Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.14.1.0 - Page 115

If counters are enabled on ACL rules that are already configured, those counters are reset when a new rule which is inserted or prepended or appended requires a hardware shift in the flow table. Resetting the counters to 0 is transient as the proginal counter values are retained after a few seconds. If there is no need to shift the flow in the hardware, the counters are not affected. This is applicable to the following features: • L2 Ingress Access list • L2 Egress Access list In the Dell EMC Networking OS versions prior to 9.13(0.0), the system does not install any of your ACL rules if the available CAM space is lesser than what is required for your set of ACL rules. Effective with the Dell EMC Networking OS version 9.13(0.0), the system installs your ACL rules until all the allocated CAM memory is used. If there is no implicit permit in your rule, the Dell EMC Networking OS ensures that an implicit deny is installed at the end of your rule. This behavior is applicable for IPv4 and IPv6 ingress and egress ACLs. NOTE: System access lists (system-flow entries) are pre-programmed in the system for lifting the control-plane packets destined for the local device which the CPU needs to process. The system access lists always override the user configured access lists. Even if you configure ACL to block certain hosts, control plane protocols such as, ARP, BGP, LACP, VLT, VRRP and so on, associated with such hosts cannot be blocked. Assigning ACLs to VLANs When you apply an ACL to a VLAN using single port-pipe, a copy of the ACL entries gets installed in the ACL CAM on the port-pipe. The entry looks for the incoming VLAN in the packet. When you apply an ACL on individual ports of a VLAN, separate copies of the ACL entries are installed for each port belonging to a port-pipe. You can use the log keyword to log the details about the packets that match. The control processor becomes busy based on the number of packets that match the log entry and the rate at which the details are logged in. However, the route processor (RP) is unaffected. You can use this option for debugging issues related to control traffic. ACL Optimization If an access list contains duplicate entries, Dell EMC Networking OS deletes one entry to conserve CAM space. Standard and extended ACLs take up the same amount of CAM space. A single ACL rule uses two CAM entries to identify whether the access list is a standard or extended ACL. Determine the Order in which ACLs are Used to Classify Traffic When you link class-maps to queues using the service-queue command, Dell EMC Networking OS matches the class-maps according to queue priority (queue numbers closer to 0 have lower priorities). As shown in the following example, class-map cmap2 is matched against ingress packets before cmap1. ACLs acl1 and acl2 have overlapping rules because the address range 20.1.1.0/24 is within 20.0.0.0/8. Therefore (without the keyword order), packets within the range 20.1.1.0/24 match positive against cmap1 and are buffered in queue 7, though you intended for these packets to match positive against cmap2 and be buffered in queue 4. In cases where class-maps with overlapping ACL rules are applied to different queues, use the order keyword to specify the order in which you want to apply ACL rules. The order can range from 0 to 254. Dell EMC Networking OS writes to the CAM ACL rules with lowerorder numbers (order numbers closer to 0) before rules with higher-order numbers so that packets are matched as you intended. By default, all ACL rules have an order of 255. Example of the order Keyword to Determine ACL Sequence DellEMC(conf)#ip access-list standard acl1 DellEMC(config-std-nacl)#permit 20.0.0.0/8 DellEMC(config-std-nacl)#exit DellEMC(conf)#ip access-list standard acl2 DellEMC(config-std-nacl)#permit 20.1.1.0/24 order 0 Access Control Lists (ACLs) 115