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Understanding Weighted ECMP Traffic Distribution on One-Hop IS-IS Neighbors

Equal-cost multipath (ECMP) is a popular technique to load balance traffic across multiple paths. With ECMP enabled, If paths to a remote destination have the same cost, then traffic is distributed between them in equal proportion. Equal distribution of traffic across multiple paths is not desirable if the local links to adjacent routers towards the ultimate destination have unequal capacity. Typically the traffic distribution between two links is equal and the link utilization is the same. However, if the capacity of an aggregated Ethernet bundle changes, equal traffic distribution results in imbalance of link utilization. In this case, weighted ECMP enables load balancing of traffic between equal cost paths in proportion to the capacity of the local links.

In figure 1, Router R1 has 5 links to Router R2, four of them are part of the aggregated Ethernet bundle. Both L3 links have the same cost. To reach the destination network N, Router R1, load balances traffic between L3 links. The distribution between the two links is equal in normal conditions and link utilization is the same. However, if AE bundle capacity changes, equal distribution results in the imbalance of link utilization. Weighted ECMP feature enables load balancing between equal cost paths in proportion to the capacity of the local inks. In this example, if AE bundle has three active links, traffic is distributed in 30/40 proportion between AE bundle and a single link.

Note:

This feature provides weighted ECMP routing to IS-IS neighbors that are one hop away. Junos OS supports this feature on immediately connected routers only and does not support weighted ECMP on multihop routers, that is, on routers that are more than one hop away.

To enable weighted ECMP traffic distribution on directly connected IS-IS neighbors, configure weighted one-hop statement at the [edit protocols isis spf-options multipath] hierarchy level. Weighted ECMP is currently supported for the IS-IS protocol only.

Note:

You must configure per-packet load balancing policy before configuring this feature. WECMP will be operational if per-packet load balancing policy is in place,

Starting in Junos OS Release 17.1R1, weighted ECMP feature also supports IS-IS SPRING based next hop addresses.

Note:

For logical interfaces, you must configure interface bandwidth to distribute traffic across equal cost multipaths based on the underlying physical interface bandwidth. If you do not configure the logical bandwidth for each logical interface, Junos OS assumes that the entire bandwidth of the physical interface is available for each logical interface.

Release History Table
Release
Description
17.1R1
Starting in Junos OS Release 17.1R1, weighted ECMP feature also supports IS-IS SPRING based next hop addresses.