Gigabit Switch

Donald Becker becker at scyld.com
Tue Nov 11 12:32:32 EST 2003


On Tue, 11 Nov 2003, Keyan Mehravaran wrote:

> I am planning to connect 8 dual Xeon PCs
> with onboard gigabit through a switch and
> I only need access to the "zeroth" node.
> I have two questions:
> 
> 1) Is there any benefit to using "managed"
>    switch rather than the "unmanaged" ones?

Frequently "managed" switches are a negative.
An Ethernet switch should "just work".
Providing configuration options just encourages setting the switch to
flawed modes, such as forced-full-duplex or filtering packet types you
thought you were not using.

> 2) Is it possible to increase bandwidth by
>    adding an extra gigabit NIC to each node?
>    If the answer is yes, then should all the
>    16 ports connect to the same switch?

Yes, you can marginally increase bandwidth.  But it's not worth it.

If you channel bond GbE, you'll likely get out-of-order packets on
the receiving side and consume much more CPU to reassemble.
If you trunk, you will not see higher peak bandwidth, and may still
suffer from bad cache or interrupt affinity effects.

You should use separate switches for channel bonding.  Although it's
possible to use VLAN to avoid this, that's brings us back to the
switch configuration issue.  And two half size switches are less
expensive than one.

Bottom line: use a single GbE channel unless there is a specific
application reason to do otherwise.

-- 
Donald Becker				becker at scyld.com
Scyld Computing Corporation		http://www.scyld.com
914 Bay Ridge Road, Suite 220		Scyld Beowulf cluster system
Annapolis MD 21403			410-990-9993

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