ACLs can provide an important network security
feature and ?¬?lter packets on inbound and outbound router interfaces.
This section describes some of the applications for ACLs on Cisco networks, identi?¬?es the
different types of ACLs that can be implemented, and explains how Cisco IOS Software
processes ACLs.
206 Chapter 6: Managing Traffic with Access Control Lists
Understanding ACLs
To be able to con?¬?gure and implement ACLs, you need to understand the capacity in which they
are used. Cisco devices use ACLs in two primary functions: classi?¬?cation and ?¬?ltering. The
following explains each of these functions:
?– Classi?¬?cation: Routers also use ACLs to identify particular traf?¬?c. After an ACL has
identi?¬?ed and classi?¬?ed traf?¬?c, you can con?¬?gure the router with instructions on how to
handle that traf?¬?c. For example, you can use an ACL to identify the executive subnet as the
traf?¬?c source and then give that traf?¬?c priority over other types of traf?¬?c on a congested WAN
link.
?– Filtering: As the number of router connections to outside networks increase and the use of
the Internet increases, access control presents new challenges.
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