Cisco documents a feature called “DSCP Preservation” in regards to ToS Byte handling on the ASA. This basically means that the ToS Byte is left in tact as packets flow through the firewall. But what happens to the ToS Byte when a VPN is in use? The video below demonstrates that the ASA actually copies the original value found in the ToS byte into the new outer IP header for transport across the public network.
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