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From: Miles Fidelman on 5 May 2010 21:10
Not sure if this is the right list, sort of a general linux networking
question (pointers to a more appropriate list welcomed)....

Setup:

- I have two servers in a datacenter, currently used for different things

- I have one gigE cable coming in from one of the datacenter's big
routers - that goes into a simple gigE switch - each box is plugged into
that switch

- I have two network /27 network blocks that are NOT contiguous - I use
one for each box

- as I understand the basic setup, any traffic from one of my boxes to
the other (one netblock to the other) end up going to the datacenter's
router and back (and the traffic gets accounted for in our bill)

Up to now, I haven't been routing any traffic between boxes, but I'm
getting ready to install some cluster software and I expect there to be
a lot of inter-box traffic. So....

I'm now looking for a way to have the inter-box traffic go directly
through the gigE switch, and not reach the datacenter's router. Which
leaves me with some questions that are just a bit beyond my general
network setup knowledge:

1. Yes, I have a cross-over cable plugged directly between the 2nd
ethernet card in each box. I plan to dedicate that for disk mirroring
traffic; but I expect I'll end up with things running on one box that
need to talk to the other, that may go through the primary ethernet ports.

2. Is there a way to use ARP and/or set up routing tables so that
inter-box traffic simply goes through the bridge?

3. If not, is this something I can do with a simple Linksys switch/router?

Any guidance would be much appreciated.

Miles Fidelman

--
In theory, there is no difference between theory and practice.
In<fnord> practice, there is. .... Yogi Berra



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From: Anand Sivaram on 5 May 2010 23:00
On Thu, May 6, 2010 at 06:36, Miles Fidelman <mfidelman(a)meetinghouse.net>wrote:

> Not sure if this is the right list, sort of a general linux networking
> question (pointers to a more appropriate list welcomed)....
>
> Setup:
>
> - I have two servers in a datacenter, currently used for different things
>
> - I have one gigE cable coming in from one of the datacenter's big routers
> - that goes into a simple gigE switch - each box is plugged into that switch
>
> - I have two network /27 network blocks that are NOT contiguous - I use one
> for each box
>
> - as I understand the basic setup, any traffic from one of my boxes to the
> other (one netblock to the other) end up going to the datacenter's router
> and back (and the traffic gets accounted for in our bill)
>
> Up to now, I haven't been routing any traffic between boxes, but I'm
> getting ready to install some cluster software and I expect there to be a
> lot of inter-box traffic. So....
>
> I'm now looking for a way to have the inter-box traffic go directly through
> the gigE switch, and not reach the datacenter's router. Which leaves me
> with some questions that are just a bit beyond my general network setup
> knowledge:
>
> 1. Yes, I have a cross-over cable plugged directly between the 2nd ethernet
> card in each box. I plan to dedicate that for disk mirroring traffic; but I
> expect I'll end up with things running on one box that need to talk to the
> other, that may go through the primary ethernet ports.
>
> 2. Is there a way to use ARP and/or set up routing tables so that inter-box
> traffic simply goes through the bridge?
>
> 3. If not, is this something I can do with a simple Linksys switch/router?
>
> Any guidance would be much appreciated.
>
> Miles Fidelman
>
> --
> In theory, there is no difference between theory and practice.
> In<fnord> practice, there is. .... Yogi Berra
>
>
>
> --
> To UNSUBSCRIBE, email to debian-user-REQUEST(a)lists.debian.org with a
> subject of "unsubscribe". Trouble? Contact listmaster(a)lists.debian.org
> Archive: http://lists.debian.org/4BE2162B.8050207(a)meetinghouse.net



Are you using static IP or using dhcp? If you are using static, then you
could try your local netmask from 255.255.255.224 (/27) to 255.255.255.0
(/24) or so in such a way that it encompasses both networks. I think this
is the simplest to start with. If this has problem, you could always alter
the packet using iptables to send it though your second network connection.
From: Kevin Ross on 6 May 2010 00:00
On 5/5/2010 6:06 PM, Miles Fidelman wrote:
> Not sure if this is the right list, sort of a general linux networking
> question (pointers to a more appropriate list welcomed)....
>
> Setup:
>
> - I have two servers in a datacenter, currently used for different things
>
> - I have one gigE cable coming in from one of the datacenter's big
> routers - that goes into a simple gigE switch - each box is plugged
> into that switch
>
> - I have two network /27 network blocks that are NOT contiguous - I
> use one for each box
>

I'm not a networking expert, but this part seems wrong to me. I don't
think you're supposed to have different subnet addresses on the same
broadcast domain. If they both had the same subnet address, they would
then talk to each other over the switch and not touch the router.

