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Configuring IPv4 ACLs
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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
Note
for the TCP-destination-port well-known numbers equaling Simple Mail Transfer Protocol (SMTP) and
Telnet, respectively.
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Configuring IPv4 ACLs
Configuring IP v4ACLs on the switch is the same as configuring IPv4 ACLs on other Cisco switches and
routers. The process is briefly described here. For more detailed information on configuring ACLs, see
the "Configuring IP Services" section in the "IP Addressing and Services" chapter of the Cisco IOS IP
Configuration Guide, Release 12.2. For detailed information about the commands, see the Cisco IOS IP
Command Reference, Volume 1 of 3: Addressing and Services, Release 12.2. The Cisco IOS
documentation is available from the Cisco.com page under Documentation > Cisco IOS Software >
12.2 Mainline > Configuration Guides or Command References.
The switch does not support these Cisco IOS router ACL-related features:
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Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
31-6
Deny ACEs that check Layer 4 information never match a fragment unless the fragment contains
Layer 4 information.
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 3 information when applied to fragments. The information in this example is that the
packet is TCP and that the destination is 10.1.1.1.
Packet B is from host 10.2.2.2, port 65001, going to host 10.1.1.2 on the Telnet port. If this packet
is fragmented, the first fragment matches the second ACE (a deny) because all Layer 3 and Layer 4
information is present. The remaining fragments in the packet do not match the second ACE because
they are missing Layer 4 information. Instead, they match the third ACE (a permit).
Because the first fragment was denied, host 10.1.1.2 cannot reassemble a complete packet, so packet
B is effectively denied. However, the later fragments that are permitted will consume bandwidth on
the network and resources of host 10.1.1.2 as it tries to reassemble the packet.
Fragmented packet C is from host 10.2.2.2, port 65001, going to host 10.1.1.3, port ftp. If this packet
is fragmented, the first fragment matches the fourth ACE (a deny). All other fragments also match
the fourth ACE because that ACE does not check any Layer 4 information and because Layer 3
information in all fragments shows that they are being sent to host 10.1.1.3, and the earlier permit
ACEs were checking different hosts.
Non-IP protocol ACLs (see
IP accounting
Inbound and outbound rate limiting (except with QoS ACLs)
Reflexive ACLs or dynamic ACLs
ACL logging for port ACLs and VLAN maps
Table 31-1 on page
31-8) or bridge-group ACLs
Chapter 31
Configuring Network Security with ACLs
OL-8915-03
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