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Dell Configuration Guide for the S4048–ON System 9.14.2.5

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IP Prefix Lists

IP prefix lists control routing policy. An IP prefix list is a series of sequential filters that contain a matching criterion (examine IP route prefix) and an action (permit or deny) to process routes. The filters are processed in sequence so that if a route prefix does not match the criterion in the first filter, the second filter (if configured) is applied. When the route prefix matches a filter, Dell EMC Networking OS drops or forwards the packet based on the filter’s designated action. If the route prefix does not match any of the filters in the prefix list, the route is dropped (that is, implicit deny).

A route prefix is an IP address pattern that matches on bits within the IP address. The format of a route prefix is A.B.C.D/X where A.B.C.D is a dotted-decimal address and /X is the number of bits that should be matched of the dotted decimal address. For example, in 112.24.0.0/16, the first 16 bits of the address 112.24.0.0 match all addresses between 112.24.0.0 to 112.24.255.255.

The following examples show permit or deny filters for specific routes using the le and ge parameters, where x.x.x.x/x represents a route prefix:
  • To deny only /8 prefixes, enter deny x.x.x.x/x ge 8 le 8.
  • To permit routes with the mask greater than /8 but less than /12, enter permit x.x.x.x/x ge 8.
  • To deny routes with a mask less than /24, enter deny x.x.x.x/x le 24.
  • To permit routes with a mask greater than /20, enter permit x.x.x.x/x ge 20.
The following rules apply to prefix lists:
  • A prefix list without any permit or deny filters allows all routes.
  • An “implicit deny” is assumed (that is, the route is dropped) for all route prefixes that do not match a permit or deny filter in a configured prefix list.
  • After a route matches a filter, the filter’s action is applied. No additional filters are applied to the route.

Implementation Information

In Dell EMC Networking OS, prefix lists are used in processing routes for routing protocols (for example, router information protocol [RIP], open shortest path first [OSPF], and border gateway protocol [BGP]).

NOTE It is important to know which protocol your system supports prior to implementing prefix-lists.

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