Why do servers have dual NICs?

Why do most servers have dual NICs? Why is two better than one?

How are they usually configured when the box must respond to multiple IP
addresses?

Chris <(E-Mail Removed)> wrote:
> Why do most servers have dual NICs? Why is two better than one?

In unsorted order:

*) It sounds better than a server with one NIC
*) NICs are rather cheap anyway
*) If you bind/bond/aggregate them together, you can remove some SPOFs
(Single Points of Failure)

rick jones
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Rick Jones <(E-Mail Removed)> wrote:
> Chris <(E-Mail Removed)> wrote:
> > Why do most servers have dual NICs? Why is two better than one?
>
> In unsorted order:
>
> *) It sounds better than a server with one NIC
> *) NICs are rather cheap anyway
> *) If you bind/bond/aggregate them together, you can remove some SPOFs
> (Single Points of Failure)

And you can use one to connect to the clients, and have one to connect
to upstream. That kind of doubles the bandwidth in practice anyway.

Servers really need three nics. The third nic holds the secure channel
to the twin server that's configured for rapid failover, and is
probably mirroring in realtime, or close.

Then of course, you really need four nics - to avoid surprises.

Peter

> Why do most servers have dual NICs? Why is two better than one?

Web serving is one of the most typical applications these days, and for
high-traffic sites, using a pool of servers to handle the load is easy and
cheap. However, those web servers typically need (a) a connection to the
outside world, (b) a connection to a file server for content, and (c) a
connection to a database server. If you want to maintain seperate networks
for security and/or congestion avoidance, that would be three network cards
right there.

> How are they usually configured when the box must respond to multiple IP
> addresses?

They're configured like you normally configure NICs. Perhaps you could
ask a more specific question?

steve

P.T. Breuer <(E-Mail Removed)> wrote:
> Rick Jones <(E-Mail Removed)> wrote:
> > Chris <(E-Mail Removed)> wrote:
> > > Why do most servers have dual NICs? Why is two better than one?
> >
> > In unsorted order:
> >
> > *) It sounds better than a server with one NIC
> > *) NICs are rather cheap anyway
> > *) If you bind/bond/aggregate them together, you can remove some SPOFs
> > (Single Points of Failure)

> And you can use one to connect to the clients, and have one to connect
> to upstream. That kind of doubles the bandwidth in practice anyway.

> Servers really need three nics. The third nic holds the secure channel
> to the twin server that's configured for rapid failover, and is
> probably mirroring in realtime, or close.

> Then of course, you really need four nics - to avoid surprises.

Probably you have a dedicated backup LAN, needing an additional
(GB) NIC.

In reality you can't have enough of them. That's why you get
multi-port cards, as you run out of PCI slots fast...



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"Steve Wolfe" <(E-Mail Removed)> wrote in message
news:bulf8c$id6es$(E-Mail Removed)...
> > Why do most servers have dual NICs? Why is two better than one?
>
> Web serving is one of the most typical applications these days, and for
> high-traffic sites, using a pool of servers to handle the load is easy and
> cheap. However, those web servers typically need (a) a connection to the
> outside world, (b) a connection to a file server for content, and (c) a
> connection to a database server. If you want to maintain seperate
networks
> for security and/or congestion avoidance, that would be three network
cards
> right there.
>
> > How are they usually configured when the box must respond to multiple IP
> > addresses?
>
> They're configured like you normally configure NICs. Perhaps you could
> ask a more specific question?

Sure. My thought was that you could get some redundancy with multiple NICs.
The trouble is that it doesn't seem possible to get two different NICs to
respond to the same IP address. So any request for a given domain will have
to go through a single card, and that becomes a single point of failure.

From the replies above, it seems like redundancy is *not* the reason to have
multiple NICs -- you do it to have a separate way to access a different
network behind the firewall.

> Sure. My thought was that you could get some redundancy with multiple
NICs.
> The trouble is that it doesn't seem possible to get two different NICs
to
> respond to the same IP address. So any request for a given domain will
have
> to go through a single card, and that becomes a single point of failure.

You're forgetting that a NIC doesn't respond to an IP address, it
responds to a *MAC* address. IP->MAC translation is done via a protocol
called ARP. If one NIC (or server) stops responding, you can craftily use
ARP to your advantage, and tell/trick the other devices on the ethernet
segment into using the new MAC address. There are various packages out
there to make it easier for you.

> From the replies above, it seems like redundancy is *not* the reason to
have
> multiple NICs -- you do it to have a separate way to access a different
> network behind the firewall.

Every once in a blue moon, a NIC will go bad. However, chances are
that you'll go through a bare *minimum* of a hundred disk or power-supply
failures before you'll ever see a NIC go bad. In my years, I have *yet*
to see a NIC go bad on any piece of equipment, although I have a fairly
impressive stack of dead disks, power supplies, fans, etc..

(Alright, I admit it. The stack of dead hard drives isn't *really* all
that big, because I keep taking them apart to get the magnets out!)

steve

Chris <(E-Mail Removed)> wrote:
[..]
> Sure. My thought was that you could get some redundancy with multiple NICs.
> The trouble is that it doesn't seem possible to get two different NICs to
> respond to the same IP address. So any request for a given domain will have
> to go through a single card, and that becomes a single point of failure.

> From the replies above, it seems like redundancy is *not* the reason to have
> multiple NICs -- you do it to have a separate way to access a different
> network behind the firewall.

Sure you can have redundancy with auto fail-over, even over
multiple switches if your equipment supports that, it's called
bonding, as already suggested in this thread.

Well documented, needs kernel sources installed:

/usr/src/linux*/Documentation/networking/bonding.txt

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Michael Heiming

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