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OT: One for the network experts
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I have two wireless cards in a machine. One is 54mb and the other
108mb. If I transfer a file from machine A -> B using a mapped drive
on B and B is the machine with the two cards. What decides which NIC
to use and can I control this?
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All sorts of things, and yes.
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Such as and how?
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Well it depends on how it's set up. If they were on different subnets
with different gateways you could set up static routes with costs,
making the lease preferred cost having a higher cost.
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Microsoft has the following to explain how it decides between wired and
wireless networks on the same machine. I would think it may be the same
for two wireless connections
I hope this helps
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Certainly does, thanks for that.
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I'm guessing routing comes into this but the route command has me
confused as it seems to want gateway addresses. Both machines are
using DHCP off a single router.
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Are these machines on the same subnet? If so, routing is irrelevant as
you're doing everything across the local network, so the gateway is
never used.
If you're routing between subnets, consider that you have two flows
here, packets from A to B, and packets back the other way. As you're
pushing data from A to B, the volume of data in that direction will be
vastly more than that flowing from B to A, so the A->B flow will be the
one limited by available bandwidth.
How are you addressing these machines? Both interfaces may be picking up
an IP from the DHCP server (in this case, the router) but is host A
referring to B via IP or hostname? If IP, then skip to the bit about TCP
below. If you're using manually configured DNS, which of the two IPs
does it resolve to? If dynamic DNS, are both interfaces configured to
register their IPs with the nameserver? As you're not teaming the
interfaces, they'll have different IPs, and you'll only be using one of
these to refer to the host at a time (unless both are registering with
DDNS, and then maybe.)
Ignore the whole "both hosts are picking up a default route" thing -
both will be picking up the same route anyway, and if it's destined for
a remote network the routing engine will decide which gateway to send
stuff to before it decides which interface to send it though.
IPs aren't actually tied to an interface, as such. Your wireless cards
operate at layer 2, whereas IP addresses are entirely logical entities
operating at layer 3. Return traffic from B to A could have the IP
address of interface 1 but go 'onto the wire' through interface 2,
regardless of which way they came in.
Assuming routed traffic, the router will forward packets for host B to
the MAC for the interface with the IP that host A was using to send
them. If you're using NetBIOS over TCP (445) then the return traffic
will originate from that same IP on host B. However, depending on
metrics, as both interfaces have IPs on the same subnet, the Ethernet
frames could go out of either interface.
If you really want to know, grab a copy of Ethereal (or tcpdump, but I'm
guessing these are Windows boxes) and sniff the network on each
interface. You may see asymmetric flows, you may not, but you'll be able
to see which way the traffic flows and it'll point you the right way.
But as you've ;robably guessed, there may not be an easy answer.
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Are they all on the same subnet? If so, why have two cards?
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Yes to the subnet and because I'm greedy.
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I'm pretty sure you won't get any benefit but see below
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Probably an easy one but I'm thick so humour me please.
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Yep it'll use the first NIC if they're on the same subnet, the only way
you could get round having an address per card is using adapter teaming
on ethernet cards, or putting some kind of load balancing device in
front of the server to front a service using a VIP address. I *think*
NT picks up the default priority from the order the cards were
installed, but I'm not 100% sure.
If you do want to keep both cards installed, what I'd suggest you do is
hard code IP addresses onto each cards, and only put a default route on
the card you want to do all the hard work..
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This confuses me a bit. Why do I need to set fixed IP addresses. ATM
they both quite happily pick up their own IP from the DHCP.
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Because you can't have more than one default route, but DHCP will try
and set one for each card.
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But anyway in summary what you're saying is it'll use the first card
installed/enabled, correct?
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Yep. But they will also pick up the gateway IP address and default route.
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I suppose you could allocate different subnet, onto the slower card,
then let your wireless router route between the 2 subnets assuming it
supports multilple IP addresses.. but I can't see what you'd gain in
doing so.. better just to unplug it in all honesty.
The only point in multiple NICs in servers where you're not using
adapter teaming is is where you've interfaces onto multiple LANs or
VLANs, which you'd only really see in a more secure environment than
you'd need (or normally have) at home.
Plus, above all else, this is a fucing tedious subject and too much like
being at work.
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well, if you will live in the office...
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Hmm, I've not actually been into the office for about 3 weeks. Perhaps
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that was, sort of, my point
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I should go in.
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Yes, disable / unplug the card you don't want to use.
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Yes I can do that but then I want to know the *techie* answer.
Where's Ginge when you need him?
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Well, the techie answer is that it's a pointless thing do be doing.
ISTBC [1] but you can't use the route command to alter the route because
there is only one route and the two cards *must* be using the same one.
[1] This is mostly based on theory for me, as opposed to my more usual
experience.
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