When a requirement exists to increase the available bandwidth for a logical link that exceeds the physical bandwidth or add redundancy for a physical link, typically one of two methods is applied: equal cost multi-path (ECMP) or Link Aggregation (LAG). A system can deploy both at the same time using ECMP of two or more Link Aggregation Groups (LAG) and, or single links.
Different types of hashing algorithms can be employed to achieve one of the following objectives:
ECMP and LAG load balancing should be influenced solely by the offered flow packet. This is referred to as per-flow hashing.
ECMP and LAG load balancing should maintain consistent forwarding within a specific service. This is achieved using consistent per-service hashing.
LAG load balancing should maintain consistent forwarding on egress over a single LAG port for a specific network interface, SAP, and so on. This is referred as per link hashing (including explicit per link hashing with LAG link map profiles). Note that if multiple ECMP paths use a LAG with per link hashing, the ECMP load balancing is done using either per flow or consistent per service hashing.
These hashing methods are described in the following subsections. Although multiple hashing options may be configured for a specific flow at the same time, only one method is selected to hash the traffic based on the following decreasing priority order:
For ECMP load balancing:
Consistent per service hashing
Per flow hashing
For LAG load balancing:
LAG link map profile
Per link hash
Consistent per service hashing
Per flow hashing