H3C Technologies H3C SecBlade NetStream Cards - Manual (Page 145)

130
Match order
The rules in an ACL are sorted in a specific order. When a packet matches a rule, the device stops the
match process and performs the action defined in the rule. If an ACL contains overlapping or conflicting
rules, the matching result and action to take depend on the rule order.
The following ACL match orders are available:
• config—Sorts ACL rules in ascending order of rule ID. A rule with a lower ID is matched before a
rule with a higher ID. If you use this approach, carefully check the rule content and order.
• auto—Sorts ACL rules in depth-first order. Depth-first ordering ensures that any subset of a rule is
always matched before the rule.
Table 11 lists the sequence of tie breakers that depth-first ordering
uses to sort rules for each type of ACL.
Table 11 Sort ACL rules in depth-first order
ACL category Sequence of tie breakers
IPv4 basic ACL
1. More 0s in the source IP address wildcard (more 0s means a narrower IP
address range)
2. Smaller rule ID
IPv4 advanced ACL
1. Specific protocol type rather than IP (IP represents any protocol over IP)
2. More 0s in the source IP address wildcard mask
3. More 0s in the destination IP address wildcard
4. Narrower TCP/UDP service port number range
5. Smaller rule ID
Ethernet frame header ACL
1. More 1s in the source MAC address mask (more 1s means a smaller MAC
address)
2. More 1s in the destination MAC address mask
3. Smaller rule ID
NOTE:
A wildcard mask, also called an "inverse mask," is a 32-bit binary and represented in dotted decimal
notation. In contrast to a network mask, the 0 bits in a wildcard mask represent 'do care' bits, and the 1
bits represent 'don’t care' bits. If the 'do care' bits in an IP address are identical to the 'do care' bits in an
IP address criterion, the IP address matches the criterion. All 'don’t care' bits are ignored. The 0s and 1s
in a wildcard mask can be noncontiguous. For example, 0.255.0.255 is a valid wildcard mask.
ACL rule numbering
What is the ACL rule numbering step
If you do not assign an ID for the rule you are creating, the system automatically assigns it a rule ID. The
rule numbering step sets the increment by which the system automatically numbers rules. For example, the
default ACL rule numbering step is 5. If you do not assign IDs to rules you are creating, they are
numbered 0, 5, 10, 15, and so on. The wider the numbering step, the more rules you can insert between
two rules.
By introducing a gap between rules rather than contiguously numbering rules, you have the flexibility of
inserting rules in an ACL. This feature is important for a config order ACL, where ACL rules are matched
in ascending order of rule ID.