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.
Taking as an example, there are two devices interconnected with an aggregated Ethernet bundle with four links and a single link of the same cost. Under normal conditions, both the AE bundle and the single link is utilized evenly to distribute traffic. However, if a link in the AE bundle goes down, there is a change in the link capacity that results in uneven link utilization. Weighted ECMP load balances traffic between the equal cost paths in proportion to the capacity of the local links. In this case, traffic is distributed in 30/40 proportion between the AE bundle and the single link.
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.
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.
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.