Tag: Load-Sharing

Load-Sharing on the SAME router

Load-balance-1router-2Assume you have either of the following setups. A single router (R3) with multiple links, either to the same upstream router (R2) or to 2 different upstream routers(R2+R4). And you want to load-share traffic across both links outbound (direction from left to right). Obviously the routing table needs multiple outgoing links as next-hops to perform the desired balancing. The command maximum-paths specifies how many paths or next hops are allowed per prefix in the routing table for a specific routing protocol, else default behavior dictates only the best route from each routing protocol which are candidate for insertion into the routing table.

Since the links terminate on the same router (R3) you have the following options:

  1. Per-Destination Load-Sharing using Fast Switching
  2. Per-Source-Destination Load-Sharing using CEF
  3. Per-Packet Load-Balancing using Process Switching
  4. Per-Packet Load-Balancing using CEF

You need to be aware that IOS makes switching decisions based on the configuration of the inbound interface first. If CEF is configured on an inbound interface, the packets will be CEF switched regardless of the configuration on the outbound interface. CEF is ONLY used if  enabled on the inbound interface. If CEF is not configured on the inbound interface, the configuration of the exit interface determines the switching method. The following table illustrates the different behaviors:

Inbound Configuration Outbound Configuration Switching Method Used
CEF CEF CEF
CEF Process CEF
CEF Fast CEF
Fast Fast Fast
Fast CEF Fast
Fast Process Process
Process Process Process
Process CEF Fast
Process Fast Fast

Refer to the following article, for more info about the Switching Types and how to enable each.

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Load-sharing vs Load-balancing

The terminology load sharing vs. load balancing are commonly misunderstood terms in the networking industry.  Most people use the term load-balancing when they in fact referring to load sharing.

So what is Load Balancing?

Definition: Load balancing is a concept that aims to make a network more efficient. Load balancing distributes of traffic load evenly across a network with multiple-paths, in order to get optimal resource utilization, maximize throughput and minimize response time. Thus load-balancing will split the traffic down the configured paths equally towards the destination. E.g., with two 768 kpbs links and 800 kpbs traffic at any point, conceptually with load-balancing each path should have 400 kpbs worth of traffic. But is that what happens?

How does Load Sharing differ?

Definition: Load sharing is inherent to the forwarding process of a router to share the forwarding of traffic, if the routing table has multiple paths to a destination. If equal paths, the forwarding process will decide the manner of forwarding and forward packets based on the load-sharing algorithm used. This still bears the possibility of unbalanced forwarding. If unequal paths, the traffic is distributed inversely proportionally to the cost of the routes. That is, paths with lower costs (metrics) are assigned more traffic, and paths with higher costs are assigned less traffic. Continue reading “Load-sharing vs Load-balancing” →