netdev - Re: [Query]: DSA Understanding

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Date:   Thu, 2 Aug 2018 15:58:10 +0100
From:   "Lad, Prabhakar" <prabhakar.csengg@...il.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [Query]: DSA Understanding

Hi Andrew,

On Thu, Aug 2, 2018 at 3:45 PM, Andrew Lunn <andrew@...n.ch> wrote:
>> 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.
>
Sorry about that. VB4-SN00000000 is the switch device and
tango-charlie.local is the PC.

> Am i reading this correct? The PC is ARPing the switch device. The
> switch device is replying?
>
Yes the PC is ARPing to the switch device and from the tcpdump on
swicth device lan4
I can see that its trying to send Reply

~$ tcpdump -i lan4 -v
[ 1675.526326] device lan4 entered promiscuous mode
[ 1675.531503] device eth1 entered promiscuous mode
tcpdump: listening on lan4, link-type EN10MB (Ethernet), capture size
262144 bytes
07:57:06.133853 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has
VB4-SN00000000 tell tango-charlie.local, length 46
07:57:06.133893 ARP, Ethernet (len 6), IPv4 (len 4), Reply
VB4-SN00000000 is-at c4:f3:12:08:fe:7f (oui Unknown), length 28
07:57:07.151100 ARP, Ethernet (len 6), IPv4 (len 4), Request who-has
VB4-SN00000000 tell tango-charlie.local, length 46
07:57:07.151133 ARP, Ethernet (len 6), IPv4 (len 4), Reply
VB4-SN00000000 is-at c4:f3:12:08:fe:7f (oui Unknown), length 28
07:57:07.703063 IP (tos 0x0, ttl 64, id 41824, offset 0, flags [DF],
proto UDP (17), length 224)
    tango-charlie.local.netbios-dgm > 169.254.255.255.netbios-dgm: NBT
UDP PACKET(138)
07:57:07.804780 IP6 (flowlabel 0x42aaa, hlim 255, next-header UDP (17)
payload length: 54) VB4-SN00000000.mdns > ff02::fb.mdns: [udp sum o)
07:57:07.804961 IP (tos 0x0, ttl 255, id 13070, offset 0, flags [DF],
proto UDP (17), length 74)
^C    VB4-SN00000000.mdns > 224.0.0.251.mdns: 0 PTR (QM)? 255.255.[
1689.046815] device lan4 left promiscuous mode
254.169.in-addr.arpa. (46)

7 packets captured
53 packets rec[ 1689.055592] device eth1 left promiscuous mode
eived by filter
40 packets dropped by kernel

> What does tcpdump on the PC show? Are the ARP replies getting to it?
> Is the PC dropping the ARP replies?
>
I dont see any Reply's on the PC with tcpdump on PC

> If the PC is dropping the ARP replies, take a look at the
> checksums. Wireshark is good at that.
>
Nor do I see anything in the wireshark.

> 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?
>
Yes the Tx counts are going up, without any errors, following is the log:

~$ ethtool -S lan4
NIC statistics:
     tx_packets: 499
     tx_bytes: 37105
     rx_packets: 462
     rx_bytes: 34138
     rx_hi: 0
     rx_undersize: 0
     rx_fragments: 0
     rx_oversize: 0
     rx_jabbers: 0
     rx_symbol_err: 0
     rx_crc_err: 0
     rx_align_err: 0
     rx_mac_ctrl: 0
     rx_pause: 0
     rx_bcast: 402
     rx_mcast: 69
     rx_ucast: 0
     rx_64_or_less: 361
     rx_65_127: 45
     rx_128_255: 34
     rx_256_511: 31
     rx_512_1023: 0
     rx_1024_1522: 0
     rx_1523_2000: 0
     rx_2001: 0
     tx_hi: 0
     tx_late_col: 0
     tx_pause: 0
     tx_bcast: 14
     tx_mcast: 196
     tx_ucast: 0
     tx_deferred: 0
     tx_total_col: 0
     tx_exc_col: 0
     tx_single_col: 0
     tx_mult_col: 0
     rx_total: 43248
     tx_total: 33759
     rx_discards: 0
     tx_discards: 0

Cheers,
--Prabhakar