Top 100+ Border Gateway Protocol (bgp) Interview Questions And Answers
Question 1. Explain Border Gateway Protocol (bgp) ?
Border Gateway Protocol advertises, learns and chooses the first-class paths within the worldwide Internet. When ISPs join, they usually use BGP to exchange routing statistics. Enterprises also once in a while uses BGP to trade routing facts with ISPs, permitting the Enterprise routers to analyze Internet routes. Whilst we've got a couple of Internet connections and we want to persuade some packets to take one direction and a few packets to take any other we use BGP.
Question 2. Can Routers On Different Subnet Become Bgp Neighbors?
BGP does now not require acquaintances to be attached to the equal subnet. Instead, BGP routers use a TCP connection among the routers to bypass BGP messages permitting neighboring routers to be at the equal or distinctive subnet.
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Question 3. What Tcp Port Number Bgp Use For Connection?
BGP makes use of TCP port 179 for the connection.
Question four. Difference Between Ebgp And Ibgp Neighbor?
In iBGP, neighborship is fashioned between routers in the equal AS (self sufficient machine) while in eBGP, neighborship is fashioned between routers inside one-of-a-kind AS.
Question 5. What Administrative Distance Bgp Uses For Ibgp & Ebgp ?
AD for iBGP = 200, AD for eBGP = 20.
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Question 6. Explain Loop Prevention Mechanism In Bgp?
BGP makes use of mechanism to prevent loops:-
When a router learns routes from an iBGP peer, that router does not promote it the equal routes to another iBGP peer.
By the usage of AS_PATH - When marketing to an eBGP peer, a BGP router provides its personal ASN to the AS_PATH. If a BGP router gets an replace and the course advertisement lists an AS_PATH with its personal ASN, the router ignores that path.
Note - A BGP router does no longer upload its ASN when marketing to an iBGP peer.
Question 7. Do We Need To Follow three Way Handshake Process To Establish Bgp Communication?
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Question 8. What Is The Difference Between Hard Reset And Soft Reset In Bgp?
In case of tough reset the local router brings down the neighborship, brings down the underlying TCP connection and all the BGP desk entries discovered from that neighbor are removed. #clear ip bgp * In case of tender reset, the router does now not convey down the BGP neighborship or the underlying TCP connection. However, the neighborhood router resends outgoing Updates and reprocesses incoming Updates adjusting the BGP table based at the cutting-edge configuration. #clean ip bgp * soft command is used for tender reset.
Question 9. What Are Different Bgp Message Types?
Open - It is Used to establish a neighbor dating and change parameters, consisting of self sufficient device wide variety and authentication values.
Keepalive - It is Sent periodically to preserve the neighbor courting. If the Keepalive message is not acquired within the negotiated Hold timer than BGP neighborship might be turned down.
Update - It exchanges Path Attributes and the related prefix/duration (NLRI) that use those attributes.
Notification - It is Used to document BGP blunders. It consequences in a reset of neighbor dating.
Question 10. Explain Various States Of Bgp?
Idle - The BGP technique is both administratively down or watching for the the next retry try.
Connect - The BGP procedure is expecting the TCP connection to be completed. If it's miles successful, it will continue to the OpenSent nation. In case it fails, it's going to continue to the Active country.
Active - BGP will strive any other TCP three-manner handshake to establish a reference to the far flung BGP neighbor. If it is a success, it'll circulate to the OpenSent kingdom.
Opensent - The TCP connection exists, and a BGP Open message has been sent to the peer, however the matching Open message has no longer yet been acquired from the opposite router.
Openconfirm - An Open message has been each sent to and received from the alternative router. Next step is to receive a BGP Keepalive message (to confirm that every one neighbor-associated parameters healthy) or a BGP Notification message (to research that there's a few mismatch in neighbor parameters).
Established - All neighbor parameters matched, the neighbor dating has been established and the peers can now trade Update messages
Question 11. Explain Bgp Path Attributes?
BGP supports a huge sort of Path Attributes. BGP use these path attributes to take a look at the competing BGP paths (routes) in BGP table to choose the fine route(course).
Next Hop - It lists the next-hop IP cope with used to attain a prefix. If Next hop is on hand? If no route to reach Next Hop, router can't use this path.
Weight - It is a numeric value set via a router while receiving updates to steer the course for a prefix. It isn't always marketed to any BGP peers. Bigger is favored
Local Preference - It is a numeric price set and communicated within a unmarried AS for the cause of selecting fine direction for all routers in that AS to attain a positive network. Bigger is favored
Locally injected routes - Locally injected routes (routes injected using network command) are higher than iBGP/eBGP learned.
AS Path - It is the range of ASNs in the AS Path. Smaller is favored.
Origin - Preferred I over E & E over ?. It implies that the course became injected into BGP as I (IGP), E (EGP) or ? (incomplete statistics).
