giving more weight to a router’s link than other links when choosing a path for outgoing traffic.

35. What is the origin type in BGP?

The origin type in BGP refers to how a route enters the routing table, and it can be I (indirect), E (interior gateway protocol), or a question mark (indicating that the route was redistributed).

36. What is the multi-exit discriminator (med) in BGP?

The multi-exit discriminator (med) is a metric used in BGP to determine the preferred outbound path, the higher the med, the less selected the route. To choose routes with a lower med, you can stamp them and advertise them to your neighbour.

37. What is a routing information-based BGP failure (rib failure)?

A routing information-based failure (rib failure) in BGP occurs when BGP fails to take route information from a more credible source, such as the route; this is not usually a significant issue and is known as an “orange” error.

38. Why was BGP synchronisation turned off in 2002?

BGP synchronisation was turned off in 2002 because it became a consensus in Cisco iOS 12 dot 280 that it is turned off by default and that a coming end in BGP will say no synchronisation.

39. What is the initial step in investigating the synchronising behaviour of an IBGP network?

They examine the live interface and check if they are receiving 3.3.3.

40. How can synchronisation in an IBGP network be fixed if there is no matching entry in an interior gateway riding protocol?

OSPF can be set up on R2 and R3, and the network command can be specified to participate in the same OSPF autonomous system.

41. How can synchronisation rules be checked in an IBGP network?

By performing a show IP OSPF neighbours command and checking the state of the network.

42. What happens when there is synchronisation in an IBGP network, and there is a matching entry in an interior gateway riding protocol?

If the rules are on, the router will not advertise the regulations to its IBGP peer of R1.

43. What happens when there is synchronisation in an IBGP network and a matching entry in an interior gateway riding protocol and OSPF?

OSPF’s interior gateway riding protocol is now synchronised with BGP’s exterior gateway protocol.

44. What is BGP summarisation, and why is it important?

BGP summarisation is a technique used in routing protocols to combine multiple networks into a single route advertisement; it is essential because it allows for the efficient use of various routes within a single route advertisement, reducing the overall size of the routing table and improving performance.

45. How does BGP summarisation work?

In BGP summarisation, the four networks share a standard set of bits, which can be mathematically represented as 22 bits, revealing that the first eight bits are the same, followed by the next eight bits and the third six bits, this results in 22 bits in common for all four networks.

46. How does BGP summarisation reduce the size of the routing table?

Combining the four networks into a single BGP summarization reduces the overall size of the routing table instead of having four separate route advertisements for each network.

47. Can BGP summarisation be used with multi-hop routing protocols?

Yes, BGP summarisation can be used with multi-hop routing protocols; the speaker demonstrates this by comparing the topology with router R1’s four loopback interfaces, which have different IP addresses but share the first 22 bits in common.

48. What is BGP summarisation?

BGP summarisation is used in routing protocols like BGP to combine multiple networks into a single route advertisement, allowing for the efficient use of numerous routes within a single route advertisement.

49. What is a topology on screen in BGP summarisation?

A topology on screen in BGP summarisation visualises the routing table used to advertise the networks; this topology is compared with router R1’s four loopback interfaces, which have different IP addresses but share the first 22 bits in common.

50. How does BGP summarisation have the potential for summarisation in the internet routing table?

BGP summarisation potential in the internet routing table is extensive due to the many networks and routes available; by combining multiple networks into a single summary advertisement, BGP can efficiently use various routes within a single route advertisement, reducing the overall size of the routing table.

51. What is the EBGP multihop feature, and how does it allow for forming neighborships with routers that are potentially several hops away?

The EBGP multihop feature allows for forming neighborships with routers potentially several hops away by allowing multiple hops between routers.

52. What is the importance of having a full mesh of relationships within an autonomous system?

A full mesh of relationships within an autonomous system is essential because it enables more efficient and secure communication between routers.

53. How do you use the EBGP multi-hop feature to create a more efficient and secure network?

EBGP multi-hop feature allows for two hops between routers, enabling more efficient and secure communication between routers.

54. What is the purpose of the binary for the aggregated address and the subnet mask?

The purpose of the binary for the aggregated address and the subnet mask is to send a summarised address to the router.

55. How do presenting an IP BGP summary and modifying the rule for building an E BGP neighbour with non-adjacent routers differ?

Setting up an IP BGP summary shows the neighbours of R1, R2, and R3 while changing the rule about forming an E BGP neighborship with non-adjacent routers allows for more than one hop away.

56. What is the best practice for setting up BGP?

The best practice for setting up BGP is to use the router ID as the BGP space router ID, which in this example is three dots, three dots, three dots, and three dots.

