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I need to create a virtual PPP connection that I can configure and capture into PCAP files. The goal is to set up a PPP link with protocol-field-compression and FCS-alternatives options in LCP (the FCS should be visible in Wireshark), I also need multilink support.

Using GNS3 I’ve run into two problems:

  1. There is no FCS checksum.
  2. I cannot configure LCP options (for example fcs-alternatives).

How can I solve my problem in GNS3?

UPD:

I tried creating a network directly with pppd pty on my system, but the problem is that I can only capture traffic after the LCP handshake, which doesn’t suit my needs.

In GNS I set up a connection between two C3600 devices and established a PPP link, but there’s no way to configure LCP flags, and the FCS is missing (I assume it’s being stripped).

My ultimate goal is to be able to create a PPP connection with arbitrary LCP settings and capture the pcap from the very beginning, including the handshake.enter image description here

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  • What have you tried so far? StackOverflow expects you to try to solve your own problem first, as your attempts help us to better understand what you want. Please edit the question to show what you've tried, and show a specific roadblock you're running into with a minimal reproducible example. For more information, please see How to Ask. Commented Aug 20, 2025 at 14:26
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