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10 Interview questions on BGP Routing Protocol

November 04, 2021 ,

Interview questions on BGP Routing Protocol

BGP routing protocol is one of the most important and widely used protocol in today's world and it is used when you want to connect two different organizations.

Let's start with the first part of interview questions in BGP ( Border Gateway Protocol). we will come up with another round of questions in BGP.

Fig 1.1- BGP Routing Protocol Sample Topology

Q1: What is BGP and the purpose of bgp ?
BGP States as Border Gateway Protocol and is used to exchange routing between two different autonomous systems (AS). The Border Gateway Protocol makes routing decisions based on paths, network policies, or rules configured

Q2: What is the Port Number of BGP ?
BGP is an application layer protocol and uses TCP port 179

Q3: What is the BGP path selection criteria ?
BGP path selected on the base of the below process

  • Choose the route with the highest weight, in case you have Cisco router.
  • If weight is not set, choose the route with the highest local preference
  • Choose routes that this router originated
  • Choose the path with the shortest Autonomous System path
  • Choose the path with the lowest origin code (i is lowest, e is next, ? is last).
  • Choose the route with the lowest MED, if the same Autonomous System advertises the possible routes
  • Choose an EBGP route over an IBGP route
  • Choose the route through the nearest IGP neighbor as determined by the lowest IGP metric.
  • Choose the oldest route

Q4: What is recursive lookup in BGP and how it works ?
The router looks up the BGP route and the BGP next hop to reach a destination in the remote AS. Then the router looks up the route to reach the BGP next hop using the IGP.

Q5: Define various types of communities and why they are used ?
There are four well-known communities that can be referenced by name

  • No-export: prevents the route from being advertised outside the local AS to eBGP peers.
  • No-advertise: prevents the route from being advertised to either internal or external peers
  • Internet: allows the route to be advertised outside the local AS.
  • Local-AS: prevents the route from being advertised outside the local AS to either eBGP or confederate peers

Q6: What is the purpose of route dampening ?
Route dampening minimizes the impact of route flaps in downstream autonomous systems upon local and upstream autonomous systems.

Q7:Do iBGP sessions modify the next hop?
iBGP sessions preserve the next hop attribute learned from eBGP peers. This is why it is important to have an internal route to the next hop. The BGP route is otherwise unreachable. In order to make sure you can reach the eBGP next hop, include the network that the next hop belongs to in the IGP or issue the next-hop-self neighbor command to force the router to advertise itself, rather than the external peer, as the next hop.

Q8: What additional command do you have to use to establish intra confederation EBGP sessions between loopback interfaces?
To establish intra confederation EBGP sessions between loopback interfaces, you must specify ebgp-multihop on the intra confederation EBGP neighbor.

Q9: What is the difference between split horizon and poison reverse?
Split-horizon routing with poison reverse is a variant of split-horizon route advertising in which a router actively advertises routes as unreachable over the interface over which they were learned by setting the route metric to infinite 

Q10: What is route reflector and why it is required?
A route reflector is BGP router that is allowed to break the iBGP loop avoidance rule. Route reflectors can advertise updates received from an iBGP peer to another iBGP peer under specific conditions.

By breaking the rules, route reflectors are used to eliminate the full mesh requirement and allow for building iBGP networks that scale easily and cleanly.