OT-hijacked. . .Re: [K12OSN] Solving the bandwidth bottleneck

Jim Kronebusch jim at winonacotter.org
Tue May 10 21:09:53 UTC 2005

Very well put Jim.  I was going to find some official documentation but
yours seems very complete.  I just vaguely remember all of the specifics
from getting my CCNA.  

The point here is it may seem okay.  But underneath you may have an
intermittent problem, or one just waiting to happen.  

Switches are cheap enough, don't take the risk.

If anyone can find information stating otherwise I would love to see it.
I tried to look for some but can't find anything stating 2 networks on
the same switch is a good idea.

> Hmm,
> Let me try to explain WHY connecting 2 nics on the same 
> machine to the same switch is a BAD IDEA.
> I'm talking about the case when you are NOT using VLANS.
> VLANs effectively split a physical switch into separate switches.
> It helps to have an understanding of how packets get from one 
> machine to another on the same lan.
> When 2 computers on a LAN talk to each other, they don't 
> address each other by IP address.  Deep down, they really 
> address each other with MAC addresses.
> The way a client gets the MAC address of the server is it 
> sends out an ARP request.  This is an Ethernet broadcast.  It 
> is to see 'Who has' a specific IP address.
> ALL nodes on the network will receive that broadcast.
> The server will receive the broadcast on both nics.  AND, 
> since the server has the IP address in question, it will send 
> out the ARP reply on both of it's interfaces.  That's because 
> it is the protocol stack in the kernel that is handling this, 
> not the NIC.
> Now, because the server sent out 2 replies, one for each nic, 
> the switch will see these ARP replies, and add both to its 
> arp table. Depending on how the arp table code is setup in 
> the switch, it might keep both, or one might step on the other.
> Then, when you try talking to the server, the packet will get 
> to the switch, and then the switch will decide which port to 
> send it out, based on its arp tables.  It might send the 
> packet out the correct port, or it might send the packet out 
> the wrong port, OR, it might send it out both ports.  Don't 
> forget, we use switches and not hubs, because switches setup 
> virtual circuits between 2 nodes on the net.  It's the arp 
> table that allows the switch to know how to setup that circuit.
> Sometimes you'll get lucky, and it will just seem to work 
> fine. Other times, you'll have un-explained packet loss or collisions.
> This has NOTHING to do with your choice if IP Subnetting.  
> This happens below the IP layer.
> So, my advice:  NEVER connect 2 nics on the same server to 
> the same switch.  Even if it seems to work fine when you do 
> it.  Trouble lurks, and it won't appear when you are sitting 
> idle with nothing else to do. The trouble will happen on a 
> friday afternoon, just as you are heading out to begin a 
> vacation, and the boss calls, because his PC on his desk 
> can't get to the internet, so he can't check his stock portfolio.
> Jim McQuillan
> jam at Ltsp.org
> On Tue, 10 May 2005, Eric Harrison wrote:
> > Jim Kronebusch wrote:
> > > > I have two NICs in a server, one on a public IP and one on a 
> > > > private one and they are both connected to the same 
> switch. . . is 
> > > > this really a problem?
> > > >
> > > > Don't seem to be from here. . .
> > >
> > >
> > > As far as know this definitely breaks more than a couple 
> networking 
> > > rules :-)
> > >
> > > If it is working I hate to say change it as the general 
> rule is "if 
> > > it ain't broke don't fix it".  But you may want to fix that.
> > >
> > > You should never have 2 networks propogating the same 
> switch.  This 
> > > should cause major slowdowns and collisions, if not more.
> > >
> > >
> >
> > Having two NICs on the same logical network is certainly a no-no.
> >
> > But in Doug's case above, he specifies that the two NICs are on 
> > different logical networks (one public, one private). I 
> have the same 
> > setup on several of my servers.
> >
> > -Eric
> >
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