[clug] Linux routing problem - two interfaces on same media segment, one seems isolated

steve jenkin sjenkin at canb.auug.org.au
Sun Aug 24 08:32:36 GMT 2008

Daniel Pittman wrote on 24/8/08 1:33 PM:

> Wow.  That is a nasty situation to inherit, because it is almost
> certainly /never/ going to work the way whoever built it originally
> expected.
>> I've setup systems before with multiple interfaces and this should be
>> easy... I'm missing something, but don't know what. 
> There is probably one key thing you are missing if you have two
> interfaces on a single physical segment:
> Linux implements the "weak end host model", in which the addresses of
> the system are a property of the *host*, not of the *interface*.  
> This means, for example, that it is going to ARP for all your IP
> addresses on a single physical device per physical segment.
> This also means that you are not going to get, for your machine, any of
> load balancing, improved performance, or improved reliability, by
> connecting multiple physical NICs to a single physical segment.
> (Without additional effort.)

Thanks - that's a good explanation for what I see.
Also says I'm stuck with the situation.

> Sure: either pull one of the physical cards, so you only have a single
> card per physical segment, or use bonding so that the cards can both
> function as if they were a single physical card for the segment.

I want to put the private and public IP networks on separate switches -
"air gapped" in fact.

Don't care about improving performance or redundancy :-)

> So ... if you want improved performance or redundancy, look into bonding
> and ensure that your switches support it.
> If you want a simple life then pull one of the cards physically out of
> the segment.
> Regards,
>         Daniel
> No, seriously, most anything else is just going to make you sad.

=> That's great advice. Thanks.

Steve Jenkin, Info Tech, Systems and Design Specialist.
0412 786 915 (+61 412 786 915)
PO Box 48, Kippax ACT 2615, AUSTRALIA

sjenkin at canb.auug.org.au http://members.tip.net.au/~sjenkin

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