netdev - Re: [Query]: DSA Understanding

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Message-ID: <20180802144550.GE7462@lunn.ch>
Date:   Thu, 2 Aug 2018 16:45:50 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [Query]: DSA Understanding

> I have PC connected to lan4(ip = 169.254..126.126) and the PC ip is
> 169.254.78.251,
> but when I ping from PC to lan4 I get Destination Host Unreachable,
> but where as I can see
> that in the tcpdump log for lan4 it does reply back, but it doesn’t
> reach the PC, Is there I am missing
> something here ?
> 
> ~$ tcpdump -i lan4 -v
> [  661.057166] device lan4 entered promiscuous mode
> [  661.061814] device eth1 entered promiscuous mode

> 07:40:21.254161 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has
> VB4-SN00000000 tell tango-charlie.local, length 46
> 07:40:21.254181 ARP, Ethernet (len 6), IPv4 (len 4), Reply
> VB4-SN00000000 is-at c4:f3:12:08:fe:7f (oui Unknown), length 28

Having names here does not help when you gave IP addresses above.

Am i reading this correct? The PC is ARPing the switch device. The
switch device is replying?

What does tcpdump on the PC show? Are the ARP replies getting to it?
Is the PC dropping the ARP replies?

If the PC is dropping the ARP replies, take a look at the
checksums. Wireshark is good at that.

If the ARP replies are not making it to the PC, look at the switch
statistics. ethtool -S lan4. Are the TX counts going up? Any error
counts going up?

       Andrew