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I have a misunderstanding, in this article, Figure 4-9, in Transport Mode, first header (IPsec Hdr), is correct? Should not be GRE IP Hdr?

https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/V3PN_SRND/V3PN_SRND/v3p_plan.html

https://www.cisco.com/c/dam/en/us/td/i/000001-100000/80001-85000/81001-82000/81613.ps/_jcr_content/renditions/81613.jpg

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer. – Ron Maupin 4 hours ago
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No, it should not be GRE.

In both cases as shown (IPSec tunnel mode and transport mode), it's always IPsec encrypting GRE (and whatever it may contain).

Therefore, the outermost header has to be IPSec.

  • Let me understand that this image is wrong researchgate.net/profile/Faycal_Bensalah/publication/316968574/… – Lucian Jun 11 at 8:48
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    @Lucian: please edit the original question to include this partial question, and the link. I cannot tell for sure if attaching these drawing to the question (possibly as one single image file) is violating copyright rules (even if you quote and attribute them correctly) - but it would certainly help to understand the question - also for stachexchange visitors searching through the archives long after the original links have gone stale. – Marc 'netztier' Luethi Jun 11 at 11:19

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