[BlueOnyx:16112] Re: Redundancy and using two network cards

[Chris Gebhardt - VIRTBIZ Internet]

Hi Richard,

On 10/7/2014 4:22 AM, Richard Morgan wrote:
> Hi, hope this is simple.

It is, but probably not what you're looking for.

> In the data centre we use, we’ve been provided two network cables to
> connect to the server which come from separate switches/routers to
> provide redundancy.  At the moment we’re only using one of these cables,
> but we’d like to be able to use both.
>
> How do I configure the network settings to allow both network ports to
> be used with the same IP address?  I’m not interested in teaming for
> speed, just the redundancy in case one of the data centre’s switches fails.

The short answer:  "you don't".

See, those network drops are most likely configured in HSRP (or 
similar).  You'll typically use that with a switching or routing device, 
not an individual host.

Since an IP address can not be "shared", it simply isn't possible to 
place the same IP on 2 NICs on the same server.   So there's the bad news.

The good news is you probably don't REALLY want to do that anyhow.  In 
my 18+ years in the hosting business I can probably count on one hand 
the number of times I have seen a NIC fail while the rest of the server 
is fine.  I'd probably still have 4 fingers left over in that count, to 
be honest.

So your other concern would be the switch that you connect to (that your 
provider supplies).   If the datacentre is worth its salt, they're 
running a carrier-grade switch, which is going to prove itself pretty 
robust and reliable.  Our average distribution switch, for example, 
carries an uptime of well over 5 years.  Switch failure is unlikely to 
be the cause for you to drop a connection, and if it is, you'll need to 
make sure that your 2 drops are supplied from separate distribution points.

If you are just super-paranoid about for some reason losing one NIC and 
you think you're going to be in a situation where having the additional 
NIC connected will be useful, my recommendation would be to use a free 
IP on the second NIC.   Then if the primary fails, you can still access 
the system and move IP's to the second NIC.

However, there's another issue that's going to bite you, and that's ARP 
cache.   It's likely that the router you are connected to that provides 
your gateway will cache the MAC address that the IP is found on.   You 
could conceivably run your IP on the primary NIC, then unplug that 
primary, put the IP over on the secondary, but since the MAC address is 
different you'll need to wait for the ARP cache to expire, or ask your 
provider to manually clear it for you.

Again... it's just a lot of trouble for no real-world gain.  I see your 
thought process, but you'll be happier forgetting about it.

-- 
Chris Gebhardt
VIRTBIZ Internet Services
Access, Web Hosting, Colocation, Dedicated
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