Cisco IOS Software Release 11.3T introduced support for IPSec and along with it the ToS byte preservation feature. With this feature, the router automatically copies the ToS header value from the original IP packet to the encapsulating IP header when the IPSec in tunnel mode is used.
Cisco PIX Firewall version 5.1 and later and VPN 3000 Series Concentrator version 3.5 and later support ToS byte copying. Section 5.1.2, "Header Construction for Tunnel Mode," in RFC 2401 mandates copying the IP ToS bits.
ToS byte preservation also applies to AH. Also note that ESP in transport mode retains the original IP header, and the original ToS value is transmitted even without ToS byte preservation.
If packets arrive at the router without a set IP precedence or DSCP values, you can mark based on class to remark the packet headers before encryption or encapsulation. When the packets reach the egress interface, the QoS output policy can then match and act on the remarked values.
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...