Cisco Systems 3750E manual ACLs and Switch Stacks, 34-6

Chapter 34 Configuring Network Security with ACLs

Understanding ACLs

•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 permit tcp any host 10.1.1.2

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 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.

ACLs and Switch Stacks

ACL support is the same for a switch stack as for a standalone switch. ACL configuration information is propagated to all switches in the stack. All switches in the stack, including the stack master, process the information and program their hardware. (For more information about switch stacks, see Chapter 5, “Configuring the Switch Stack.”)

The stack master performs these ACL functions:

•It processes the ACL configuration and propagates the information to all stack members.

•It distributes the ACL information to any switch that joins the stack.

•If packets must be forwarded by software for any reason (for example, not enough hardware resources), the master switch forwards the packets only after applying ACLs on the packets.

•It programs its hardware with the ACL information it processes.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

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