Brocade Communications Systems Brocade ICX 6650 6650 - Manual (Page 104)

84 Brocade ICX 6650 Security Configuration Guide
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How hardware-based ACLs work
Default ACL action
The default action when no ACLs are configured on a device is to permit all traffic. However, after
you configure an ACL and apply it to a port, the default action for that port is to deny all traffic that
is not explicitly permitted on the port:
• If you want to tightly control access, configure ACLs consisting of permit entries for the access
you want to permit. The ACLs implicitly deny all other access.
• If you want to secure access in environments with many users, you might want to configure
ACLs that consist of explicit deny entries, then add an entry to permit all access to the end of
each ACL. The software permits packets that are not denied by the deny entries.
How hardware-based ACLs work
When you bind an ACL to inbound or outbound traffic on an interface, the device programs the
Layer 4 CAM with the ACL. Permit and deny rules are programmed. Most ACL rules require one
Layer 4 CAM entry. However, ACL rules that match on more than one TCP or UDP application port
may require several CAM entries. The Layer 4 CAM entries for ACLs do not age out. They remain in
the CAM until you remove the ACL:
• If a packet received on the interface matches an ACL rule in the Layer 4 CAM, the device
permits or denies the packet according to the ACL.
• If a packet does not match an ACL rule, the packet is dropped, since the default action on an
interface that has ACLs is to deny the packet.
How fragmented packets are processed
The descriptions above apply to non-fragmented packets. The default processing of fragments by
hardware-based ACLs is as follows:
• The first fragment of a packet is permitted or denied using the ACLs. The first fragment is
handled the same way as non-fragmented packets, since the first fragment contains the Layer
4 source and destination application port numbers. The device uses the Layer 4 CAM entry if
one is programmed, or applies the interface's ACL entries to the packet and permits or denies
the packet according to the first matching ACL.
• For other fragments of the same packet, they are subject to a rule only if there is no Layer 4
information in the rule or in any preceding rules.
The fragments are forwarded even if the first fragment, which contains the Layer 4 information,
was denied. Generally, denying the first fragment of a packet is sufficient, since a transaction
cannot be completed without the entire packet.
For tighter control, you can configure the port to drop all packet fragments. Refer to “Enabling strict
control of ACL filtering of fragmented packets” on page 108.
Hardware aging of Layer 4 CAM entries
Rule-based ACLs use Layer 4 CAM entries. The device permanently programs rule-based ACLs into
the CAM. The entries never age out.