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-
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 .
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Access Control Lists (ACLs)