57. What is the issue with the link speed on R3 in this BGP configuration?

The issue with the link speed on R3 in this BGP configuration is that the user is experiencing problems with it, even though the weight and local preference are the same and the autonomous system path length is different.

58. What is the purpose of peer groups in networks?

Peer groups in networks simplify configuration by grouping routers with various policies or routing commands, this approach allows for a more straightforward application of policies, summarisation commands, or filters, potentially impacting overall performance.

59. How can a Route reflector be used in networks?

Route reflectors can be used in networks to reflect advertisements to other rat reflector clients, allowing for improved autonomy and communication between routers; this approach can simplify the configuration of networks and improve performance.

60. What is the concept of confederations in networks?

Network confederations involve grouping routers to simplify configuration and improve autonomy, this approach allows for a more straightforward application of policies, summarisation commands, or filters, potentially impacting overall performance.

61. How can private IP addresses be blocked in networks?

Private IP addresses can be blocked in networks by applying a prefix list to ISPs, for example, R3 can block any personal IP address from ISP1 and use the same prefix list for both ISP1 and ISP2.

It ensures that the network does not receive advertisements for private IP addresses, preventing them from being transmitted over the Internet.

62. What is applying a prefix list to a peer group?

Applying a prefix list to a peer group involves filtering out private IP addresses from network advertisements by creating a group called BGP hyphen and using the prefix list; this allows for more efficient configuration and better network performance.

63. What happens to the peer group after applying the prefix list?

After the prefix list is applied to the peer group, the group is removed from the network, and the individual routers are then put back into the group, ensuring that the correct autonomous systems are assigned to each neighbour.

64. How does applying a prefix list to a peer group work?

When a prefix list is applied to a peer group, all neighbours are part of the group, for example, if a neighbour of size P1 belongs to the BGP hyphen P G peer group, the speaker can apply the prefix list to that neighbour.

65. Why apply the route map outward while setting up a multi-protocol BGP (MPBGP) network using the A S path route map?

Send an exaggerated autonomous system path to ISP1, improving the network’s appearance and allowing the network to use ISP two for outbound and inbound connections.

66. Why reset all connections and delete the IPBGP star when setting up an MPBGP network using the A S path route map?

To ensure that the preferred path is the shorter autonomous system path length for ISP one, which is faster than the massively long path with ISP one.

67. Why is IPv6 addressing used in address families to implement a multi-protocol BGP (MPBGP) network utilising the A S path route map?

To add IPv6 addressing to ISP three and ISP two, with an address family for IPP version four and an address family for IPP version six.

68. Why switch off R3’s prior BGP router configuration while creating an MPBGP network using the A S path route map?

To allow IPv6 version six unicast routing that is supported now.

69. Why is the router process removed while setting up a multi-protocol BGP (MPBGP) network using the A S path route map?

Ensure the IPv6 address is on the same network as R3 and shares the same link.

70. What is the purpose of creating a route map on R3 in this scenario?

Creating a route map on R3 will advertise the egress IPv6 address as the next hop for any advertisement sent to ISP two.

71. Why is the IPv6 neighbour manually activated in this configuration?

The purpose of manually activating the neighbour for the IPv6 address family is to ensure that R3 is advertised as the next hop for any advertisement sent to ISP two.

72. What is the purpose of applying the route map in this configuration?

The purpose of applying the route map is to advertise the egress IPv6 address as the next hop for any advertisement sent to ISP two.

73. What is the IPv6 following hop address advertised in this configuration?

The IPv6 following address advertised in this configuration is 00, 2:22.

74. Why does this setup prioritize 203.0.113.4 with a 30-bit subnet mask of 255.255.255.252?

The purpose of prioritising the network of 203.0.113.4 with a 30-bit subnet mask of 255.255.255.252 in this configuration is to influence inbound path selection by pre-pending a bunch of autonomous systems to the neighbour as it advertises the network.

75. Why are rubber-stamp routes promoted into this configuration’s greater local preference?

The purpose of rubber-stamping local preference values on routes being advertised into the higher local preference in this configuration is to influence inbound path selection by rubber stamping local preference values on routes advertised into the higher local preference, the better.

76. What is the purpose of using multi-protocol BGP in this configuration?

The purpose of using multi-protocol BGP in this configuration is to do everything in one structure, and this can be done with the IGRP and WSPF with BGP, all using address families.

Conclusion

BGP is a link-state routing protocol with multiple variants; the newest version, BGPv5, is extensively used; it supports IPv6, VPNs, and greater security, while older versions have drawbacks.

In convergence, BGP exchanges routing information between autonomous systems, and it may also communicate routing information inside an independent system using OSPF and EIGRP.