netdev - Re: Debugging Ethernet issues

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Date:   Sun, 13 Nov 2016 04:09:19 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Mason <slash.tmp@...e.fr>
Cc:     netdev <netdev@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Mans Rullgard <mans@...sr.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Zach Brown <zach.brown@...com>,
        Shaohui Xie <shaohui.xie@....com>,
        Tim Beale <tim.beale@...iedtelesis.co.nz>,
        Brian Hill <brian@...ston-radar.com>,
        Vince Bridgers <vbridgers2013@...il.com>,
        Balakumaran Kannan <kumaran.4353@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Sebastian Frias <sf84@...oste.net>,
        Kirill Kapranov <kapranoff@...ox.ru>
Subject: Re: Debugging Ethernet issues

> When connected to a Gigabit switch
> 3.4 negotiates a LAN DHCP setup instantly
> 4.7 requires over 5 seconds to do so

When you run tcpdump on the DHCP server, are you noticing the first
request is missing?

What can happen is the dhclient gets started immediately and sends out
its first request before auto-negotiation has finished. So this first packet
gets lost. The retransmit after a few seconds is then successful.

     Andrew