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NE40E V800R010C00 Feature Description - WAN Access 01

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HDLC Flapping Suppression

HDLC Flapping Suppression

Background

Due to unstable signals on physical links or incorrect configurations at the data link layer on live networks, an interface on which High-Level Data Link Control (HDLC) is enabled may frequently experience HDLC negotiation, and the HDLC protocol status of the interface may alternate between Up and Down, causing routing protocol or MPLS flapping. As a result, devices and networks are severely affected. Worse still, devices are paralyzed and networks become unavailable.

HDLC flapping suppression restricts the frequency at which the HDLC protocol status of an interface alternates between Up and Down. This restriction minimizes the impact of flapping on devices and networks.

Implementation Principles

HDLC flapping suppression involves the following concepts:

  • Penalty value: This value is calculated based on the HDLC protocol status of the interface using the suppression algorithm. The core of the suppression algorithm is that the penalty value increases with the changing times of the interface status and decreases exponentially.
  • Suppression threshold: The HDLC protocol status of an interface remains Down when the penalty value is greater than the suppression threshold.
  • Reuse threshold: The HDLC protocol status of an interface is no longer suppressed when the penalty value is smaller than the reuse threshold.
  • Ceiling threshold: The penalty value no longer increases when the penalty value reaches the ceiling threshold, preventing the HDLC protocol status of an interface from being suppressed for a long time. The ceiling value can be calculated using the following formula: ceiling = reuse x 2(MaxSuppressTime/HalfLifeTime).
  • Half-life-period: period that the penalty value takes to decrease to half. A half-life-period begins to elapse when the HDLC protocol status of an interface goes Down for the first time. If the specific half life expires, the penalty value decreases by half. Once a half life ends, another half life starts.
  • Max-suppress-time: maximum period during which the HDLC protocol status of an interface is suppressed. After a max-suppress-time elapses, the HDLC protocol status of the interface is renegotiated and reported.

Figure 3-2 shows the relationships between these parameters.

Figure 3-2  HDLC flapping suppression

At t1, the HDLC protocol status of an interface goes Down, and its penalty value increases by 1000. Then, the interface goes Up, and its penalty value decreases exponentially based on the half-life rule. At t2, the HDLC protocol status of the interface goes Down again, and its penalty value increases by 1000, reaching 1600, which has exceeded the suppression threshold of 1500. The HDLC protocol status of the interface is therefore suppressed. As the interface keeps flapping, its penalty value keeps increasing until it reaches the ceiling threshold of 10000 at tA. As time goes by, the penalty value decreases and reaches the reuse value of 750 at tB. The HDLC protocol status of the interface is then no longer suppressed.

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Updated: 2018-07-04

Document ID: EDOC1100027168

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