Cisco NM-8B-U - HW ROUTERS L-M User Manual page 27

16- and 36-Port Ethernet Switch Module for Cisco 2600 Series, Cisco 3600 Series, and Cisco 3700 Series
Figure 13
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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 port
numbers, ICMP type and code, and so on. All other fragments are missing this information.
Some 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.
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 deny tcp any any
Note 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.
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
Using ACLs to Control Traffic to a Network
Cisco router with
Ethernet switch
network module
Human
network
= ACL denying traffic from Host B
and permitting traffic from Host A
= Packet
Host A
Host B
Research &
Development
network
Cisco IOS Release 12.2(2)XT, 12.2(8)T, and 12.2(15)ZJ
Feature Overview
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