Prior to ExtremeXOS 16.1, when two user rules in two separate slices are matched by a packet, the non-conflicting actions from both of the rules are executed. This feature allows you to put all user rules into a single virtual group. When rules are in a single virtual group, even when two rules in two separate virtual slices are matched, only the actions of the highest precedence rule are executed. In effect, in this mode, multiple slices behave as a big single virtual slice.
Normally ACL hardware works in the following way: on arrival of a packet, all N slices are searched in parallel to find a possible match in each of these N slices. In each slice, upon finding the first match, the search within that slice stops. In other slices the search continues until a match is found or the end of the slice is reached. Thus, a single slice can produce only a single match, but all N slices combined together can produce up to N matches for a given packet.
In the case of multiple matches in multiple slices, all the actions of the rule in the highest priority virtual slice are executed. In addition, all the actions from the lower priority rules from the lower priority virtual slices are executed if those additional actions do not conflict with the actions of the highest priority rule. An example of non-conflicting with each other actions would be "permit" and "count". An example of conflicting with each other actions would be "permit" and "deny".
However, in more recent chipsets a new mode of operation was introduced where you can combine a few virtual slices into one big virtual group. In this mode of operation, even if a packet gets multiple matches from multiple virtual slices within the same virtual group, only the actions of the highest priority rule are executed, whereas the actions from the lower priority rules are not executed at all, even if those actions do not conflict with the actions of the highest priority rule.
This feature allows choosing between the old way of operation where every virtual slice is in its own virtual group and multiple matches are possible, and the new way, where all user ACL‘s virtual slices are in the same virtual group and multiple matches are not possible.
Note
Some platforms that do not support virtual groups. On those chipsets even if single virtual group feature is enabled, ACL will operate in the old way and multiple matches would be still possible.