Reference Guide

Access Control Lists (ACLs) | 79
IP Prefix Lists
Prefix Lists are supported on platforms: s z
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, FTOS 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.
Below are some examples that 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
le 12
• 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 FTOS, prefix lists are used in processing routes for routing protocols (for example, RIP, OSPF, and
BGP).
FTOS Behavior: VRRP hellos and IGMP packets are not affected when egress ACL filtering for CPU
traffic is enabled. Packets sent by the CPU with the source address as the VRRP virtual IP address
have the interface MAC address instead of VRRP virtual MAC address.
Note: The S-Series platform does not support all protocols. It is important to know which protocol you are
supporting prior to implementing Prefix-Lists.