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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 25 Configuring Network Securi t y w it h ACLs Understa nding ACLs
Figure 25-1 Using ACLs to Control Traffic to a Network

Handling Fragmented and Unfragmented Traffic

IP packets can be fragment ed as they cross the ne twork. Whe n this happens, only the fr agme nt
conta ining th e beginning of the packet co nta ins the Laye r 4 infor mation, suc h as TCP or UD P port
numbers, In tern et C ontro l Messa ge Pro to col (IC MP) t ype a nd co de, an d so on. A ll ot her frag me nts a re
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
fragme nted IP packet. Whe n the frag ment con tains no Layer 4 info rmat ion and the ACE tests some
Layer 4 infor mat ion, the matchi n g rul es a re m odi fied:
•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
inform ation mi ght have been.
•Deny ACEs that check Layer 4 information never match a fragment unless the fragment contains
Layer 4 info rmat ion.
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 key word after the desti nation address means to test
for the TCP-destination-port well-kno wn numbers equaling Simple Mail Transfer Protocol (SMTP) and
Telnet, respectively.
Host A
Host B
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= ACL denying traffic from Host B
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