Depending on platform of PIX, there can be different upperbound limit for # of XLATE entries that the hardware can support.
For a PIX 515E 400 NAT entries should not be much of a problem and 20% memory usage is also quite normal at high usage I have seen memor rise upto 90 % and above. Normal memeory usage for your firewall is around 70-80 %. And usually firewalls can support around 10s of 10000s of NAT/ACL entries. There may have been stress testing done for getting the exact number , but I guess there is no document to testify the figures (datasheets may help)..
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...