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Top 25+ Most Asked BGP Interview Questions and Answers (2022)

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Top 25+ Most Asked BGP Interview Questions and Answers

Following is a list of BGP interview questions and answers.

1) What is BGP? What is the full form of BGP?

BGP stands for Border Gateway Protocol. It is a standardized exterior gateway protocol designed to exchange routing and provide information among autonomous systems (AS) on the Internet. BGP is mainly used for routing within an autonomous system. In this application, it is called Interior Border Gateway Protocol, Internal BGP, or iBGP. As the other use, the internet application of this protocol is called Exterior Border Gateway Protocol, External BGP, or eBGP.

We can classify the Border Gateway Protocol as a path vector protocol. It makes routing decisions according to paths, network policies, or rule sets configured by a network administrator. It is also involved in making core routing decisions. For example, when two ISPs connect, they typically use BGP to exchange routing information.

2) Can Routers on different subnets become BGP neighbours?

BGP is usually configured between two directly connected routers that belong to different autonomous systems. BGP routers do not need neighbours to be attached to the same subnet. Instead, they use a TCP connection between the routers to pass BGP messages and allowing neighbouring routers to be on the same or different subnet.

3) What is the main purpose of BGP?

The main purpose of BGP is to exchange routing updates like other routing protocols, but generally, it does not exchange individual network routes. It exchanges summaries of network routes. That’s why it is typically used over very large networks, including the Internet.

4) What are some basic characteristics of BGP?

BGP is a path-vector protocol or an exterior gateway protocol (EGP) used in routing on the Internet. Following are the most common characteristics of BGP:

  • BGP uses TCP port 179 to establish connections with neighbours and transfer data to ensure reliable protocol updates delivery.
  • It sends updates only when changes occur in the network (no periodic updates).
  • It periodically sends keepalive messages to verify TCP connections.
  • The metric of the protocol is called path-vector or attributes.
  • BGP uses several attributes in the routing-decision algorithm.
  • BGP is an interdomain routing protocol.
  • BGP is a path-vector routing protocol that is best suited for strategic routing policies.
  • It uses confederations and route reflectors to reduce BGP peering overhead.
  • BGPv4 implements CIDR.
  • eBGP is used for external neighbours between different autonomous systems.
  • iBGP is used for internal neighbours within the same autonomous system.
  • It uses MED (metric) attributes between autonomous systems to influence inbound traffic.
  • It uses weight to influence the path of outbound traffic from a single router configured locally.

5) What TCP port number does BGP use for connection?

BGP uses TCP port 179 for the connection.

6) What is the main difference between eBGP and iBGP neighbours?

The eBGP is used for external neighbours between different autonomous systems. On the other hand, iBGP is used for internal neighbours within the same autonomous system.

7) Is it possible to run two BGP processes on a single router?

No, it is not possible to run two BGP processes on a single router.

8) Can we use BGP instead of any IGP?

No, we cannot use BGP instead of any IGP because BGP works between different autonomous systems, but IGP works inside an autonomous system.

9) How can we check the BGP status?

There are two ways to check BGP status:

  • To view the BGP configuration, enter show config in CONFIGURATION ROUTER BGP mode.
  • To view the BGP status, you should use the show IP BGP summary command in EXEC Privilege mode.

10) What do you understand by peers in BGP?

In BGP, any two routers that formed a TCP connection to exchange BGP routing information are called peers. Peers are also called BGP neighbours.

11) What do you understand by poison reverse in BGP?

In a computer network that uses the Routing Information Protocol (RIP) or other distance vector routing protocols, poison reverse is how a gateway node tells its neighbour gateways that one gateway is unreachable over the interface or no longer connected.

12) What are some BGP timers?

There are mainly two BGP timers:

  • Keepalive Interval: It specifies the time in seconds between sending keepalive messages. The default keepalive timer is 60 seconds.
  • Hold Time: It specifies the interval time in seconds, after which the neighbour will be considered unavailable. The default hold time is 180 seconds.

13) What do you understand by loop prevention mechanism in BGP?

There are two types of loop prevention mechanisms in BGP:

  • When an iBGP peer specifies routes for a router, that router does not advertise the same routes to another iBGP peer.
  • By using AS_PATH: When we advertise to an eBGP peer, a BGP router adds its own ASN to the AS_PATH. If a BGP router receives an update and the route advertisement lists an AS_PATH with its ASN, it ignores that route.

Note: A BGP router does not add its ASN when advertising to an iBGP peer.

14) What is the difference between Hard Reset and Soft Reset in BGP?

In BGP terminology, the administrators are required to reset BGP peers. There are two ways to reset a peer:

  • Hard Reset
  • Soft Reset

Let’s see the differences between Hard Reset and Soft Reset in a tabular form:

