Establishing LDP Neighborship Session Failure Prevention Mechanism | Cisco CCIE Service Provider Exam 400-201

Preventing Unthrottled Session Setup Failures between LSRs


An engineer finds out that two LSRs do not reach an agreement on LDP neighborship session.

Which mechanism prevents these two LSRs from engaging in an unthrottled sequence of session setup failures?



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A. B. C. D.


When two Label Switching Routers (LSRs) fail to establish an LDP neighborship session, they will continue to attempt to connect to each other in an unthrottled sequence of session setup failures. This can lead to excessive network congestion, instability, and other issues.

To prevent this from happening, LDP uses a mechanism known as "LDP backoff." This mechanism introduces a delay between each session setup attempt, which helps to prevent the unthrottled sequence of failures.

LDP backoff works by implementing a simple exponential backoff algorithm. Each time a session setup attempt fails, the LSR increases the delay before the next attempt according to a predefined formula. For example, the delay might start at 1 second, and double with each successive failure, until it reaches a maximum value of 64 seconds.

This delay gives the network time to recover from any congestion or other issues that might be preventing the session setup from succeeding. Once the delay period has elapsed, the LSR will attempt to establish the session again, using the normal LDP negotiation process.

In contrast, the other options mentioned in the question are not directly related to preventing unthrottled sequence of session setup failures:

  • "Discovery transport-address" is a feature that enables LDP to discover the transport address of its LDP peers. This information is used to determine the source address of LDP messages that are sent to the peer.

  • "Instance-tlv" is a feature used by MPLS-TE to help identify and differentiate between different instances of a given LSP.

  • "LCC" (Label Controlled Crossconnect) is a technique used in MPLS networks to establish a direct path between two LSRs without the need for intermediate label switching.