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Re: [lwip-users] Routing and link up/down vs. netif up/down


From: Simon Goldschmidt
Subject: Re: [lwip-users] Routing and link up/down vs. netif up/down
Date: Wed, 4 Jun 2014 16:53:26 +0200

Fabian Koch wrote:
> I have two netifs and they are both in the same subnet.

This is the first problem. A setup like this is not really supported in lwIP. 
We do have additional hooks to enable this, but it's not meant to work 
out-of-the-box.

> [..]
> But when I pull the plug on netif one, all established connections no longer
> work and I also can’t establish new ones, because the ACKs are still tried
> to be sent out on the first netif in ip_route() because even though the link
> is down, the netif is still “up”, so lwip tries to send on that first one.

In recent versions (or in git master only?) you can provide a routing hook 
function to help lwIP implement more sophisticated routing like you need here.

The problem with having 2 netifs in one subnet is that it is not a standard 
configuration and that the behaviour a user expects when configuring lwIP like 
this can largely vary: I'm just guessing you're trying to implement media 
redundancy this way...; but one could also be trying to implement link 
aggregation with that setup, just for an example. This is why we don't really 
want to support it in the core code.

Plus you have to pay attention that anything you change to make your 'two 
netifs in one subnet' work doesn't break the default case of having two netifs 
each in their own subnet.


Simon



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