[ewg] Multiple IP subnets in a single partition

[Or Gerlitz]

On 10/12/2011 3:28 PM, richard Croucher wrote:
> I understand that's it not good practice however I'm seeking to 
> understand whether  actual problems have been observed.
>
> The only issues I can suggest will be because of ARP is in the shared 
> broadcast domain.    Is there any IPoIB state in the SM other  than 
> QoS?   I can't think of any reason why there should be. Even though 
> ARP requests will be seen  by interfaces in a different subnet, they 
> should not respond with it's GUID since they will not match the 
> requested IP address.

wrong, see 
http://lxr.linux.no/#linux+v3.0/Documentation/networking/ip-sysctl.txt#L926

>
> I think this refers back to behaviour seen many years ago, when 
> multihomed hosts were rare.  There was a tendency for them to respond 
> on  all interfaces to ARP requests for their nodename and  cause ARP 
> resolution problems..  Is there a reproducable test case of this 
> problem, since I certainly know of systems which are configured like 
> this and appear to be working fine.  Maybe, they've just been lucky, 
> but so far I've seen numerous messages saying don't do it and none to 
> say what actually goes wrong.

For basic testing and/or PoC you can set net.ipv4.conf.*.arp_ignore  to 
1 or alike (2). For production, I wouldn't
do that or at least do it after conducting a deeper study (I gave you 
the heads-up, so please share your findings...),
Indeed, I know that in the iscsi multipathing world people use 
multi-homed NICs on the same IP subne, though.

Or.


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openfabrics.org/pipermail/ewg/attachments/20111016/4ce7cf6a/attachment.html>