Cisco Systems IE 2000 Handling Fragmented and Unfragmented Traffic

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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 37 Configuring Network Security with ACLs
Information About Network Security with ACLs
The switch examines ACLs associated with all inbound features co nfigured on a given interface and
permits or denies packet forwarding based on how the packet matches the entries in the ACL. In this way,
ACLs control access to a network or to part of a network. Figure 37-1 is an example of using port ACLs
to control access to a network when all workstations are in the same VLAN. ACLs applied at the Layer
2 input would allow Host A to access the Human Resources network, but prevent Host B from accessing
the same network. Port ACLs can only be applied to Layer 2 interfaces in the inbound direction.
Figure 37-1 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.
Host A
Host B
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Research &
Development
network
= ACL denying traffic from Host B
and permitting traffic from Host A
=Packet
Human
Resources
network