BGP is a protocol that allows autonomous systems on the Internet to share routing and reachability information. This post covers the frequently asked BGP Interview Questions and Answers in detail for basic, intermediate, and advanced BGP professionals.
The Internet's global routing system is based on the BGP (Border Gateway Protocol) protocol. It manages how packets are routed from network to network by exchanging routing and reachability information among edge routers.
We have categorized BGP Interview Questions - 2023 (Updated) into 2 levels they are:
Frequently requested BGP questions, such as scenario-based BGP interview questions and BGP troubleshooting interview questions and answers, have been addressed in this blog.
Border Gateway Protocol (BGP) is a protocol that allows two networks to communicate with one other. BGP is a standardized outside gateway protocol that allows autonomous systems (AS) on the Internet to share routing and reachability information. A route vector protocol is what the protocol is called. The Border Gateway Protocol is engaged in fundamental routing choices and determines routing decisions depending on pathways, network regulations, or rule sets a network administrator provides.
Within an autonomous system, BGP may be utilized for routing. Interior Border Gateway Protocol, often known as Internal BGP or iBGP, is used in this application. The protocol's Internet implementation, on the other hand, is known as Exterior Border Gateway Protocol, External BGP, or eBGP.
A strategy used by distance vector protocols to prevent network routing loops is the split horizon. The core idea is simple: routing information should never be sent back in the same direction it came from.
A split-horizon is essential because distance vector protocols like Routing Information Protocol (RIP) are prone to routing loops, which occur when a data packet is locked in an unending loop and routed via the same routers over and again. Split horizon is a technique for avoiding loops in protocols. Other protocols, such as Open Shortest Path First, employ various ways to eliminate packet looping. When split horizon is enabled, a router cannot advertise a route back to learn. To put it another way, if a router gets routing information from another router, the first router will not broadcast it back, eliminating routing loops.
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The Poison Reverse method is a distance-vector routing technique that is commonly utilised. Poison reverse is used to solve the count-to-infinity issue. Poison reverse is the inverse of the split horizon, to put it another way. Route advertising that would be silenced by split horizon is instead advertised with poison reverse at a distance of infinity. Poison reverse is based on the RIP (Routing Information Protocol).
Routers do not immediately erase erroneous path information from the routing database; instead, they broadcast a hop-count of 16, an unreachable metric value. This enhances the routing table's size while also assisting in the reduction of loops. It can immediately break any loop between neighbouring routers. Poison reverse's primary goal is to prevent a path from returning to the same node if the network's cost has changed.
The following are the many types of Timers found in BGP:
Keep-Alive Timer: This is the heartbeat timer. A local neighbour periodically sends a BGP heartbeat packet to a distant neighbour to monitor reachability and availability. This interval is set to '30' seconds by default.
Hold down timer: This is the amount of time the local neighbour must wait before declaring the far neighbour unavailable. This interval is set at "90" seconds by default, three times the Keep-Alive Interval.
In other words, if a local neighbour misses three Keep-Alive packets in a row from a remote neighbour, the local neighbour considers the remote neighbour unavailable and changes the neighbourship's status and removes all associated routes advertised by the neighbour from the routing table/BGP table—the BGP neighbour status changes before and after the hold-down timeout end. By default, the hold-down period is set to 90 seconds, after which the local neighbour proceeds through stages such as 'idle,' 'connect,' and 'functional.' After a few seconds, the status changes to 'Idle,' then to 'Connect,' then to 'Active,' after 10 seconds.
The Advertisement Interval timer is a timer that defines how much time must pass between the time a route is advertised and the time it is deleted from a BGP Peer. The default is 30 seconds for eBGP peers and 5 seconds for iBGP peers. On a per-neighbour basis, this can be changed.
The values of the following characteristics are used to choose BGP paths:
BGP provides several Path Attributes that may be used to evaluate competing BGP paths (routes) in the BGP table to find the optimum path (route).
Some BGP Path Attributes are as follows:
A BGP community is an optional transitive BGP attribute recognised and communicated between BGP peers. A BGP community is added to BGP routes sent between two BGP peers.
A community is made up of two 16-bit portions of a 32-bit integer. The first 16 bits contain the AS number for the community, while the next 16 bits constitute a unique number assigned by the AS. Because each AS number is unique, each online community is also unique. This means that an AS with the ASN 9999 (or 0x270F in hex) might have communities ranging from 0x270F0000 to 0x270FFFFF.
BGP aims to minimise the number of potential pathways to only one optimal path; by default, it does not load balance. To do so, it examines the path attributes of every loop-free, synchronised (if synchronisation is enabled) route with an available next-hop:
The following are the various loop avoidance strategies used in BGP:
Other four well-known communities can be mentioned by name:
The BGP ConnectRetry timeout is set to 120 seconds by default. The BGP procedure checks to determine if the passive TCP session is formed only after this time has passed. If the passive TCP session fails, the BGP process initiates a new active TCP connection attempt with the distant BGP speaker. The remote BGP peer can establish a BGP session during the ConnectRetry timer's idle 120 seconds. The Cisco IOS ConnectRetry timeout cannot be adjusted from its default setting of 120 seconds.
