White Papers

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 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
DellEMC(config-std-nacl)#exit
DellEMC(conf)#class-map match-all cmap1
DellEMC(conf-class-map)#match ip access-group acl1
DellEMC(conf-class-map)#exit
DellEMC(conf)#class-map match-all cmap2
DellEMC(conf-class-map)#match ip access-group acl2
DellEMC(conf-class-map)#exit
DellEMC(conf)#policy-map-input pmap
DellEMC(conf-policy-map-in)#service-queue 7 class-map cmap1
DellEMC(conf-policy-map-in)#service-queue 4 class-map cmap2
DellEMC(conf-policy-map-in)#exit
DellEMC(conf)#interface te 10/1
DellEMC(conf-if-te-10/1)#service-policy input pmap
IP Fragment Handling
Dell EMC Networking OS supports a configurable option to explicitly deny IP fragmented packets, particularly second and
subsequent packets.
It extends the existing ACL command syntax with the fragments keyword for all Layer 3 rules applicable to all Layer protocols
(permit/deny ip/tcp/udp/icmp).
● Both standard and extended ACLs support IP fragments.
● Second and subsequent fragments are allowed because a Layer 4 rule cannot be applied to these fragments. If the packet is
to be denied eventually, the first fragment would be denied and hence the packet as a whole cannot be reassembled.
● Implementing the required rules uses a significant number of CAM entries per TCP/UDP entry.
● For IP ACL, Dell EMC Networking OS always applies implicit deny. You do not have to configure it.
● For IP ACL, Dell EMC Networking OS applies implicit permit for second and subsequent fragment just prior to the implicit
deny.
● If you configure an explicit deny, the second and subsequent fragments do not hit the implicit permit rule for fragments.
● Loopback interfaces do not support ACLs using the IP fragment option. If you configure an ACL with the fragments
option and apply it to a Loopback interface, the command is accepted but the ACL entries are not actually installed the
offending rule in CAM.
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Access Control Lists (ACLs)