[CentOS] Redundant LAN routing possible?

Sun Jul 11 20:34:13 UTC 2010
Gordon Messmer <yinyang at eburg.com>

On 07/11/2010 10:25 AM, Emmanuel Noobadmin wrote:
> I think I must had made a mistake in my setup example because the
> multipath thing seems to be what I had in mind. i.e. all machines have
> at least 2 paths to the others so as long as one switch is alive,
> functionality remains.

A critical element of your configuration involves whether or not your 
managed switch links the two unmanaged switches.

If it does not, then there are two paths from any host to the other 
hosts and to the managed switch/router.  One path will be eth0->switch 
1->destination host or router.  The other path will be eth1->switch 
2->destination host or router.  You'll need to use "active-bonding" mode 
in order for this to work.

If it does link them and the two are not directly linked, you have 
mostly the same situation.  Any switch or ethernet failure will continue 
to allow communication between hosts, using active-bonding.

If you then link the two unmanaged switches together, there become an 
infinite number of paths between any two hosts.  A packet can be sent 
from eth0->switch 1->switch 2->switch 1->...->destination.  Broadcast 
packets *will*.  That's why your network failed the last time you tried 
cross-connecting the switches.  You can't do this with unmanaged 
switches.  Not even with STP.  STP is used when you have a mesh of 
managed switches to locate the shortest path between hosts and to handle 
the failure of a switch in the mesh.

> Would connect bond0 to both switches still work without STP in this
> kind of a setup, or is this when STP comes in?

STP comes in when you're linking multiple managed switches in a topology 
other than daisy-chain or star.  It has nothing to do with the 
configuration of network nodes which are not switches.