Concept Guide

If counters are enabled on ACL rules that are already congured, those counters are reset when a new rule which is inserted or prepended
or appended requires a hardware shift in the ow 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 ow in the hardware, the counters are not aected. 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. Eective 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-ow 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 congured
access lists. Even if you congure 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 unaected. You
can use this option for debugging issues related to control trac.
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 Trac
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 buered in queue 7, though you intended for these
packets to match positive against
cmap2 and be buered in queue 4.
In cases where class-maps with overlapping ACL rules are applied to dierent 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 lower-
order 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)
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