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ACL memory usage on ACE module





ACL memory on the ACE module is used for programming traffic processing policy in the data plane. ACLs allow users to control network connection setups rather than processing each packet. The ACE supports a maximum of 8,192 unique ACLs and 64,000 ACL entries. Some ACLs use more memory than others, such as an ACL that uses large port number ranges or overlapping networks. Even using object groups in ACL entries, you enter fewer actual ACL entries, but the same number of expanded ACL entries as you did when you entered entries without object groups. Expanded ACL entries count toward the system limit. Complicated ACL, NAT and load balancing configurations can exponentially increase it's usage to the point that new configuration changes won't be able to fit into the available memory.



ACL memory overview and resource allocation


ACL memory consists of different type of nodes - Compressed, Uncompressed, Leaf Head, Leaf Parameter and Policy action nodes. In a system we have fixed number of those various nodes. For example, ACE has 262143 compressed nodes, 19999 uncompressed nodes, 204799 Leaf Head Nodes, 409600 Leaf Parameter Nodes and 204800 Policy Action nodes. “Show np 1 access-list resource” command from *Admin* context will show the max-limit for each type of nodes except the Policy Action nodes. When users configure ACL resources such as min 10% and max *equal to min*, ACE guarantees 10% of each nodes, except action nodes, to that resource class. What that means is ACE configurations for that resource class need to be accommodated with 10% of each node. If any of the nodes consumption goes beyond the 10% limit, ACL resource allocation would fail even though the other nodes usages are well below 10%.



Recommendation for ACL resource allocation


ACL nodes usages depend on the configurations. It doesn’t depend on the number of lines of configurations and also it doesn’t increase linearly with the number of lines of configurations. Nodes allocation for a given configurations is handled by ACL compiler using a complex data structure and it is extremely difficult to calculate the node usage for a large configurations before applying the configurations. So the recommendation would be


  1. Apply the configurations on the system.
  2. Find the maximum used node, in terms of percentage, for the applied configuration from the “show np 1 access-list resource” command.
  3. While configuring ACL resource, make sure the max resource percentage is above the percentage calculated in step 2.



Reducing ACL memory usage


One common cause for high ACL memory usage is per server NAT ACLs, where a single line of ACL is written for each server that must be processed by a policy. As this policy gets applied to multiple interfaces, across multiple policies, it begins to grow exponentially and uses up a lot of ACL memory. One method to reduce the usage is to summarize ACL entries, so instead of having an ACL for server,, and, these can be summarized into, reducing 4 entries to 1.


Another method to reduce the exponential growth of ACL memory usage is to break up large single context configurations into smaller multiple contexts. Following the simplified math, if a single context has 8*8 entries, that may expand to 64 total used, whereas if that configuration is broken down into two contexts, both containing 4*4, the total usage is 16+16=32. This math is greatly simplified and it's much more difficult to calculate the actual ACL memory usage, however is meant to demonstrate the logic behind exponential vs smaller linear growth.


You can instruct the ACE to check the health of servers and server farms by configuring health probes (sometimes referred to as keepalives). After you create a probe, you assign it to a real server or a server farm. A probe can be one of many types, including TCP, ICMP, Telnet, HTTP, and so on. The ACE sends out probes periodically to determine the status of a server, verifies the server response, and checks for other network problems that may prevent a client from reaching a server. Based on the server response, the ACE can place the server in or out of service, and, based on the status of the servers in the server farm, can make reliable load-balancing decisions.


Helpful Commands

Use "show resource usage" command, in Admin context, to display resource utilization in your ACE and check acl-memory usage.


ACE_module/Admin# show resource usage
        Resource         Current       Peak        Min        Max       Denied
Context: Admin
  conc-connections             10         18          0    8000000          0
  mgmt-connections              2         10          0     100000          0

  ---------- snip ------------

acl-memory                33448      33448    7858944   70749384          0                           
  regexp                        0          0          0    1048576          0
 ---------- snip -------------

Use "limit-resource acl-memory" command to allocate ACL resources to all member contexts of a resource class.

ACE_module/Admin# limit-resource acl-memory minimum <percentage> maximum <percentage>




Related Information


Configuring Security Access Control Lists on ACE

Troubleshooting Access Control Lists on Cisco ACE

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