This value is the external ifIndex that inbound packets matching this ACL rule are copied (mirrored) to, while still continuing along their normal forwarding path. Admins set it to capture specific traffic for analysis, for example mirroring traffic from a suspect source to a packet-capture appliance without disrupting the original flow. A value of zero (the default) means mirroring is off, and mirroring is mutually exclusive with redirection on the same rule via aclRuleRedirectIntf.