Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 124

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. 124 Access Control Lists (ACLs)