Use LCM to Mitigate Congestion Locally
Local Congestion Mitigation (LCM) checks the capacity locally, in and around the congested area, at an interface level. LCM computes the shortest paths for one or more tactical policies to divert the minimal amount of traffic on a congested interface to alternate paths with sufficient bandwidth. It attempts to keep as much of the traffic on the original IGP path. If the user approves, LCM performs the mitigation through the deployment of Tactical Traffic Engineering (TTE) SR policies. LCM will not modify paths of existing deployments of SR policies to mitigate congestion.
TTE tunnel recommendations are listed in the LCM Operational Dashboard. From the dashboard, you can visually preview the TTE SR policy recommendations before deployment. TTE SR policy deployment to resolve congestion is not automated. You must approve and commit LCM recommended actions. LCM also recommends removal of previous TTE SR policies (instantiated by LCM) if they are no longer needed.
LCM Important Notes
Consider the following information when using LCM:
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LCM evaluates network utilization on a regular, configurable cadence of 10 minutes or more. The cadence is typically set to be greater than or equal to the SNMP traffic polling interval.
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LCM leverages ECMP across parallel TTE SR policies and assumes roughly equal splitting of traffic. The degree to which actual ECMP splitting adheres to this assumption depends on the presence of large elephant flows and the level traffic aggregation.
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Traffic that can be optimized must not be carried on existing SR-TE policies.
Platform Requirements
The following is a non-exhaustive list of high-level requirements for proper LCM operation:
Congestion Evaluation:
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LCM requires traffic statistics from the following:
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SNMP interface traffic measurements
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SNMP headend SR-TE policy traffic measurements
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Strict SID labels should be configured for SR.
Congestion Mitigation:
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Headend device should support Equal Cost Multi-Path (ECMP) across multiple parallel SR-TE policies
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Headend device must support PCE-initiated SR-TE policies with autoroute steering
Devices should be configued with
force-sr-iinclude
to enable traffic steering into SR-TE policies with autoroute. For example:segment-routing traffic-eng pcc profile <id> autoroute force-sr-include
LCM Calculation Workflow
This example walks you from congestion detection to the calculations LCM performs prior to recommending tactical tunnel deployment.
Procedure
Step 1 |
LCM first analyzes the Optimization Engine Model (a realtime topology and traffic representation of the physical network) on a regular cadence. |
Step 2 |
In this example, after a congestion check interval, LCM detects congestion when Node 2 utilization goes above the 70% utilization threshold. |
Step 3 |
LCM estimates how much traffic is eligible to divert. LCM only diverts traffic that is not on an existing SR policy (for example: unlabeled, IGP routed, or carried via FlexAlgo-0 SIDs). SR-TE policy traffic is not included in LCM calculation as eligible traffic and will continue to travel over the original programmed path. Eligible traffic is computed by taking the interface traffic stats that account for all traffic on the interface and subtracting the sum of traffic stats for all SR-TE policies that flow over the interface. Total interface traffic – SR policy traffic = Eligible traffic that can be optimized This process must account for any ECMP splitting of SR policies to ensure the proper accounting of SR policy traffic. In this example, the total traffic on congested Node 2 is 800 Mbps. The total traffic of all SR policies routed over Node 2 is 500 Mbps. The total traffic that LCM can divert in this example is 300 Mbps: 800 Mbps – 500 Mbps = 300 Mbps |
Step 4 |
LCM calculates the amount that must be sent over alternate paths by subtracting the threshold equivalent traffic from the total traffic on the interface. In this example, the amount to be diverted is 100Mbps: 800 Mbps – 700 Mbps (70% threshold) = 100 Mbps LCM must route 100 Mbps of 300 Mbps (eligible traffic) to another path. |
Step 5 |
LCM determines how many TTE SR policies are needed and their paths. The ratio of how much LCM eligible traffic can stay on the shortest path to the amount that must be detoured, will determine the number of TTE SR policies that are needed on the shortest versus alternate paths, respectively. In this example, LCM needs to divert 1/3 of the total eligible traffic (100Mbps out of 300Mbps) away from congested link. Assuming a perfect ECMP, LCM estimates 3x tactical SR-TE policies in total to create this traffic split: 1 tactical SR-TE policy will take the diversion path and 2 tactical SR-TE Policies will take the original path. There is sufficient capacity in the path between Node 2 and Node 4. Therefore, LCM recommends 3 TTE SR policies (each expected to route approximately 100Mbps ) to be deployed from Node 2 to Node 3 via SR-PCE:
These recommendations will be listed in the LCM Operational Dashboard. |
Step 6 |
Assuming you deploy these TTE SR policies, LCM continues to monitor the deployed TTE policies and will recommend modifications or deletions as needed in the LCM Operational Dashboard. TTE SR policy removal recommendations will occur if the mitigated interface would not be congested if these policies were removed (minus a hold margin). This helps to avoid unnecessary TTE SR policy churn throughout the LCM operation. |
Mitigate Congestion on Local Interfaces Example
In this example, we will enable LCM and observe the congestion mitigation recommendations to deploy TTE SR polices when utilization surpasses a defined threshold. We will preview the recommended TTE SR policies before committing them to mitigate the congestion. The following image shows the initial topology before congestion occurs.
Procedure
Step 1 |
View initial topology and utilization prior to LCM configuration. |
Step 2 |
Enable LCM and configure the global utilization threshold. |
Step 3 |
View TTE SR policy recommendations in the LCM Dashboard. |
Step 4 |
Validate TTE SR policy deployments. |
Step 5 |
Remove the TTE SR policies upon LCM recommendation. |
Configure LCM
To enable and configure LCM:
Procedure
Step 1 |
From the main menu, choose Traffic Engineering > Local Congestion Mitigation. |
Step 2 |
Toggle the Enable switch to True. |
Step 3 |
Enter the required information. Hover the mouse pointer over to view a description of each field. The following list describes additional field information:
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Step 4 |
Click Commit Changes. |
Monitor LCM Operations
View the LCM Dashboard (
) to monitor LCM operations. The LCM Operational Dashboard shows congested interfaces as defined by the configured utilization threshold. For each interface, it lists details such as current utilization, recommended action, status, expected utilization after committing recommendations, and so on. Hover the mouse pointer over to view a description of what type of information each column provides. From this dashboard, you can also preview and deploy TTE policy recommendations.In addition to the LCM Operational Dashboard, you can click to view LCM events.