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 active switch, process the
information and program their hardware.
Active Switch and ACL Functions
The active switch 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 active switch forwards the packets only after applying ACLs on the packets.
•
It programs its hardware with the ACL information it processes.
Stack Member and ACL Functions
Stack members perform these ACL functions:
•
They receive the ACL information from the active switch and program their hardware.
•
A stack member configured as a standby switch, performs the functions of the active switch in the event
the active switch fails.
Active Switch Failure and ACLs
Both the active and standby switches have the ACL information. When the active switch fails, the standby
takes over. The new active switch distributes the ACL information to all stack members.
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Configuring IPv4 ACLs
ACLs and Switch Stacks