Implementation Information; Configuration Task List For Prefix Lists - Dell S4820T Configuration Manual

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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-

To configure a prefix list, use commands in PREFIX LIST, ROUTER RIP, ROUTER OSPF, and ROUTER BGP

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.

Access Control Lists (ACLs)

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