Ip Prefix Lists; Implementation Information; Configuration Task List For Prefix Lists - Dell S6100 Configuration Manual

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 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 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.

Configuration Task List for Prefix Lists

To configure a prefix list, use commands in PREFIX LIST, ROUTER RIP, ROUTER OSPF, and ROUTER BGP modes.
Create the prefix list in PREFIX LIST mode and assign that list to commands in ROUTER RIP, ROUTER OSPF and ROUTER BGP modes.
The following list includes the configuration tasks for prefix lists, as described in the following sections.
• Configuring a prefix list
• Use a prefix list for route redistribution
For a complete listing of all commands related to prefix lists, refer to the Dell Networking OS Command Line Interface Reference Guide.
Creating a Prefix List
To create a prefix list, use the following commands.
1 Create a prefix list and assign it a unique name.
You are in PREFIX LIST mode.
122 Access Control Lists (ACLs)