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

Doug Simpson veewee77 at alltel.net
Wed May 11 04:10:15 UTC 2005


This explanation seems ok, but then if you connect the NICs to different 
switches that are subsequently connected together, you get the same thing.

So, how then do you get the ability to put public and private IP 
addresses on the same physical wire?

For example, a Printer in the office has a JetDirect in it and is 
available on a public IP address, and connected to a small switch.  
Another port on that switch is connected to a workstation that gets it's 
IP address via DHCP (a private number).  so, now there are two different 
IP ranges on the same physical wire, connected back at the TC to a 
single port on a switch, which is connected to the two NICs in the 
server, one public and one private.  The printer is available to the 
machine on the private network via NAT and it is shared through the 
server, to both public IP machines and private IP machines.

 From what I gather, it would be required to run individual cables for 
each IP range to every connection so that the public and private would 
always be separate.

But, in the case of dual-boot terminals. . .

Example #2:

Let's say the workstation mentioned above is set up to dual-boot, and 
get's it's IP when booted as a LTSP terminal from the DHCP on the 
private range NIC.

But, this same workstation, when run in Winders, requires a public IP 
address.  Then what do you do? Change the cable  each time you want to 
use the other OS?  Unhandy! Especially for users!

So far, having two NICs on the same switch hasn't seemed to adversely 
affect it . . .


Jim McQuillan wrote:

>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
>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
>>>>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.
>K12OSN mailing list
>K12OSN at redhat.com
>For more info see <http://www.k12os.org>

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