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[MX] Different values observed for "Underflow Max AvgBW" under MPLS LSP auto-bandwidth

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Article ID: KB35067 KB Last Updated: 27 Sep 2019Version: 1.0
Summary:

Users may notice that a label switched path (LSP) under MPLS LSP auto-bandwidth shows both zero and non-zero values for "Underflow Max AvgBW:" in the show mpls lsp autobandwidth name <lsp-name> extensive or show mpls lsp name <lsp-name> extensive output.

This article explains when users will notice these different values and indicates if any action is required following a value change.

Symptoms:

When the underflow count is zero, that is when there are no more underflow events, the "Underflow Max AvgBW: " value will be set to zero.

When the underflow count is non-zero, the "Underflow Max AvgBW:" value will be set according to the LSP bandwidth used in the MPLS statistics file.

Let’s say that the LSP reserves 3 Gbps bandwidth and that the underflow limit is set as 6 with the difference as 20%. This means that if the LSP traffic rate is less than 2.4 Gbps (3 – 3*20% ), it will be counted as one underflow event. When the LSP bandwidth usage drops to 1.9226 Gbps with six continuous underflows, the LSP auto-bandwidth is triggered and the current LSP bandwidth is reserved as 1.9226 Gbps.

When traffic continues to flow at the rate of 1.9226 Gbps with 100% usage of the reserved bandwidth, there will be no usage difference with the current LSP reserved bandwidth of 1.9226Gbps. This means that there are no underflow or overflow events for this LSP following which the underflow count is reset to zero.

auto-bandwidth {
       adjust-interval 10800;
       adjust-threshold 20;    <<< difference > +- 20% to count as an underflow or overflow event
       minimum-bandwidth 1m;
       maximum-bandwidth 15g;
       adjust-threshold-overflow-limit 4;
       adjust-threshold-underflow-limit 6;  <= underflow
   }

Example

labroot@mx2020-GNF1-re0> show mpls lsp autobandwidth name test-1 extensive | refresh 30 
Sep 16 15:47:37
---(refreshed at 2019-09-16 15:47:37 PDT)---
To: 140.222.241.18, 
  From: 140.222.240.205, LSPname: test-1 
  Min BW: 1000kbps, Max BW: 15Gbps, Max AvgBW util: 4.5933Gbps
  Overflow limit: 4, Overflow sample count: 0
  Underflow limit: 6, Underflow sample count: 0, Underflow Max AvgBW: 0bps    <===     0bps  
  Adjustment Timer: 10800 sec, Adjustment Threshold: 20, Adjustment Threshold Absolute Bandwidth: 0bps
  Adjustment Threshold Activate Bandwidth: 0bps
  Time for Next Adjustment: 8378 sec, Time of Last Adjustment: Mon Sep 16 15:07:16 2019
  Last BW: 1.83725Gbps, Last Requested BW: 4.5934Gbps, Last Signaled BW: 4.5934Gbps, Highest Watermark BW: 8.34996Gbps
  Total AutoBw Adjustments: 19, Successful Adjustments: 19, Failed Adjustments: 0
...
---(refreshed at 2019-09-16 15:49:37 PDT)---
To: 140.222.241.18, 
  From: 140.222.240.205, LSPname: test-1 
  Min BW: 1000kbps, Max BW: 15Gbps, Max AvgBW util: 4.5933Gbps
  Overflow limit: 4, Overflow sample count: 0
  Underflow limit: 6, Underflow sample count: 0, Underflow Max AvgBW: 0bps  <===     0bps 
  Adjustment Timer: 10800 sec, Adjustment Threshold: 20, Adjustment Threshold Absolute Bandwidth: 0bps
  Adjustment Threshold Activate Bandwidth: 0bps
  Time for Next Adjustment: 8258 sec, Time of Last Adjustment: Mon Sep 16 15:07:15 2019
  Last BW: 1.83725Gbps, Last Requested BW: 4.5934Gbps, Last Signaled BW: 4.5934Gbps, Highest Watermark BW: 8.34996Gbps
  Total AutoBw Adjustments: 19, Successful Adjustments: 19, Failed Adjustments: 0
---(refreshed at 2019-09-16 15:50:07 PDT)---
To: 140.222.241.18, 
  From: 140.222.240.205, LSPname: test-1 
  Min BW: 1000kbps, Max BW: 15Gbps, Max AvgBW util: 4.5933Gbps
  Overflow limit: 4, Overflow sample count: 0
  Underflow limit: 6, Underflow sample count: 1, Underflow Max AvgBW: 1.9226Gbps <===     1.9226Gbps now
  Adjustment Timer: 10800 sec, Adjustment Threshold: 20, Adjustment Threshold Absolute Bandwidth: 0bps
  Adjustment Threshold Activate Bandwidth: 0bps
  Time for Next Adjustment: 8198 sec, Time of Last Adjustment: Mon Sep 16 15:07:16 2019
  Last BW: 1.83725Gbps, Last Requested BW: 4.5934Gbps, Last Signaled BW: 4.5934Gbps, Highest Watermark BW: 8.34996Gbps
  Total AutoBw Adjustments: 19, Successful Adjustments: 19, Failed Adjustments: 0
...
---(refreshed at 2019-09-16 16:02:09 PDT)---
To: 140.222.241.18, 
  From: 140.222.240.205, LSPname: test-1 
  Min BW: 1000kbps, Max BW: 15Gbps, Max AvgBW util: 4.5933Gbps
  Overflow limit: 4, Overflow sample count: 0
  Underflow limit: 6, Underflow sample count: 5, Underflow Max AvgBW: 1.9226Gbps    <= 1.9226Gbps 
  Adjustment Timer: 10800 sec, Adjustment Threshold: 20, Adjustment Threshold Absolute Bandwidth: 0bps
  Adjustment Threshold Activate Bandwidth: 0bps
  Time for Next Adjustment: 7506 sec, Time of Last Adjustment: Mon Sep 16 15:07:16 2019
  Last BW: 1.83725Gbps, Last Requested BW: 4.5934Gbps, Last Signaled BW: 4.5934Gbps, Highest Watermark BW: 8.34996Gbps
  Total AutoBw Adjustments: 19, Successful Adjustments: 19, Failed Adjustments: 0
---(refreshed at 2019-09-16 16:02:39 PDT)---   With the same rate of 1.9226Gbps traffic, no more underflow events exist (auto-bw resignals lsp bw to 1.9226 Gbps). Underflow count will be reset to zero.

To: 140.222.241.18, 
  From: 140.222.240.205, LSPname: test-1 
  Min BW: 1000kbps, Max BW: 15Gbps, Max AvgBW util: 4.5933Gbps
  Overflow limit: 4, Overflow sample count: 0
  Underflow limit: 6, Underflow sample count: 0, Underflow Max AvgBW: 0bps     <= underflow count =0 (no underflow event) and this value will be set as 0bps.
  Adjustment Timer: 10800 sec, Adjustment Threshold: 20, Adjustment Threshold Absolute Bandwidth: 0bps
  Adjustment Threshold Activate Bandwidth: 0bps
  Time for Next Adjustment: 10776 sec, Time of Last Adjustment: Mon Sep 16 16:02:16 2019
  Last BW: 4.5934Gbps, Last Requested BW: 1.9226Gbps, Last Signaled BW: 1.9226Gbps, Highest Watermark BW: 8.34996Gbps
  Total AutoBw Adjustments: 20, Successful Adjustments: 20, Failed Adjustments: 0

Solution:

The 0bps display itself is expected behavior and no action is required with reference to this behavior. The bandwidth value is displayed only when underflow is in use.

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