Cisco Systems 2960 Handling Fragmented and Unfragmented Traffic

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Chapter28 Configuring Network Securi ty with ACLs
Understanding ACLs
Figure28-1 Using ACLs to Control Traffic to a Network
When you apply a port ACL to a trunk port, the ACL filters traffic on all VLANs present on the trunk
port. When you apply a port ACL to a port with voice VLAN, the ACL filters traffic on both da ta and
voice VLANs.
With port ACLs, you can filter IP traffic by using IP access lists and non-IP traffic by using MAC
addresses. You can filter both IP and non-IP traffic on the same Layer 2 interface by applying both an IP
access list and a MAC access list to the interface.
Note You cannot apply more than one IP access list and one MAC access list to a Layer 2 interfac e. If an IP
access list or MAC access list is already configured on a Layer 2 interface and you apply a new IP access
list or MAC access list to the interface, the new ACL replaces the previously configured one.
Handling 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 po rt
numbers, ICMP type and code, and so on. All other fragm ents are missing this information.
Some ACEs do not check Layer 4 information and therefore can be applied to all pa cket 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 infor mation 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.
Host A
Host B
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= ACL denying traffic from Host B
and permitting traffic from Host A
= Packet
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