Soft Clearing of a BGP Session

A

A refresh message is used to perform a soft clearing of a BGP session. Normally a BGP speaker cannot be made to readvertise routes that have already been sent and acknowledged. The route refresh message supports soft clearing of BGP sessions by allowing a peer to readvertise routes that have already been sent.

The correct answer is A. Refresh.

In BGP, a soft clearing of a BGP session refers to the process of gracefully resetting the session without tearing it down completely. A soft clearing can be performed in situations where there is a need to update the BGP session information, such as when new prefixes are added or existing ones are withdrawn.

To perform a soft clearing of a BGP session, the BGP Refresh capability is used. This capability allows a BGP speaker to request that its peer send updated route information. The Refresh capability is negotiated during the BGP session establishment process, and if both speakers support it, it can be used at any time during the session.

When a BGP speaker wants to perform a soft clearing, it sends a BGP Refresh message to its peer, requesting that the peer send updated route information. The Refresh message can be sent for specific address families or for all address families supported by the session.

The Refresh message contains a Request Code field, which specifies the type of refresh that is requested. There are two types of refresh requests:

• Route Refresh (code 2): This request asks the peer to send all BGP routes that it has for a specific address family.
• End-of-RIB (code 3): This request asks the peer to send all BGP routes that it has for a specific address family, but only those that are not already known by the requesting speaker.

Once the Refresh message is received, the peer BGP speaker sends updated route information to the requesting speaker. This allows the session to be updated without tearing it down and establishing a new session.

In conclusion, the BGP Refresh message is used to perform a soft clearing of a BGP session by requesting updated route information from a peer speaker. The Refresh message is negotiated during the session establishment process and can be sent at any time during the session.