BGP - [Part 4] - BASIC INTERVIEW QUESTIONS

Question - 1 Can BGP neighbors have different subnet ?

BGP neighbor does not require to be attached to the same subnet. Actually BGP peers use a TCP connection between the routers to pass BGP messages , so neighboring router can be in same or different subnet.

Question - 2 How BGP is discovered routing neighbor ? 

• Unlike EIGRP and OSPF, BGP router will not discover another BGP neighbor device automatically. 
• You will have to configure it manually. Suppose you have two router R1 and R2, and you want to establish neighbor-ship between both, you will have to configure network command on the R1 router to tell him R2 is neighbor and also you will have configure network command on R2 to tell him R1 is neighbor. 
• Both side network command should match and proper configuration required, then neighbor-ship will be established.

Question - 3 What is Recursive Lookup ?

The router looks up the BGP route and the next hop to reach a destination remote AS. Then the router initially looks up the route to reach the next hop. So it means router will perform lookup two times to reach to a destination, this process is known as recursive lookup.

Question - 4 What is the difference between hard reset and soft reset in BGP?

• Hard Reset - 

1. In hard reset neighbor-ship will be down in local router.
2. TCP connection will be down.
3. All the BGP table entries, whiched learn from that neighbor are removed.
4. Command for hard reset - Clear IP bgp *

• Soft Reset - 

1. In soft reset BGP neighbor ship will not be down in local router.
2. TCP connection will not be down also.
3. But local router will resend all outgoing update and reprocess all incoming update.
4. Command for soft reset - Clear IP BGP * soft

Qusetion - 5 When we need to reset a BGP session?

If you changed the inbound/outbound policy then you need to clear the BGP session to apply the policy changes.

• Use the clear ip bgp x.x.x.x soft out command to reset the BGP session to apply the outbound policy changes.
• Use the clear ip bgp x.x.x.x command to reset the BGP session to apply the inbound policy changes. you can also use the clear ip bgp x.x.x.x soft in command if neighbor has soft reconfiguration capability 

Question - 6 Can BGP session cleared automatically ?

Yes, via Optimized Edge Routing, BGP session can be cleared automatically for both inbound and outbound policy. But you will have to setup Optimized Edge Routing manually.

Question - 7 Which command we use to administratively disable BGP neighbor-ship?

•  Command to admin disable BGP neighbor-ship - neighbor shutdown
• Command to admin disable BGP neighbor-ship - no neighbor shutdown

Question - 8 Which command you will use to show which entries in the BGP table of the local router have been propagated to a specific neighbor ?

• Command - show ip bgp advertised

Question - 9 Which command you will use to display, which of the routes in the local BGP table have been received and accepted from an indicated neighbor ?

• Command - show ip bgp routes

Question - 10 What is router-id ?

A router-id is a 32-bit IP address, which is uniquely identifies a router in an Autonomous System. So a router-id is uniquely identifies a device in an Autonomous System.

Question - 11 Explain router-id selection process in BGP.

BGP will follow the below listed way to obtain the correct router ID-

• We can configure the router id manually, so if you will set it manually , BGP will prefer this.
•  If you will not configure router-id, the BGP will prefer highest IP address on loop-back interface as router-id.
• If there is no any loop-back interface, then BGP will prefer highest IP address on physical interface as router-id.
• If you did not set any router-id manually, and there is no any loop-back and physical interface then BGP will show 0.0.0.0 as router-id.

Question - 12 How can we configure BGP router-id ? 

Router(config-router)#bgp router-id 4.1.1.48

Router(config-router)#end

Router#show ip bgp summary

BGP currently operating:

AS: 200

Router ID: 4.1.1.48

Note-you can Use the no bgp router-id to use the default BGP router-id for this router

Question - 13 What happen if we will change the router-id in live network ?

• If you will use the bgp router-id command in live network, then BGP processes will restart, so your network will flactuate. BGP will recomputes the router-id based on the process.
• If BGP use the IP address of physical interface as router id, and you want to change the IP in live network then route flapping may occur.
• If BGP use the IP address of loop-back interface as router id, and you want to change the IP in live network then route flapping will not occur.

Question - 14 How can you check, BGP router advertise the routes to neighbor ?

The show ip bgp neighbors advertised-routes command shows which routes are being advertised.

Question - 15 How can you check all accepted received routes in BGP ?

The show ip bgp neighbors  routes command will display all accepted received routes.

Question - 16 How can you check all received routes (both accepted and rejected ) in BGP ?

The show ip bgp neighbors received-routes command will show all BGP received routes (both accepted and rejected).

Question - 17 BGP is showing 0.0.0.0 as router-id, what is the reason behind this?

If BGP is showing 0.0.0.0 as router-id means router has non-IP interfaces, all available physical or loop-back interfaces configured without IP addresses, so BGP cannot get the router-id. In this situation BGP will use 0.0.0.0 as router-id.

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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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