You should be able to get what you want by adding a second IP address to
each machine, preferably something in the 192.168.x.x or 10.x.x.x range
that doesn't conflict with anything else on the network. Then when you
access the other machine by the new IP address, since both machines
would then have the same subnet address, it should just go over the
switch and not the router.

Another option is to change the subnet mask so that the mask then allows
both computers to appear to be on the same subnet. This will have the
side effect of making other computers that happen to fall within that
same subnet inaccessible, since the networking layer will think those
computers are on the same subnet and not attempt to go through the
router, even though they aren't, and going through the router is required.

I hope this makes sense.

-- Kevin


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From: Alex Samad on 6 May 2010 00:00
On Wed, 2010-05-05 at 21:06 -0400, Miles Fidelman wrote:
> Not sure if this is the right list, sort of a general linux networking
> question (pointers to a more appropriate list welcomed)....
>
> Setup:
>
> - I have two servers in a datacenter, currently used for different things
>
> - I have one gigE cable coming in from one of the datacenter's big
> routers - that goes into a simple gigE switch - each box is plugged into
> that switch
>
> - I have two network /27 network blocks that are NOT contiguous - I use
> one for each box
>
> - as I understand the basic setup, any traffic from one of my boxes to
> the other (one netblock to the other) end up going to the datacenter's
> router and back (and the traffic gets accounted for in our bill)
>
> Up to now, I haven't been routing any traffic between boxes, but I'm
> getting ready to install some cluster software and I expect there to be
> a lot of inter-box traffic. So....
>
> I'm now looking for a way to have the inter-box traffic go directly
> through the gigE switch, and not reach the datacenter's router. Which
> leaves me with some questions that are just a bit beyond my general
> network setup knowledge:
>
> 1. Yes, I have a cross-over cable plugged directly between the 2nd
> ethernet card in each box. I plan to dedicate that for disk mirroring
> traffic; but I expect I'll end up with things running on one box that
> need to talk to the other, that may go through the primary ethernet ports.
>
> 2. Is there a way to use ARP and/or set up routing tables so that
> inter-box traffic simply goes through the bridge?
>
> 3. If not, is this something I can do with a simple Linksys switch/router?
>
> Any guidance would be much appreciated.

if the 2 servers are in the same ethernet broadcast domain - just
because they are in the same switch doesn't mean they can talk directly
- they could be vlan'ed apart.

Presuming they are then

for example
if server a has ip 192.168.11.10/24 on eth0 - belongs to
192.168.0/255.255.255.0 network
and server b has 10.0.0.254/24 on eth0 - belongs to
10.0.0.0/255.255.255.0



then what you can do is (iproute package is your friend)
on server A
ip r a 10.0.0.254/32 dev eth0 src 192.168.11.10

on server B
ip r a 192.168.11.20/32 dev eth0 serc 10.0.0.254

that should work for you.

you could add these to post-up instructions in /etc/network/interfaces
for eth0

Alex



>
> Miles Fidelman
>
> --
> In theory, there is no difference between theory and practice.
> In<fnord> practice, there is. .... Yogi Berra
>
>
>



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From: Alex Samad on 6 May 2010 00:20
On Wed, 2010-05-05 at 20:55 -0700, Kevin Ross wrote:
> On 5/5/2010 6:06 PM, Miles Fidelman wrote:
[snip]

> > - I have two network /27 network blocks that are NOT contiguous - I
> > use one for each box
> >
>
> I'm not a networking expert, but this part seems wrong to me. I don't
> think you're supposed to have different subnet addresses on the same
> broadcast domain. If they both had the same subnet address, they would
> then talk to each other over the switch and not touch the router.

Why do you think this, reason I ask is I had a rather long discussion
with a work college about this and I am wondering were this thinking
comes from.

>

> Another option is to change the subnet mask so that the mask then allows

careful you might loose connectivity with the router.

> both computers to appear to be on the same subnet. This will have the
> side effect of making other computers that happen to fall within that
> same subnet inaccessible, since the networking layer will think those
> computers are on the same subnet and not attempt to go through the
> router, even though they aren't, and going through the router is required.
>
> I hope this makes sense.
>
> -- Kevin
>
>



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