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B-9
Cisco Secure Router 520 Series Software Configuration Guide
OL-14210-01
Appendix B Concepts
Access Lists
Access Lists
With basic standard and static extended access lists, you can approximate session filtering by using the
established keyword with the permit command. The established keyword filters TCP packets based on
whether the ACK or RST bits are set. (Set ACK or RST bits indicate that the packet is not the first in the
session and the packet therefore belongs to an established session.) This filter criterion would be part of
an access list applied permanently to an interface.