BGP - [Part 3] - BASIC INTERVIEW QUESTIONS

Question - 1 What are the Single Homed, Dual Homed, Single Multi-homed and Dual Multi-Homed ?

• Single Homed - An organization is connected to single ISP using a single links.

• Dual Homed - An organization is connected to single ISP using a dual links.

• Single Multi-homed - An organization is connected to two ISP using a single links.

• Dual Multi-Homed - An organization is connected to two ISP using a dual links.

Question - 2 When can we use BGP ?

• AS working as transit AS (Example- ISP)

Example 1 - See the below diagram, you can see here are multiple customers that belongs to different AS and connected with same ISP, So we will have to configure BGP on ISP, because there are requirement of path manipulation due to multiple customer and each customer belong to different AS, BGP allow the path manipulation and allow the communication between different AS.

Example 2 - See the second diagram, In this example, ISP is going to maintain, where is your yahoo server and routes of the yahoo server, yahoo server is somewhere , it may be different location or different company or organization and User is also belong to different location ,so user and yahoo server both are belong to different different AS,  so ISP will have to maintain different AS, so here we will have to use BGP

• When the AS is multi-homed

Example 1 -  Now continue with second example, In this scenario Organization connected with two different ISP, and there are two exit path , we are using two ISPs for redundancy. In this scenario we can also use default route to access internet and static route to ISP from ISP and IGP in Organization, but  , but note that only BGP provides path manipulation means if organization have network 10.0.0.0, 20.0.0.0, 30.0.0.0 and 40.0.0.0 and you want to apply condition ,  10.0.0.0 and 20.0.0.0 should use ISP-1 and 30.0.0.0 and 40.0.0.0 should use  ISP-2 so you will have to use BGP for path manipulation

Question - 3 When, we should not use BGP ?

• If it is Single-home AS

Example 1 - Check the below example , if an organization which have planety of routers and connected to an ISP and ISP connected to internet, you can see that in this scenario only single connection to ISP so here is no any requirement of BGP. You can configure default route to access internet, Static route to ISP and from ISP. You can use IGP within organization. This type scenario is known as single homing which have single exit path. So in this case no requirement of BGP

• Lack of resources like memory and less processing power in routers.
• Lack of knowledge about BGP route filtering and path selection process.

Question - 4 BGP Path selection criteria ?

BGP path selection priorities are given below in sequence.

1. Route with the highest weight.
2. Route with the highest local preference.
3. Routes which is originated from this router.
4. Route with the shortest Autonomous System path.
5. Route with the lowest origin code (i is lowest, e is next, ? is last).
6. Route with the lowest MED.
7. An EBGP route over an IBGP route.
8. Route through the nearest IGP neighbor as determined by the lowest IGP metric.
9. Oldest route
10. Route through the neighbor with the lowest router ID.
11. Route through the neighbor with the lowest IP address.

Question - 5 Explain BGP communities.

A community is a group of prefixes that will be treated or behaving the same way. If you have 50 or 100 prefix that require the same weight or local preference then you will match all prefixes using an access list or prefix-list but using BGP communities is more convenient.

There are four well known BGP communities.

• Internet Community - Advertise the prefix to all BGP neighbors.
• No-Advertise Community - Do not advertise the prefix to any BGP neighbors. If you will add the No-Advertise community to a prefix,  then the receiving BGP router will use and store the prefix in its BGP table, but it will not advertise the prefix to any other neighbors.
• No-Export Community - Do not advertise the prefix to any eBGP neighbors, only advertise a prefix to iBGP neighbors.
• Local-AS Community - Do not advertise the prefix outside of the sub-AS, basically it is used for BGP confederations.

Question - 6 BGP community attribute format, which we can use to configure.

We can configure communities in three different formats, it is a 32-bit number -
Decimal - 1966100
Hexadecimal - Example - 0x1E0014
AA:NN - Example - 30:20 (AS 30, number 20)

Question - 7 How can you enable community propagation ?

Enable the BGP community propagation with the neighbor send-community command.

Question - 8 How many types of BGP routing table available ?

• Adj-RIB-in - 

1. Storing the incoming routes from neighbor.
2. It will also store the unprocessed information received from the its peer
3. In the Adj-RIB-in best path selection process occurs and after selection of best path , path is entered into local BGP table (Loc-RIB) 

• Adj-RIB-out - 

1. Adj-RIB-out table basically use to send the routes to neighbor, neighbor will receive the routes from this table.

• Loc-RIB - 

1. Next hop address will be confirmed from Loc-RIB table, if it is reachable by IGP then the route will be entered into to main routing table.

Question - 9 What does r RIB-Failure mean in the show ip bgp command output?

Router1# show ip bgp

BGP table version is 5, local router ID is 100.100.100.1

Status codes: s suppressed, d damped, h history, * valid,  best, i - internal, r  RIB-failure

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

r 3.3.3.0/24       10.10.12.1               0    135      0 30 i

*4.4.4.0/24       10.10.12.1               0    130      0 30 i

When BGP tries to install the best path prefix into Routing Information Base (RIB), RIB can be reject the BGP route due to below reasons:

• Already route available with better administrative distance in IGP.
• Memory failure.
• Route limit exceeds in VPN routing or VRF instance

In these situation, the prefixes which are rejected, marked by r RIB Failure,

Question - 10 How much memory required in router to receive the complete BGP routing table from ISP?

As per Cisco, minimum 512 MB of RAM should be available in the router to store complete global BGP routing table from one peer.

But I want to inform you , "How much memory required in router to receive the complete BGP routing table from ISP" it depend on the multiple parameter .

• Router model
• Community
• Attributes
• Route Dampening
• Total available alternate path and other things

So it is difficult to calculate the memory required to store the complete BGP routing table from ISP

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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