Cisco MWR 1941-DC - 1941 Mobile Wireless Router, Software Configuration Manual

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Cisco MWR 1941-DC Mobile Wireless Edge Router Software Configuration Guide
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Chapter 6 Configuring the MWR 1941-DC in a Cell Site DCN
Filtering IP Packets Using Access Lists
Enabling Turbo Access Control Lists
The Turbo Access Control Lists (Turbo ACL) feature processes access lists more expediently than
conventional access lists. This feature enables a Cisco router to evaluate ACLs for more expedient packet
classification and access checks.
ACLs are normally searched sequentially to find a matching rule, and ACLs are ordered specifically to
take this factor into account. Because of the increasing needs and requirements for security filtering and
packet classification, ACLs can expand to the point that searching the ACL adds a substantial amount of
time and memory when packets are being forwarded. Moreover, the time taken by the router to search
the list is not always consistent, adding a variable latency to the packet forwarding. A high CPU load is
necessary for searching an access list with several entries.
The Turbo ACL feature compiles the ACLs into a set of lookup tables, while maintaining the first match
requirements. Packet headers are used to access these tables in a small, fixed number of lookups,
independently of the existing number of ACL entries. The benefits of this feature include the following:
•
For ACLs larger than three entries, the CPU load required to match the packet to the predetermined
packet-matching rule is lessened. The CPU load is fixed, regardless of the size of the access list,
allowing for larger ACLs without incurring any CPU overhead penalties. The larger the access list,
the greater the benefit.
• The time taken to match the packet is fixed, so that latency of the packets is smaller (substantially
in the case of large access lists) and, more importantly, consistent, allowing better network stability
and more accurate transit times.
Note Access lists containing specialized processing characteristics such as evaluate and time-range entries
are excluded from Turbo ACL acceleration.
The Turbo ACL builds a set of lookup tables from the ACLs in the configuration; these tables increase
the internal memory usage, and in the case of large and complex ACLs, tables containing 2 MB to 4 MB
of memory are usually required. Routers enabled with the Turbo ACL feature should allow for this
amount of memory usage. The
show access-list compiled EXEC command displays the memory
overhead of the Turbo ACL tables for each access list.
To configure the Turbo ACL feature, perform the tasks described in the following sections. The task in
the first section is required; the task in the remaining section is optional:
• Configuring Turbo ACLs (Required)
•
Verifying Turbo ACLs (Optional)
Configuring Turbo ACLs
To enable the Turbo ACL feature, use the following command in global configuration mode:
Command Purpose
Router(config)# access-list compiled Enables the Turbo ACL feature.