What is the difference between Load Balancing and Load Sharing?

[Meddane]

Load balancing and load sharing are two terms often used interchangeably in Routing. What is the difference between Load Balancing and Load Sharing?

[Richard Burts]

Load Balance and Load Share are indeed quite similar terms and frequently seem to be used interchangeably. But there is a subtle difference and we should be aware of this and perhaps not use the terms interchangeably. Both terms refer to situations where a source sends packets to a destination and the routing process uses multiple paths to get to the destination. Load Balance has a meaning emphasizing the Balance aspect and implies that equal amount of traffic goes over each link. Load Share has a meaning that some traffic goes over each link but the amount of traffic on each link may be different.

When you use Cisco ECMP (equal cost multi path) by having more than one static routes for a particular destination, or by having a dynamic routing protocol that discovers more than one path to the same destination and equal cost of each path then what you really get is Load Sharing but many people do refer to this as Load Balance which is technically not correct because the amount of traffic on each path may very well not be the same.

[Giuseppe Larosa]

Hello @Meddane ,

Load balancing and Load sharing are not the same .

Both attempt to use multiple links in a topology.

We can use per flow load balancing when we build ECMP : there are multiple paths of equal costs to the destination prefixes. In this case CEF uses its algorythm to decide what link to use and a flow is identified by source IP address and destination IP address. They are put in exor at bit level ( the last meaningful bits of the last octet) and then are in exor with the seed that is a value chosen at router startup that is device specific to avoid polarization.

In this case I would say that if we have many flows ( > 100) of comparable traffic volume we get a fair use of available paths.

However, as noted if there are some flows that have a very big traffic volume  ( elephant flows) like an exchange between two DBs this can cause unequal use of the links.

For me load sharing happens when for example using BGP and we make a path preferred over link 1 for prefix1 and the opposite for another prefix prefix 2.  In cases like this again there is not exact load balancing but we control what path is taken based on destination.

we used this form of load sharing in MPLS L3 VPN for multi homed sites served by two PE nodes before the introduction of commands that allowed multipath from the point of view of a remote PE  ( eibgp maximum-paths 2 ) by manipulating local preference on colocated PE nodes on a per prefix basis.

Hope to help

Giuseppe

[Meddane]

 Thanks @Giuseppe Larosa