Concept Guide

(if congured) is applied. When the route prex matches a lter, the system drops or forwards the packet based on the lter’s designated
action. If the route prex does not match any of the lters in the prex list, the route is dropped (that is, implicit deny).
A route prex is an IP address pattern that matches on bits within the IP address. The format of a route prex 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 rst 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 lters for specic routes using the le and ge parameters, where x.x.x.x/x represents a route
prex:
• To deny only /8 prexes, 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 prex lists:
• A prex list without any permit or deny lters allows all routes.
• An “implicit deny” is assumed (that is, the route is dropped) for all route prexes that do not match a permit or deny lter in a
congured prex list.
• After a route matches a lter, the lter’s action is applied. No additional lters are applied to the route.
Implementation Information
In the Dell Networking OS, prex lists are used in processing routes for routing protocols (for example, router information protocol [RIP],
open shortest path rst [OSPF], and border gateway protocol [BGP]).
NOTE
: Dell Networking OS does not support all protocols. It is important to know which protocol you are supporting prior to
implementing prex lists.
Conguration Task List for Prex Lists
To congure a prex list, use commands in PREFIX LIST, ROUTER RIP, ROUTER OSPF, and ROUTER BGP modes.
Create the prex 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 conguration tasks for prex lists, as described in the following sections.
• Conguring a prex list
• Use a prex list for route redistribution
For a complete listing of all commands related to prex lists, refer to the Dell Networking OS Command Line Interface Reference Guide.
Creating a Prex List
To create a prex list, use the following commands.
1 Create a prex list and assign it a unique name.
You are in PREFIX LIST mode.
CONFIGURATION mode
ip prefix-list prefix-name
2 Create a prex list with a sequence number and a deny or permit action.
CONFIG-NPREFIXL mode
seq sequence-number {deny | permit} ip-prefix [ge min-prefix-length] [le max-prefix-length]
Access Control Lists (ACLs)
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