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Because LLC wasn't complicated/annoying enough, there's 2 more "ethertypes" being used for it: - 0x8870 is pretty "normal", it got standardized in 802.1AC-2016/Cor1-2018 for transporting LLC frames > 1500 bytes. It simply replaces the length value (which is no longer encoded, and must now be derived from the packet.) The actual value dates back to 2001; https://datatracker.ietf.org/doc/html/draft-ietf-isis-ext-eth-01 (it was used without "proper" standardization for a long time) - 0x00fe is a doozy - actually "invalid" depending on how you look at it; it's used in GRE (and possibly GENEVE) tunnels to transport the IS-IS routing protocol. https://seclists.org/tcpdump/2002/q4/61 is the best/oldest source I could find. It's inspired by the 0xfe SAP value, a GRE packet with protocol 0x00fe is followed by a payload "as if" it was Ethernet with "<length> 0xfe 0xfe 0x03". (Again the length isn't encoded explicitly anymore.) The 0x00fe value is quite close to other values the kernel is using internally for various things (after all they "won't clash for 1500 types"). Except this one does clash, and if someone unknowingly starts using it for something internal... we end up in a world of pain in getting IS-IS running on GRE tunnels. Hence the "WARNING". Signed-off-by: David 'equinox' Lamparter <equinox@diac24.net> Cc: Andrew Lunn <andrew+netdev@lunn.ch> Link: https://patch.msgid.link/20260605164144.81184-1-equinox@diac24.net Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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