Users Guide

ACL Optimization
If an access list contains duplicate entries, the system 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, the system 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. The system 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 254.
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
Dell(conf-class-map)#match ip access-group acl1
Dell(conf-class-map)#exit
Dell(conf)#class-map match-all cmap2
Dell(conf-class-map)#match ip access-group acl2
Dell(conf-class-map)#exit
Dell(conf)#policy-map-input pmap
Dell(conf-policy-map-in)#service-queue 7 class-map cmap1
Dell(conf-policy-map-in)#service-queue 4 class-map cmap2
Dell(conf-policy-map-in)#exit
Dell(conf)#interface tengig 1/0
Dell(conf-if-te-1/0)#service-policy input pmap
IP Fragment Handling
The system 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.
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Access Control Lists (ACLs)