Reference Guide

104 | Access Control Lists (ACLs)
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Figure 6-12. Applying an ACL to the Loopback Interface
IP Prefix Lists
Prefix Lists are supported on platforms: c e 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
Note: See also the section VTY Line Local Authentication and Authorization.
FTOS(conf)#interface loopback 0
FTOS(conf-if-lo-0)#ip access-group abcd in
FTOS(conf-if-lo-0)#show config
!
interface Loopback 0
no ip address
ip access-group abcd in
no shutdown
FTOS(conf-if-lo-0)#end
FTOS#configure terminal
FTOS(conf)#ip access-list extended abcd
FTOS(config-ext-nacl)#permit tcp any any
FTOS(config-ext-nacl)#deny icmp any any
FTOS(config-ext-nacl)#permit 1.1.1.2
FTOS(config-ext-nacl)#end
FTOS#show ip accounting access-list
!
Extended Ingress IP access list abcd on Loopback 0
seq 5 permit tcp any any
seq 10 deny icmp any any
seq 10 deny icmp any any
Use the in keyword.
Add rules to the ACL
named “abcd.”
Display the ACL.