14) Performance: Load Balancing

This blog is is last blog of Hierarchical Network Design, kindly check all blog to understand Network Design.
  Redundant links are not cheap to operate, but they are called for in some situations. If you are going to pay for redundant links, you would likely want to use both lines when they are both available, and that brings us to load balancing.
A good design rule is to keep bandwidth consistent within a layer of hierarchy whenever possible and to use technologies such as DDR when purchasing equal links is not possible. This will avoid the pinhole congestion issue and generally facilitates predictability within the network. Of course, this is not always possible, especially when the redundant paths are expensive leased lines!

Internet Protocol (IP)
  With most IP routing protocols, load balancing is automatic. Dynamic routing protocols are supposed to find the redundant paths, and dynamic IP routing protocols will use both available paths. This can, however, not always be a good thing.

Pinhole Congestion:
  Difficulties can arise when the multiple paths out do not have the same bandwidth or cost. Suppose that you have a T1 and a 56Kbps line (for backup) connecting your access-layer router into distribution layer routers, as shown in bellow picture

  Some routing protocols (for example, those that use hop count) could see these two paths and load balance across them just fine until the 56Kbps line is full. At that point, the load is equally balanced. These protocols, however, are not smart enough to realize that more than 90 percent of the total bandwidth is going unused on the T1! Once any link is operating at capacity, these routing protocols are not capable of sending additional traffic across links still not at capacity, because they do not understand capacity as a metric. This problem is called pinhole congestion, which you can avoid by using advanced routing protocols such as Enhanced IGRP.

Internetwork Packet eXchange (IPX):
  By default, IPX will not load balance across multiple links; however, Cisco provides a way to enable IPX load balancing. You can use the ipx maximum-paths command, which specifies a number of links to load balance across.
  Use the command ipx m? to access all the commands that start with ipx m. Notice that the ipx maximum-paths and ipx maximum-hops are the commands listed. If you change the default parameters on one router, you need to change these parameters on all your routers.
  The command ipx maximum-paths tells IPX to consider that there might be more than one link to the same location. By default, IPX will not consider that a second link could exist and will not provide a round-robin load balance. The ipx maximum-paths command solves this problem.

AppleTalk:
  AppleTalk, like IPX, considers only one path to a remote network. You can set the number of parallel routing paths that can be used by AppleTalk by using the appletalk maximum-paths command. Remember to set this on all your routers, not just one router.

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What is Core Distribution & Access Hierarchical Model in Networking?

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