DELL S6000-ON User 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.
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:
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.
• 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
• 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]).
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.
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.
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.
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
• Use a prefix list for route redistribution
Access Control Lists (ACLs)
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