Acl Matching Order - H3C S5600 SERIES Operation Manual

Layer 2 ACL. Rules are created based on the Layer 2 information such as source and
destination MAC addresses, VLAN priorities, type of Layer 2 protocol, and so on.
User-defined ACL. An ACL of this type matches packets by comparing the strings
retrieved from the packets with specified strings. It defines the byte it begins to perform
"and" operation with the mask on the basis of packet headers.

ACL Matching Order

An ACL can contain multiple rules, each of which matches specific type of packets. So the
order in which the rules of an ACL are matched needs to be determined.
The rules in an ACL can be matched in one of the following two ways:
config: where rules in an ACL are matched in the order defined by the user.
auto: where rules in an ACL are matched in the order determined by the system,
namely the "depth-first" rule (Layer 2 ACLs and user-defined ACLs do not support this
feature).
For depth-first rule, there are two cases:
Depth-first match order for rules of a basic ACL
1) Range of source IP address: The smaller the source IP address range (that is, the more
the number of zeros in the wildcard mask), the higher the match priority.
2) Fragment keyword: A rule with the fragment keyword is prior to others.
3) If the above two conditions are identical, the earlier configured rule applies.
Depth-first match order for rules of an advanced ACL
1) Protocol range: A rule which has specified the types of the protocols carried by IP is
prior to others.
2) Range of source IP address: The smaller the source IP address range (that is, the more
the number of zeros in the wildcard mask), the higher the match priority.
3) Range of destination IP address. The smaller the destination IP address range (that is,
the more the number of zeros in the wildcard mask), the higher the match priority.
4) Range of Layer 4 port number, that is, TCP/UDP port number. The smaller the range,
the higher the match priority.
5) Number of parameters: the more the parameters, the higher the match priority.
If rule A and rule B are still the same after comparison in the above order, the weighting
principles will be used in deciding their priority order. Each parameter is given a fixed
weighting value. This weighting value and the value of the parameter itself will jointly
decide the final matching order. Involved parameters with weighting values from high to
low are icmp-type, established, dscp, tos, precedence, fragment. Comparison rules are
listed below.
The smaller the weighting value left, which is a fixed weighting value minus the
weighting value of every parameter of the rule, the higher the match priority.
If the types of parameter are the same for multiple rules, then the sum of parameters'
weighting values of a rule determines its priority. The smaller the sum, the higher the
match priority.
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