OS10 Enterprise Edition User Guide Release 10.4.0E(R3)

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IP prefix-lists

IP prefix-lists control the routing policy. An IP prefix-list is a series of sequential filters that contain a matching criterion and an permit or deny action to process routes. The filters process in sequence so that if a route prefix does not match the criterion in the first filter, the second filter applies, and so on.

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 match the dotted decimal address.

When the route prefix matches a filter, the system 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 drops (implicit deny).

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. Use permit or deny filters for specific routes with the le (less or equal) and ge (greater or equal) 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 le 12
  • 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 permit or deny filters allows all routes
  • An “implicit deny” is assumed — the route drops for all route prefixes that do not match a permit or deny filter
  • After a route matches a filter, the filter’s action applies and no additional filters apply to the route

Use prefix-lists in processing routes for routing protocols such as OSPF, RTM, and BGP.

To configure a prefix-list, use commands in PREFIX-LIST and ROUTER-BGP modes. Create the prefix-list in PREFIX-LIST mode and assign that list to commands in ROUTER-BGP modes.


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