Multi-Exit Discriminator (MED) - Allows an AS to tell a neighboring AS the exceptional course to ahead packets into the primary AS. Smaller is preferred.
Neighbor kind - eBGP is favored over iBGP.
IGP metric - Route with nearest IGP neighbor (lowest IGP metric) is preferred.
10. EBGP direction - Oldest (longest regarded) direction is desired.
Neighbor Router ID - Lowest is desired.
Neighbor IP cope with - Lowest is favored.
Question 12. Explain Bgp Weight Attribute?
The weight attribute is a Cisco proprietary characteristic this is used in the course choice method when there's multiple direction to the same destination. A route with the Higher weight cost is preferred. The default value for weight is 0. The weight characteristic is local to the router and isn't propagated to any BGP friends. Weight characteristic is ready with the aid of a router when receiving Updates influencing that one router’s route for a prefix.
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Question thirteen. Explain Bgp Local Preference?
Local choice is an indication to the AS approximately which direction has preference to exit the AS as a way to reach a sure community. A course with a better local desire is desired extra. By default price for local desire is a hundred and can be changed manually. Unlike the load attribute, that is simplest relevant to the local router, nearby preference attribute is communicated during a unmarried AS for the motive of influencing the choice of quality course to go out the AS.
Question 14. Explain Bgp Med?
The motive of MED is to persuade how other independent systems enters into your AS to attain a sure prefix. BGP MED is an characteristic which isn't always propagated at some stage in the entire community however just to adjacent AS. The lower the MED the extra the path will be preferred.
Question 15. What Is Recursive Lookup?
The router seems up the BGP route and the following hop to attain a destination within the far flung AS. Then the router seems up the route to reach the next hop. In this way router has to carry out lookup two times to attain to a vacation spot, this method is called recursive lookup.
Question sixteen. What Is Route Reflector And Why It Is Required?
In BGP, route learned from an iBGP neighbor will now not be advertised to any other iBGP neighbor. To triumph over this situation course reflector is used. It acts as a course reflector server and makes IBGP associates as path reflector clients enabling path commercials among them.
Question 17. What Is The Difference Between Local Preference And Med?
The Local Preference attribute is to influence your personal AS a way to get or exit to every other AS. MED is to persuade different AS how to input your personal AS.
Question 18. What Is The Command To Administratively Disable Bgp Neighborship?
# neighbor neighbor-ip shutdown
# no neighbor neighbor-ip shutdown (to allow it once more)
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Question 19. Types Of Bgp Routing Table?
Till now all of us trust that BGP is having best a unmarried routing table wherein it used to store the routes and method for the best route calculation. But we all are mistaken here clearly BGP keeps 3 table one for storing incoming routes from neighbours, one for sending the routes to neighbours and one for putting in the routes wherein you absolutely discover the routes with next-hop cope with. The tables are given below:-
Adj-RIB-in stores the unprocessed records acquired from its peers. Here the best route selection occurs as according to BGP attributes and after conformation path is entered into the neighborhood bgp desk i.E Loc-RIB. From the local RIB desk it conform the following-hop address if it handy by means of IGP then the course is entered into the principle routing desk.
Question 20. What Do You Mean By Mpls? What Are The Benefits Of Using Mpls?
This is one of the most not unusual questions requested at some stage in the MPLS interviews, so this question should be organized for before going through the interview. Here you need to begin answering via introducing MPLS.
Multiprotocol label switching (MPLS) provide a mechanism to procedure packets on the basis of labels. It is a records forwarding service that offers excessive-stop network communications from one community node to the next on the premise of labels rather than routing desk lookups. Routing desk research for every packet makes communication slow however MPLS makes use of LFIB/forwarding desk to technique label from one node to every other.
The main blessings of the usage of MPLS are:
QoS integration: helps multiple ranges of QoS, to control latency, jitter and packet loss for diverse styles of visitors (voice, video, e mail, bulk record transfers, and so forth.)
Fast convergence: label-based totally switching, no need to appearance up routing tables
Scalability to IP VPNs
Low community congestion the usage of MPLS TE
Reliable and secure
Question 21. Describe Push, Swap, Pop Functions In Mpls And Also Explain Penultimate Hop Popping (Hypertext Preprocessor)?
Label switching is a discrete mechanism from IP forwarding; right here LSR regulates what label function needs to be carried out, such as Push, Pop, or Swap, to system a packet during the MPLS spine the usage of label switching.
Push: Add a label
Pop: Remove a label
Swap: Exchange/ update a label
Question 22. What Is Any Transport Over Mpls (atom)? Do We Require Vrf Or Bgp-vpn To Form Atom Connectivity?
Any Transport over MPLS (AToM) allows carrier carriers to supply Layer 2 connectivity over a MPLS backbone to connect distinctive customers’ sites with current facts hyperlink layer (Layer 2) networks. BGP-VPN or VRF isn't always required to configure AToM; to form AToM connectivity we do use xconnect (move join) peering with particular VC-ID to differentiate more than one customers.
