This article explores the adaptive update-threshold knob, its need, uses, configuration.

We will also explore how it does impact the ISIS Link State Database, the TED database in the network and overall improvement in LSP re-optimization in high scaled scenarios

update-threshold

• When a link’s BW changes, RSVP informs the IGPs, which then floods the new bandwidth information throughout the domain
• Network nodes then know how much bandwidth is available on the TE-link (local or remote), and CSPF can correctly compute the paths
• RSVP update-threshold can have a single % value for when to trigger an IGP update

Configuration

[edit logical-systems logical-system-name protocols rsvp interface interface-name],
[edit protocols rsvp interface interface-name]
update-threshold {
    threshold-percent;
     adaptive
    threshold-value threshold-value;
}

threshold-percent : Specify the percentage change in reserved bandwidth to trigger IGP update.

• Default: 10 percent
• Range: 0.001 through 20 percent

threshold-value : Specify the change in reserved bandwidth to trigger IGP update. (is capped at 20% of link bandwidth).

If the threshold-value is configured to greater than 20% of bandwidth on that link, the threshold-value is capped at 20% of bandwidth.

adaptive update-threshold

• IGP flooding updates can consume excessive system resources.
• If RSVP update-threshold is configured too-low, RSVP may trigger an IGP update even for a small change in bandwidth
• If RSVP update-threshold is configured with a high percentage, RSVP will not trigger an update when Available bandwidth on the interface is low and requested bandwidth doesn’t reach update-threshold leading to inconsistent TED.
• Adaptive update thresholds provide a way to adaptively pace IGP updates, i.e., frequency of IGP updates can be lower when available bandwidth is higher, and frequent IGP update would be required when available BW is lower.

Configuration

[edit logical-systems logical-system-name protocols rsvp interface interface-name],
[edit protocols rsvp interface interface-name]
   update-threshold {
       adaptive {
           limit 10g threshold-percent 0.5;
           limit 20g threshold-percent 1;
           limit 200g threshold-percent 20;
       }
   }

limit : Specify the upper limit of available bandwidth for the range.

threshold-percent : Specify the percentage change in reserved bandwidth to trigger IGP update.

threshold-value : Specify the change in reserved bandwidth to trigger IGP update. (is capped at 20% of link bandwidth).

Example:

• For the above example, if the Available Bandwidth is between 20G to 200G, send 1 new Link State Update after every 20% change in Available BW.
• When the Available BW is between 20G to 10G, send 1 new Link State Update after every 1% change in Available BW.
• When the Available BW is less than 10G, send 1 new Link State Update after every 0.5% change in Available BW.

Practical Usage:

R1 --------- R2 --------- R3 --------- R4--------- R5 --------- R6
|-------------------------- 90 RSVP LSPs ----------------------|

• Assuming the linear topology of 6 routers with each link is 100G bandwidth.
• RSVP subscription is also 100%. Which means the RSVP LSPs can consume full 100G of the Available BW on the ports.
• The update-threshold is set at 1%, which means that any BW change greater than 1% of 100G, in other words, any BW change greater than 1G will trigger a new IGP Link state update.
• There are 90 RSVP LSPs, each of 1.01G, which are which are being created one by one and then deleted one by one in such a way that the change in bandwidth in each step is greater than the set update threshold of 1%.
  • Link is 100G. Setting 1% update threshold will trigger a new IGP update if a LSP of 1.01 G is either being turned up or torn down.
• With normal update-threshold 1%, the above task will generate 180 new IGP Link state updates (90 LSP updates while creation, and 90 LSP updates while deletion, as each of these steps is more than the 1% threshold set)
• With adaptive update-threshold set as above, this will generate just 27 new IGP Link State Updates.