Introduction

When using Per flow load sharing algorithm, a special condition could occur where certain links are not utilized at all because of the layered network topology and the same hashing algorithm on each router.

CEF Polarization

• When relying on per flow load balancing, CEF Polarization condition could occur, where a number of links are not utilized at all in a multi-trier topology (distribution layer).
• This particular problem occurs when all routers have the same hashing algorithm configured using the same parameters, where each network tier groups the traffic and doesn't load balance across different links.
• See the example topology sourced from http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080c15610.shtml

• Router A splits some flows to Router C, while others to Router B. Since the same flows were split into Router B, those are already grouped in the same harsh output value (i.e. link number). Router B will always put them on the same link and not utilize the other path, which in this case is Router B to Router D.
• The reasons this occurs is that all routers are using the same hash algorithms and using the same static fields for the hash function.

Resolution

• To solve the problem Cisco recommends the following solutions:
  • Alternate the load sharing algorithms between network tiers. For example use source IP/destination IP on access then on distribution layer use layer 4 information as well..etc. This option will produce a different result on each network trier and break apart the grouping by using different links for forwarding.
  • Alternate between even and odd number of ECMP links between each network layer.
  • Use the universal algorithm. The universal algorithms adds another field to the hash parameters. This field is a unique ID generated as router boot or manually configured to eliminate the grouping at each network tier. Some platform uses a Router ID as well. By having a different value at each hashing router, changes of picking the same output interface for a flow are less likely.

Additional Resources

CEF Polarization - Cisco IP Switching

Comments