Prefix Lists - Operators

When using Prefix Lists to filter or match particular routes, the following operators are used:

  • le - Less Than or Equal To
  • ge - Greater Than or Equal To

The following context sensitive help from the Cisco IOS command line shows these options:

R1(config)#ip pre R1(config)#ip prefix-list TEST permit ? ge Minimum prefix length to be matched le Maximum prefix length to be matched R1(config)#ip prefix-list TEST permit

The names can be somewhat counterintuitive when you take into account that larger subnets are denoted by smaller prefix values, while smaller subnets are denoted by larger prefix values.

In the context of IPv4 and subnet masks, a smaller slash notation (like /19) actually represents a larger network because it includes more IP addresses. Conversely, a larger CIDR notation (like /21) represents a smaller network with fewer IP addresses.

The “le” and “ge” operators used by the prefix lists always refer to “less than or equal to” and “greater than or equal to” in a numerical context. In other words, /20 is considered “ge” /19.