1.1.b Identify Cisco express forwarding concepts

1.1.b (iii) Polarization concept and avoidance

when there is an even number of ecmp (equal cost multi path), traffic will not be distributed evenly.

polarization in cef occurs when traffic uses per destination load balancing and the same algorithm (the default). this will cause traffic to be unevenly load balanced after the initial distribution.

from http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/15-mt/isw-cef-15-mt-book/isw-cef-load-balancing.html

  • Original algorithm–The original Cisco Express Forwarding load-balancing algorithm produces distortions in load sharing across multiple routers because the same algorithm was used on every router. Depending on your network environment, you should select either the universal algorithm (default) or the tunnel algorithm instead.
  • Universal algorithm–The universal load-balancing algorithm allows each router on the network to make a different load sharing decision for each source-destination address pair, which resolves load-sharing imbalances. The router is set to perform universal load sharing by default.
  • Tunnel algorithm–The tunnel algorithm is designed to balance the per-packet load when only a few source and destination pairs are involved.
  • Include-ports algorithm–The include-ports algorithm allows you to use the Layer 4 source and destination ports as part of the load-balancing decision. This method benefits traffic streams running over equal cost paths that are not load shared because the majority of the traffic is between peer addresses that use different port numbers, such as Real-Time Protocol (RTP) streams. The include-ports algorithm is available in Cisco IOS Release 12.4(11)T and later releases.