|Border Gateway Protocol (BGP)||Open Shortest Path First (OSPF)|
|The BGP protocol is a sluggish concurrency protocol.||OSPF is a short concurrency protocol.|
|It employs a mesh topology or design in some way.||OSPF is a hierarchical network architecture or design.|
|An external gateway protocol is what it's called.||Another term for it is internal gateway protocol.|
|BGP implementation is complex.||OSPF is a simple protocol to implement|
|It uses port 179 for communication.||It establishes a connection with port 89.|
|In this case, the Transmission Control Protocol is used.||OSPF makes use of the internet protocol.|
|BGP is a kind of Vector State.||OSPF is a Link State protocol.|
|In this scenario, the best route algorithm is used.||OSPF makes use of the Dijkstra algorithm.|
The amount of Memory needed to hold BGP routes is determined by various parameters, including the router, the number of alternate pathways accessible, route dampening, community, the number of full paths defined, BGP characteristics, and VPN configurations. It's impossible to estimate the amount of RAM needed to keep a specific number of BGP routes without knowing these factors.
To store a whole global BGP routing table from one BGP peer, Cisco typically recommends a router with at least 512 MB of RAM. However, it's critical to comprehend how to save Memory and accomplish effective routing without acquiring the entire Internet routing table.
The next-hop property learnt from eBGP peers is preserved in iBGP sessions. This is why having an internal route to the next hop is critical. Otherwise, the BGP route is inaccessible. Include the network that the next hop belongs to in the IGP or use the next-hop-self neighbor command to compel the router to promote itself as the next hop rather than the external peer.
Administrative Distance (AD) is a metric used by routers to determine the optimum path.
When there are two or more distinct routes to the same destination from two different routing protocols, it assists the router in selecting the optimum way.
The AD value is used to rate routes in Preference, from most desired to least liked.
The AD value for the most-preferred route is the lowest, while the AD value for the least-preferred route is the highest.
OSPF uses link State Advertisement (LSA) to communicate for the Internet Protocol (IP). OSPF makes use of a Link State Database (LSDB), which is filled with LSA.
LSAs in OSPF come in a variety of shapes and sizes. Here are several examples:
This LSA contains a list of all of this router's directly linked connections. These routers are always kept inside the confines of the region.
This LSA is designed for use in multi-access networks. DR creates network LSAs.
The ABR is in charge of creating the summary LSA. As a result, ABR will provide a summary of ASBR LSA. It will be made up of the ASBR's router ID.
The ASBR LSA contains the ASBR's router ID in the link-state routing field. It facilitates the discovery of ASBR by other routers.
The external ASBR is responsible for generating these.
It is neither used nor supported.
It's also known as NSSA LSA (not-so-stubby LSA). External Type 5 LSAs are not permitted in NSSAs. You can allow them to use LSA Type 7.
A network in the BGP table with the following hop address of 0.0.0.0 indicates that the network was created locally through IGP redistribution into BGP or by a network or aggregation command in the BGP setup.
If your AS is responsible for routing traffic from another AS to a third AS, BGP should not announce a route until all routers in your AS have learned about it through IGP. BGP waits for the route to propagate within the AS before advertising it to external peers. If a BGP router with synchronization enabled cannot validate iBGP learnt routes in its IGP, it does not install them into its routing table.
To disable synchronization, issue the no synchronization command under router BGP. BGP cannot validate iBGP routes in IGP due to this.
It's used to maintain track of BGP neighbors and detect inactive neighbors. Only the packet header appears in keepalive messages (19 octets in length). Keepalive messages are not delivered when the frequency of sending them is set to 0.
The acronym DUAL refers to the Diffusing Update Algorithm. EIGRP uses the DUAL to determine the optimal routes to a destination. It allows for classless routing. As a result, subnet mask information will be included in EIGRP routing updates. This enables discontinuous networks and variable-length subnet masks (VLSM).
Exterior Gateway Protocol (EGP) stands for "Exterior Gateway Protocol." It's one of the protocols for exchanging data between gateway hosts in autonomous systems close to one another.
EIGRP stands for Enhanced Interior Gateway Routing Protocol and is a distance vector routing protocol for IP, AppleTalk, and NetWare networks.
Autonomous systems may be divided into three categories:
The goal of MED is to control how other autonomous systems enter your AS to reach a specific prefix. BGP MED is a network attribute that is only propagated to adjacent ASs rather than the entire network. The lower the MED, the more likely the path is to be chosen.
When there are many routes to the same destination, the weight attribute is a Cisco proprietary attribute employed in the path selection process. It is preferable to take the path with the more excellent weight value. Weight is set to 0 by default. Weight is a router-specific property that is not propagated to BGP peers. When a router receives updates, it sets the weight property, influencing its prefix route.
To reach a location in the distant AS, the router searches for the BGP route and the next hop. The router then searches up the path to the next hop. The technique is called recursive lookup since the router must execute lookup twice to reach a destination.
Local preference informs the AS which path prefers exiting the AS to access a particular network. A path with a stronger liking for the local environment is desired more. Local preference is set to 100 by default and may be modified manually. Unlike the weight parameter, which is only relevant to local routers, the local preference attribute is conveyed throughout an AS to determine the optimum exit path decision.
We've addressed the most frequently asked interview questions about Border Gateway Protocol in this post (BGP). We hope the information will be more helpful for clearing BGP interviews.
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Madhuri is a Senior Content Creator at MindMajix. She has written about a range of different topics on various technologies, which include, Splunk, Tensorflow, Selenium, and CEH. She spends most of her time researching on technology, and startups. Connect with her via LinkedIn and Twitter .
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