Describes how ECMP distributes traffic evenly across equal-cost paths and how UCMP applies path weights to better match traffic distribution to link capacity.
ECMP load balancing is a forwarding method that
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distributes traffic across multiple equal-cost paths
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assumes the available links have similar speed and share hash buckets evenly, and
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aims to provide nearly equal load sharing across those paths.
| Feature Name |
Release Information |
Feature Description |
|---|---|---|
| ECMP vs. UCMP Load Balancing |
Release 25.1.1 | Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*) *This feature is supported on Cisco 8011-4G24Y4H-I routers. |
| ECMP vs. UCMP Load Balancing |
Release 24.4.1 |
Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only); Modular Systems (8800 [LC ASIC: P100])(select variants only*) Enhanced ECMP improves routing scalability by supporting a higher number of equal-cost paths, facilitating efficient load distribution across the network. This feature allows you to maximize throughput by evenly distributing traffic across available paths, which minimizes potential bottlenecks. Enhanced ECMP is designed to work seamlessly with various routing protocols, ensuring consistent performance and reliability in large-scale network environments. *This feature is supported on:
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Equal bucket distribution in ECMP
In ECMP, all available links are typically of similar speed, so hash values are shared equally across the available paths.
For example, if two paths are available, the hash buckets are divided equally for a 50 percent load share in each path. If one path is a 10G link and the other is a 1G link, equal distribution may not be desirable. In such cases, a distribution closer to 90/10 may be preferred. Because BGP does not consider bandwidth, the 10G and 1G paths are still each selected 50% of the time.
How UCMP addresses unequal links
UCMP applies a weight to a path, assigning more hash buckets to the path with higher weight. The applied weight is static , which means it is derived from the DMZ bandwidth extended community and assigned to a peer or configured using Route Policy Language (RPL) route settings.
Traffic distribution based on link capacity
A routing protocol generally selects the best path to a destination based on a metric. This metric usually depends on circuit bandwidth. For example, if there are three paths (1G, 10G, and 100G), routing protocols often use only the highest-capacity path. In some situations, you may want to distribute traffic over the circuits based on the load each can carry. For example, you might distribute traffic 1%, 10%, and 89% among the 1G, 10G, and 100G paths.