[CentOS] OT: CentOS server with 2 GbE links to 2 GbE switches

[Bryan J. Smith]

Patrick <centos at puzzled.xs4all.nl> wrote:
> I am trying to come up with an architecture that has some
> redundancy.  The idea is to hook up the two GbE LAN
> interfaces of a CentOS server to two Gigabit Ethernet
> switches. In case one switch goes down, there is a
> redundant path (the server is redundant too).
> How would I acomplish this?

First off, doing it at the layer-3/IP level with dynamic
routes is far more overhead than is required.  In your case,
you're just looking for layer-2/802 level.  So leverage what
standard 802 offers if you can.

I'm more of an academic, so the first thing I recommend to
people is that they get familar with the standard
capabilities of 802.  More explicitly, research 802.1d
Spanning Tree Protocol (STP) as well as newer standards like
802.3ad Link Aggregation.  In fact, it's this latter addition
that really makes things very easy.

In the "good old days," you'd setup a single, virtual UNIX
interface bridged to two.  Your system only knows about the
single, virtual UNIX interface.  But it would then leverage
two interfaces, only bringing the other interface up if one
failed.  With support for STP, loops would be avoided.  The
only thing to worry about with STP is the maximum number of
hops in a layer-2 network -- 7.  This, of course, requires
both your host (software) and network stack (firmware) to
support STP.

In the "new, better days" we now have 802.3ad Link
Aggregation.  Now you can get more bandwidth and failover at
the same time.  Again, both your host (NIC firmware) and
network stack (firmware) need to support 802.3ad Link
Aggregation.  But if it does, it becomes very, very easy to
configure a single IP address to a pair of NICs, and
aggregate both to two different ports in a network stack.

Now if you're using cheap network equipment, I don't know
what to tell you.  Layer-2 is probably out then.

> Can I use IP addresses from one IP network (say
10.0.0.0/24)
> to assign to the 2 LAN ports on the CentOS server and a
port
> on each of the GbE switches and then use something like
OSPF
> on the switches and the CentOS box to do the routing?

You can maybe use layer-3 hacks and tweaks to deal with
things, but it's very difficult to handle the failover
without support at the concentrator end.  Your idea to use
different IP addresses and dynamic routing tables is probably
the best way.  But it's definitely not as clean.

Especially with 802.3ad Link Aggregation being supported more
and more.

> Any other ideas?

If you let me know what your networking equipment and/or
budgetary constraints are, I can help you further.  You'd be
surprised how little this actually costs, but if you're using
$200 GbE switches, then I can't help you with layer-2.


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