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

Shahms King shahms at shahms.com
Tue May 10 21:36:54 UTC 2005

Hash: SHA1

Jim McQuillan wrote:
| 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.


That's not entirely true. Linux (at least as of August 2003), only sends
out an ARP reply from the interface that recieved the request.  Of
course, when you have two NICs on the same (physical) network, both NICs
will recieve the request and both will respond. This LWN article has
more details: http://lwn.net/Articles/45373/

That article also includes a link the sysctl documentation describing
how to change the ARP behavior to allow for two NICs on the same subnet
(with some caveats).

- --
Shahms E. King <shahms at shahms.com>
Multnomah ESD

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