Re: [lwip-users] IP6 address autoconfig problem when using hub

[Zach Smith]

Hmmm, I didn't know that. Now you have me wondering what is going on then in my 
setup.  I have a switch connected to my LAN.  Then a hub connected directly to 
the switch.  Then my IP6 device connected directly to the hub.  There are no 
other devices in the picture.  
(LAN)----(switch)----(hub)-----(my IP6 device)

When I connect the IP6 device to the switch it works fine, when I connect to 
the hub, the IP6 address chosen for auto-configuration is marked as invalid 
because of a DAD response.


-----Original Message-----
From: address@hidden [mailto:address@hidden On Behalf Of Sylvain Rochet
Sent: Monday, December 10, 2012 10:12 AM
To: Mailing list for lwIP users
Subject: Re: [lwip-users] IP6 address autoconfig problem when using hub

Hi,

On Sun, Dec 09, 2012 at 10:53:58AM +0100, Simon Goldschmidt wrote:
> Zach Smith wrote:
> 
> > I am having a problem when I connect my IPv6 device running lwip to 
> > a hub instead of a switch. [..] Since a hub forwards all packets 
> > received onto all ports it is echoing my devices neighbor discovery 
> > messages back.
> 
> I haven't had the chance to test lwIP on a hub, yet (we only have 
> switches, all hubs were thrown away a long time ago...). However, if 
> your hub really sends back all your frames, I guess IPv6 autoconfig 
> will not be the only protocol having problems.

Added to that, a hub being half duplex by design is not supposed to bounce back 
packets, because it can only have one RX port at a time while all other should 
be TX ports (or there is a collision).

So, a hub DOES NOT forwards all packets received onto all ports, a hub DOES 
forwards all packets received onto all ports EXCEPT the sender port, if this is 
not the case, this is because there is an Ethernet loop somewhere or a faulty 
device that should be fixed/replaced.

Anyway, a switch without a populated MAC forwarding table is working like a hub 
and bounce packets to all ports except the sender' one.

Sylvain