Comparison Parameter Hard Reset Soft Reset
Primary Goal Hard Reset kills the TCP session with the BGP neighbour forcing it to restart. It kills the TCP connection between the peers, re-establishes the connection with a BGP open message, and proceeds with the regular message exchanges between peers. A soft reset is of 2 types:
1. Soft reset with Route refresh: Soft reset with Route refresh allows BGP peers to request an update without killing the entire neighbour relationship.
2. Soft-reconfiguration: Soft-reconfiguration stores a copy of BGP routes delivered in the BGP update from our peer because the route-refresh request is not sent to the BGP neighbour.
Direction Both In and Out Either In or Out
Impact The hard reset can severely disrupt the reachability of all networks. The soft reset can impact only those prefixes that are affected by the policy change.
Usage Hard Reset should be used only as a last option when soft reset cannot address the issue. Soft is the most preferred approach, and it is mostly used.
Memory usage It doesn’t require any additional memory requirement for storing another table. It requires additional memory since the router now stores another table for each neighbour instead of one BGP table.
Prerequisite It doesn’t require any prerequisites. Both peers may be required to support the Route Refresh capability in soft reset with the Route refresh feature.
Convergence time It requires a very long convergence time since the processing of the full table takes a long time. Its convergence time is much shorter than a hard reset.
Related commands clear ip bgp {* | neighbor ip | peer-group} clear ip bgp {neighbor ip} soft out
clear ip bgp {neighbor ip} soft in

15) What are BGP Message types? What are the different BGP Message types?

BGP Message types are used to establish a neighbour relationship and exchange parameters, including autonomous system number and authentication values. Following is a list of some BGP Message types:

  • Open: The open message type creates a neighbour relationship and exchanges BGP parameters, including AS number and authentication values.
  • Keepalive: The Keepalive BGP message type is sent periodically to maintain the neighbour relationship. If the Keepalive message is not received within the negotiated Hold Time, the BGP neighborship is turned down.
  • Update: This message type is used to exchange the path attributes and the associated prefix/length that uses those attributes.
  • Notification: This message type is used to report BGP errors. Finally, it resets the neighbour relationship.

16) What are some well known BGP metric’s attributes?

BGP path selection depends on the values of the attributes. Following is a list of some well known BGP metric’s attributes:

  • Weight (Cisco proprietary attributes)
  • Local Preference (highest local value will be preferred, the default value is 100)
  • Originate
  • AS path length
  • Origin code
  • MED
  • eBGP path over iBGP path
  • Shortest IGP path to BGP next-hop
  • Oldest path
  • Router ID
  • Neighbor IP address

17) What do you understand by RIP in networking?

RIP stands for Routing Information Protocol. It is one of the oldest distance-vector routing protocols used to employ the hop count as a routing metric. RIP also prevents the routing loops by limiting the number of hops allowed in a path from source to destination.

18) What is the purpose of BGP MED?

The main purpose of BGPMED is to influence how other autonomous systems enter into your autonomous system to reach a certain prefix. BGP MED is an attribute that is not propagated throughout the whole network but just to adjacent AS. The lower the MED, the more the path will be preferred.

19) What are the different BGP Path Attributes?

BGP supports several Path Attributes varieties and uses these path Attributes to check the competing BGP paths (routes) in the BGP table to find out the best possible path (route).

Following is a list of some BGP Path Attributes:

  • Next Hop: The Next Hop Path Attributes is used to list the next-hop IP address to reach a prefix. It checks if the Next hop is reachable. If no route is able to reach Next Hop, the router does not use this route.
  • Weight: The weight Path Attributes is a numeric value set by a router when you receive the updates to influence the route for a prefix. It is not advertised to any BGP peers, and a bigger weight is preferred.
  • Local Preference: This is also a numeric value set. It is communicated within a single autonomous system to choose the best route for all routers in that autonomous system to reach a certain network. The bigger value is preferred.
  • Locally injected routes: Locally injected routes are the routes injected using the network command. These are better than iBGP/eBGP.
  • Multi-Exit Discriminator (MED) allows an autonomous system to tell the neighbouring autonomous system the best path to forward packets. Smaller is preferred.
  • AS Path: It specifies the number of ASNs in the AS Path. Smaller is preferred.

20) What is the order of preference in BGP?

In BGP, the order of preference varies on whether the attributes are applied for inbound updates or outbound updates.

For inbound updates, the order of preference is:

  • Route-map
  • Filter-list
  • Prefix-list
  • distribute-list

For outbound updates, the order of preference is:

  • Filter-list
  • Route-map | unsuppress-map
  • Advertise-map (conditional-advertisement)
  • Prefix-list
  • distribute-list

21) What are the various types of communities used in BGP? Why are they used?

There are four well-known communities used in BGP. The following list specifies their usage:

  • No-export: It is used to prevent the route from being advertised outside the local autonomous system to eBGP peers.
  • No-advertise: It is used to prevent the route from being advertised to either internal or external peers.
  • Internet: It is used to allow the route to be advertised outside the local autonomous system.
  • Local-AS: It is used to prevent the route from being advertised outside the local autonomous system to either eBGP or confederate peers.

22) Does BGP support authentication?

Yes. BGP supports MD5 authentication.

23) Does BGP require full mesh?

There are two types of BGP:

  • eBGP (External Border Gateway Protocol): eBGP does not require a Full Mesh topology.
  • iBGP (Internal Border Gateway Protocol): iBGP requires a Full Mesh topology

24) What do you understand by a route reflector? Why is it required?

A route reflector is a BGP router that can break the iBGP loop avoidance rule. A route reflector can advertise updates received from an iBGP peer to another iBGP peer under specific conditions. Route reflectors are used to eliminate the full mesh requirement by breaking the rule and building iBGP networks that scale easily and cleanly.

25) What is the Admin Distance (AD) of eBGP & iBGP?

Admin Distance of eBGP is 20, and Admin Distance of iBGP is 200.

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