Question 23. Describe The Roles Of Rd And Rt In Mpls L3vpn? Is It Possible To Assign A Same Rd To Multiple Customers?
Route distinguisher (RD) is used to differentiate same routes in distinctive VRFs, whereas route target (RT) is used to share routes among them. Route target additionally specifies which prefixes get imported or exported on the PE router.
No, we are able to’t assign identical RD to a couple of clients on a PE router because the principle reason of defining RD is limited to offer distinctiveness in exclusive VRFs.
Question 24. How Can You Differentiate Vpnv4 And Ipv4 Address-own family?
Address-circle of relatives VPNv4 is used to transport VRF site visitors, that is pretty heavier (96 bits) than normal IPv4 packets, and cope with-family IPv4 is used to move conventional 32-bit IPv4 packets. When we installation L3VPN, we need VPNv4 peering among the PEs to take the VRF routes (labeled with RD, RT) to the corresponding destination PE router.
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Question 25. What Are Lib And Lfib? If We Disable Cef (on A Cisco Device) Will Mpls Work?
The difference among two is almost like RIB and FIB, as RIB makes use of FIB to ahead traffic. Label Information Base (LIB) is a desk in which prefix to label bindings are constructed and saved inside the control aircraft and may be seen with “display mpls ldp bindings.” Label Forwarding Information Base (LFIB) is another MPLS table used to ahead label packets at some point of the MPLS network and construct within the information aircraft. To take a look at LFIB, run “show mpls forwarding-table.” The lookups are in truth carried out within the LFIB, no longer the LIB.
If CEF is disabled, MPLS will not work, as it makes use of LFIB to forward categorised packets and LFIB constructed from the FIB table.
Question 26. Name Bgp Path Attributes To Control Incoming And Outgoing Traffic?
BGP has more than one route attributes to influencing site visitors float. In the maximum not unusual exercise we do control incoming site visitors the usage of AS-Path prepending and MED (multi-go out discriminator). And, to influence outgoing site visitors, weight and neighborhood desire are the satisfactory selections.
Question 27. My Bgp Is Showing zero.0.0.0 As Router-id; What Could Be The Possible Reason?
This happens whilst we configure BGP on a router that has non-IP interfaces, which means that if a router’s interfaces aren't configured with any IP deal with, then BGP can't get the router-identity. In that case, BGP assumes 0.0.0.0 as router-id.
Question 28. If My Bgp Neighbor Is Stuck In Idle Or Active State, What Should I Do?
If BGP peer is in idle country, then it may be due to physical connectivity failure or the neighbor isn't described properly with respective AS. In connect nation, BGP tries to establish a TCP consultation over port wide variety 179; if it fails to set up the connection, then it goes to energetic kingdom, where it attempts again to establish a TCP connection. By the use of “debug ip bgp” and “debug ip tcp transactions” instructions, we are able to research the precise reason of TCP connection failure. Lack of the “replace-supply” or “ebgp-multihop” command also can be a cause of neighbor lively nation.
Question 29. Explain Bgp Recursive Lookup And Site Of Origin (soo)?
BGP recursive lookup includes the router searching up the BGP direction and BGP subsequent hop to reach a vacation spot within the far flung AS. Then the router will appearance up the course into IGP to attain the BGP next hop. This mechanism is known as BGP recursive research.
BGP web site of origin (SoO) is a loop prevention method in the MPLS spine environment. BGP SoO is related to a CE-neighbor; that’s why whenever the PE router advertise a prefix to a CE neighbor, it test the SoO of the prefix with the SoO of the BGP neighbor to keep away from loops.
Question 30. What Do You Understand By Bgp Split-horizon Rule?
This rule states, “Network prefixes found out from an iBGP peer will not be shared with every other iBGP peer” to save you routing loops. BGP makes use of the AS_PATH attribute to keep away from routing loops, this means that a route will no longer be ordinary if the AS_PATH includes the equal AS range of the router where it's miles receiving. Route-reflector and confederation techniques are used to share iBGP learned networks with some other iBGP neighbor.
Question 31. Describe Bgp Communities. Name Well-recognised Communities?
BGP groups are usually used to govern course regulations inside the BGP surroundings and used as flags in an effort to mark it over the set of community prefixes. BGP groups’ information may be shared with a neighbor through the usage of the command “neighbor x.X.X.X ship-network trendy prolonged both” and may be applied the usage of path-maps to a described neighbor. Some famous BGP groups are:
Local-AS: Share the prefixes with iBGP-confederation friends simplest.
No-Advertise: Do now not percentage the prefixes with absolutely everyone.
No-Export: Do no longer proportion the prefixes with another Global AS, proportion inside the Public-AS handiest.
Internet: Share the prefixes with all.