Administrator Guide

Implementing ACLs on Dell Networking OS
You can assign one IP ACL per interface with Dell Networking OS. If you do not assign an IP ACL to an interface, it is not used
by the software in any other capacity.
The number of entries allowed per ACL is hardware-dependent. For detailed specification on entries allowed per ACL, refer to
your line card documentation.
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
disturbed. This is applicable to the following features:
● L2 Ingress Access list
● L2 Egress Access list
NOTE: IP ACLs are supported over VLANs in Dell Networking OS version 6.2.1.1 and higher.
ACLs and VLANs
There are some differences when assigning ACLs to a VLAN rather than a physical port.
For example, when using a single port-pipe, if you apply an ACL to a VLAN, one copy of the ACL entries is installed in the ACL
CAM on the port-pipe. The entry looks for the incoming VLAN in the packet. Whereas if 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.
When you use the log keyword, the CP has to log the details about the packets that match. Depending on how many packets
match the log entry and at what rate, the CP might become busy as it has to log these packets’ details. However, the other
processors (RP1 and RP2) are unaffected. This option is typically useful when debugging some problem related to control
traffic. We have used this option numerous times in the field and have not encountered problems so far.
ACL Optimization
If an access list contains duplicate entries, Dell 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 whether it is
identified as 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 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 such as these, 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 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
Dell(conf)#ip access-list standard acl1
Dell(config-std-nacl)#permit 20.0.0.0/8
Dell(config-std-nacl)#exit
Dell(conf)#ip access-list standard acl2
Dell(config-std-nacl)#permit 20.1.1.0/24 order 0
Dell(config-std-nacl)#exit
Dell(conf)#class-map match-all cmap1
Access Control Lists (ACLs)
81