BGP INTERVIEW QUESTIONS & ANSWERS

 

 

Question 1 - Tell us something about BGP ?

Answer -

    Biggest Routing Protocol
    Most Complex Routing Protocol
    Need to have lot of Patience
    Can’t compare with IGPs – RIP ,EIGRP,OSPF etc.
    BGP – FLOW CONTROL
    Route Manipulation
    Can Handle Huge Routing Table
    A.K.A ROUTING PROTOCOL OF INTERNET
    Exterior Gateway Protocol (EGP) – Runs between Autonomous System
    Uses TCP Port 179


Question 2 -What is an Autonomous System ?

Answer -

An Autonomous System (AS) is a group of networks under a single administrative control.
An AS can be Internet Service Provider (ISP) or a large Enterprise Organization.

Each AS will have an ASN

Autonomous System Number (ASN) - 16 bit binary number & we can represent (2 16) numbers, which is equal to 65536 in decimals.
Range of ASN – 0 to 65535
0 , 65535 - Reserved
1 – 64511 - Internet Routing – ISP AS
64512 – 65534 Private Use - Private AS

Question 3 - What is the difference between iBGP & eBGP ?

Answer -

eBGP -  External Border Gateway Protocol

It runs between two BGP routers in different autonomous system.

iBGP -  Internal Border Gateway Protocol

It runs between two BGP routers in the same autonomous system

The AD Values
EBGP = 20
IBGP = 200

Question 4 - Why BGP is known as SLOWEST Routing Protocol ?

Answer-

Convergence is Very Slow

Timers – Hello – 60 Secs and Dead – 180 Secs

Question 5 -What is BGP Router Identifier (BGP RID) ?

Answer -

It is the highest loopback IP address and in absence of loopback interface/s it will be the highest IP address of an active physical interface.

BGP Router must have BGP RID to establish BGP Peer session.

BGP router id can be also manually configured using bgp router-id command .

But once it is configured the bgp session will be reset.

Question 6 - What are Keepalive Messages in BGP ?

Answer -

Once the BGP Neighorship is up and running ,these Keepalive messages are send continuously between BGP Speaker in a
specific interval which is known as Keepalive Interval or Keepalive Time.

Keepalive messages are send between BGP Speakers to verify whether peering BGP Neighbor is live or not .

By default Keepalive Time is 60 Seconds.

Question 7 - How to make a BGP Router member of two Autonomous Systems ?

Answer -

No we can't make it !

Only one BGP AS can be configured on the device.

Hence BGP Router can be member of only one Autonomous System .


Question 8 -What is " BGP SYNCHRONIZATION RULE "

Answer -

 "Don't use or advertise the route/s learned via an iBGP neighbor to an eBGP neighbor unless &
until the same is/are learned via some other IGP like RIP , OSPF ,EIGRP etc . "


This feature is off by default in Cisco IOS Software release 12.2(8)T and later.

To disable BGP Synchronization

R1(config)#router bgp 100                    -------> AS is 100
R1(config-router)#no synchronization

To enable BGP Synchronization

R1(config-router)#synchronization

Question 9 - On a BGP Router how can we check the list of BGP Neighbor/s and the routes received and advertised from/to a particular neighbor ?

Answer -

sh ip bgp summary
sh ip bgp neigbhor x.x.x.x received-routes
sh ip bgp neighbor x.x.x.x adverstied routes

Question 10 - Tell us something about BGP Attributes ?

Answer -

As we are aware that each routing protocol uses some metric for path selection
e.g. RIP uses Hop count , EIGPR uses a composite metric which includes Bandwidth and Delay (by default) ,
OSPF uses Cost i.e Bandwith to calculate the best route.

Similarly BGP also uses something known as "Attributes" to the find the best route to reach a destination.

 There are 4 categories of BGP Attributes :-

1) Well Known Mandatory -  

    Supported (Known/Understand) by all BGP Routers
    Must be present in BGP Update Message.
    AS Path, Origin ,Next Hop.


2) Well Known Discretionary -

    Supported (Known/Understand) by all BGP Routers.
    May or May not be present in BGP Update Message.
    Local Preference


3) Optional Transitive -

    May or May not be supported  a BGP Routers.
    Passed on the other BGP Routers.
    Aggregator , Community.

4) Optional Non-Transitive-

    May or May not be supported by a BGP Routers.
    If not supported may ignore the update and may not pass on to the other BGP Routers.
    MED ,Router Cluster list , Router Originator List.

Thanks for Reading 

amartechstuff