Cisco Catalyst 2960 Series, Configuration Manual

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As with port ACLs, the switch examines ACLs associated with features configured on a given interface. As
packets enter the switch on an interface, ACLs associated with all inbound features configured on that interface
are examined. After packets are routed and before they are forwarded to the next hop, all ACLs associated
with outbound features configured on the egress interface are examined.
ACLs permit or deny packet forwarding based on how the packet matches the entries in the ACL, and can be
used to control access to a network or to part of a network.
Access Control Entries
An ACL contains an ordered list of access control entries (ACEs). Each ACE specifies permit or deny and a
set of conditions the packet must satisfy in order to match the ACE. The meaning of permit or deny depends
on the context in which the ACL is used.
ACEs and Fragmented and Unfragmented Traffic
IP packets can be fragmented as they cross the network. When this happens, only the fragment containing the
beginning of the packet contains the Layer 4 information, such as TCP or UDP port numbers, ICMP type and
code, and so on. All other fragments are missing this information.
Some access control entries (ACEs) do not check Layer 4 information and therefore can be applied to all
packet fragments. ACEs that do test Layer 4 information cannot be applied in the standard manner to most
of the fragments in a fragmented IP packet. When the fragment contains no Layer 4 information and the ACE
tests some Layer 4 information, the matching rules are modified:
• Permit ACEs that check the Layer 3 information in the fragment (including protocol type, such as TCP,
UDP, and so on) are considered to match the fragment regardless of what the missing Layer 4 information
might have been.
• Deny ACEs that check Layer 4 information never match a fragment unless the fragment contains Layer
4 information.
ACEs and Fragmented and Unfragmented Traffic Examples
Consider access list 102, configured with these commands, applied to three fragmented packets:
Switch(config)# access-list 102 permit tcp any host 10.1.1.1 eq smtp
Switch(config)# access-list 102 deny tcp any host 10.1.1.2 eq telnet
Switch(config)# access-list 102 permit tcp any host 10.1.1.2
Switch(config)#
access-list 102 deny tcp any any
In the first and second ACEs in the examples, the eq keyword after the destination address means to test
for the TCP-destination-port well-known numbers equaling Simple Mail Transfer Protocol (SMTP) and
Telnet, respectively.
Note
• Packet A is a TCP packet from host 10.2.2.2., port 65000, going to host 10.1.1.1 on the SMTP port. If
this packet is fragmented, the first fragment matches the first ACE (a permit) as if it were a complete
packet because all Layer 4 information is present. The remaining fragments also match the first ACE,
even though they do not contain the SMTP port information, because the first ACE only checks Layer
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About Access Control Lists