6. Explain Loop prevention mechanism in BGP?
BGP uses two mechanism to prevent loops:-
1. When a router learns routes from an iBGP peer, that router does not advertise the same routes to another iBGP peer.
2. By using AS_PATH - When advertising 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 own ASN, the router ignores that route.
Note - A BGP router does not add its ASN when advertising to an iBGP peer.
7. Do we need to follow 3 way handshake process to establish BGP communication?
Yes
8. What is the difference between hard reset and soft reset in BGP?
In case of hard reset the local router brings down the neighborship, brings down the underlying TCP connection and all the BGP table entries learned from that neighbor are removed. #clear ip bgp * command is used for hard reset.
In case of soft reset, the router does not bring down the BGP neighborship or the underlying TCP connection. However, the local router resends outgoing Updates and reprocesses incoming Updates adjusting the BGP table based on the current configuration. #clear ip bgp * soft command is used for soft reset.
9. What are different BGP Message Types?
1. Open - It is Used to establish a neighbor relationship and exchange parameters, including autonomous system number and authentication values.
2. Keepalive - It is Sent periodically to maintain the neighbor relationship. If the Keepalive message is not received within the negotiated Hold timer than BGP neighborship will be turned down.
3. Update - It exchanges Path Attributes and the associated prefix/length (NLRI) that use those attributes.
4. Notification - It is Used to report BGP error. It results in a reset of neighbor relationship.
10. Explain various states of BGP?
1. Idle - The BGP process is either administratively down or waiting for the the next retry attempt.
2. Connect - The BGP process is waiting for the TCP connection to be completed. If it is successful, it will continue to the OpenSent state. In case it fails, it will continue to the Active state.
3. Active - BGP will try another TCP three-way handshake to establish a connection with the remote BGP neighbor. If it is successful, it will move to the OpenSent state.
4. Opensent - The TCP connection exists, and a BGP Open message has been sent to the peer, but the matching Open message has not yet been received from the other router.
5. Openconfirm - An Open message has been both sent to and received from the other router.
Next step is to receive a BGP Keepalive message (to confirm that all neighbor-related parameters match) or a BGP Notification message (to learn that there is some mismatch in neighbor parameters).
6. Established - All neighbor parameters matched, the neighbor relationship has been established and the peers can now exchange Update messages
Read More Questions:
BGP Interview Questions Part1
BGP Interview Questions Part2
BGP Interview Questions Part3
BGP Interview Questions Part4
BGP uses two mechanism to prevent loops:-
1. When a router learns routes from an iBGP peer, that router does not advertise the same routes to another iBGP peer.
2. By using AS_PATH - When advertising 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 own ASN, the router ignores that route.
Note - A BGP router does not add its ASN when advertising to an iBGP peer.
7. Do we need to follow 3 way handshake process to establish BGP communication?
Yes
8. What is the difference between hard reset and soft reset in BGP?
In case of hard reset the local router brings down the neighborship, brings down the underlying TCP connection and all the BGP table entries learned from that neighbor are removed. #clear ip bgp * command is used for hard reset.
In case of soft reset, the router does not bring down the BGP neighborship or the underlying TCP connection. However, the local router resends outgoing Updates and reprocesses incoming Updates adjusting the BGP table based on the current configuration. #clear ip bgp * soft command is used for soft reset.
9. What are different BGP Message Types?
1. Open - It is Used to establish a neighbor relationship and exchange parameters, including autonomous system number and authentication values.
2. Keepalive - It is Sent periodically to maintain the neighbor relationship. If the Keepalive message is not received within the negotiated Hold timer than BGP neighborship will be turned down.
3. Update - It exchanges Path Attributes and the associated prefix/length (NLRI) that use those attributes.
4. Notification - It is Used to report BGP error. It results in a reset of neighbor relationship.
10. Explain various states of BGP?
1. Idle - The BGP process is either administratively down or waiting for the the next retry attempt.
2. Connect - The BGP process is waiting for the TCP connection to be completed. If it is successful, it will continue to the OpenSent state. In case it fails, it will continue to the Active state.
3. Active - BGP will try another TCP three-way handshake to establish a connection with the remote BGP neighbor. If it is successful, it will move to the OpenSent state.
4. Opensent - The TCP connection exists, and a BGP Open message has been sent to the peer, but the matching Open message has not yet been received from the other router.
5. Openconfirm - An Open message has been both sent to and received from the other router.
Next step is to receive a BGP Keepalive message (to confirm that all neighbor-related parameters match) or a BGP Notification message (to learn that there is some mismatch in neighbor parameters).
6. Established - All neighbor parameters matched, the neighbor relationship has been established and the peers can now exchange Update messages
Read More Questions:
BGP Interview Questions Part1
BGP Interview Questions Part2
BGP Interview Questions Part3
BGP Interview Questions Part4
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