1.1.b Identify Cisco express forwarding concepts

1.1.b [iii] Polarization concept and avoidance

CEF polarization occurs when traffic uses per destination load balancing and the same algorithm, which is default , is used throughout the network which causes traffic to not be load balanced after the first distribution.

As an example think of a layer 3 network with multiple layers or levels each with a possible path to the right or left. If 100Mbps of traffic was coming into a router, it would be load balanced 50/ 50, with 50Mbps to Router-right and 50Mbps to Router-left, but as Router-level-1right & Router-level1-left will use the same algorithm to determine which path the traffic will take, but as the algorithm is identical it will be a 100/ 0 split, with 50Mbps going to Router-level2-right and Router -level2-left and no data going to other paths. Whenever there is an even number of ECMP available, traffic will not be distributed evenly.

To counter this issue, a newer algorithm called the universal algorithm was developed where a 32-bit value is added to the hashing algorithm , this value can be manually set but defaults to the highest loopback IP on the router. This is based on the concept called unique-ID/universal-ID. Hash function is known as universal-ID, a randomly generated value at the time of the router or layer 3 switch boot up that can be manually controlled . This seeds the hash function on each router with a unique ID, which ensures that the same source/ destination pair hash into a different value on different routers along the path within the network. This process provides a better network-wide load-sharing and avoids the polarization issue. In order to configure a custom ID, you can use the following CLI:
Router( config)# ip cef load-sharing algorithm universal < id >

Another way to avoid polarization would be to use alternate between default (Source IP and Destination IP) and full (Source IP + Destination IP + Layer 4 ports) hashing inputs configuration at each layer of the network. Of course, this is not practical if we’re talking about a large network with many layers some possibly outside the control of the given network administrator.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 942-945).  . Kindle Edition.