Wireless AND ethernet to same router

Ever wonder what happens when you connect BOTH via wireless and wired
ethernet to the same router? Probably not, but it's still a good
question. Which interface gets priority? Where does the traffic go?
How does it work? Is it safe to leave both connected?

I had a handy laptop (that belongs to a customer) with a fresh install
of XP SP2, so I thought I would see how it works.

IPCONFIG

| Ethernet adapter Wireless Network Connection 2:
| Connection-specific DNS Suffix . :
| IP Address. . . . . . . . . . . . : 192.168.1.102
| Subnet Mask . . . . . . . . . . . : 255.255.255.0
| Default Gateway . . . . . . . . . : 192.168.1.1
|
| Ethernet adapter Local Area Connection:
| Connection-specific DNS Suffix . :
| IP Address. . . . . . . . . . . . : 192.168.1.105
| Subnet Mask . . . . . . . . . . . : 255.255.255.0
| Default Gateway . . . . . . . . . : 192.168.1.1

OK, that looks normal. Each interface gets a seperate IP address.
Both interfaces are active and enabled.

ROUTE PRINT

| ================================================== =========================
| Interface List
| 0x1 ........................... MS TCP Loopback interface
| 0x2 ...00 14 a5 18 56 71 ...... Broadcom 802.11b/g WLAN - Packet Scheduler Miniport
| 0x3 ...00 0a e4 d8 2c 38 ...... Realtek RTL8139/810x Family Fast Ethernet NIC - Packet Scheduler Miniport
| ================================================== ========================
|
| Active Routes:
| Network Destination Netmask Gateway Interface Metric
| 0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.102 30
| 0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.105 20
| 127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
| 192.168.1.0 255.255.255.0 192.168.1.102 192.168.1.102 30
| 192.168.1.0 255.255.255.0 192.168.1.105 192.168.1.105 20
| 192.168.1.102 255.255.255.255 127.0.0.1 127.0.0.1 30
| 192.168.1.105 255.255.255.255 127.0.0.1 127.0.0.1 20
| 192.168.1.255 255.255.255.255 192.168.1.102 192.168.1.102 30
| 192.168.1.255 255.255.255.255 192.168.1.105 192.168.1.105 20
| 224.0.0.0 240.0.0.0 192.168.1.102 192.168.1.102 30
| 224.0.0.0 240.0.0.0 192.168.1.105 192.168.1.105 20
| 255.255.255.255 255.255.255.255 192.168.1.102 192.168.1.102 1
| 255.255.255.255 255.255.255.255 192.168.1.105 192.168.1.105 1
| Default Gateway: 192.168.1.1
| ================================================== =========================
| Persistent Routes:
| None

Note the value for Metric for each interace. The wireless at .102 has
a higher "cost" of 30 than the ethernet connection at .105 which has a
cost of only 20. That means that all the traffic will go via the
lower cost ethernet route. Checking the bytes moved on the network
device status pages confirms this as all the traffic is going via
ethernet. Nice.

Unfortunately, my BEFW11s4 v4 isn't too smart about multiple
connections from the same computer. All it knows is that each IP
address is probably a different computer. When I start a download
with both the wired and wireless connected, the download goes via the
wired. If I then pull the plug on the ethernet during the download,
the wireless sorta takes over, but the download just stops. The
router is not smart enough to move the traffic to the wireless
interface. Bummer.

Therefore, it's safe to leave both wired and wireless connected. As
long as the routing table shows a different metric for the two
interfaces, the traffic will go via the fastest route. Some laptops
(i.e. Toshiba Config-Free) have a utility that will switch interfaces
on and off, but that appears to be un-necessary.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann <(E-Mail Removed)> wrote:
>Ever wonder what happens when you connect BOTH via wireless and wired
>ethernet to the same router? Probably not, but it's still a good
>question. Which interface gets priority? Where does the traffic go?
>How does it work? Is it safe to leave both connected?
[...]
>Note the value for Metric for each interace. The wireless at .102 has
>a higher "cost" of 30 than the ethernet connection at .105 which has a
>cost of only 20. That means that all the traffic will go via the
>lower cost ethernet route. Checking the bytes moved on the network
>device status pages confirms this as all the traffic is going via
>ethernet. Nice.

Very nice, I often find myself in this circumstance, and it's nice to
know it's doing "the right thing".

Thanks, Jeff!

I have indeed pondered this, so thank you for this reserach.

A question though; how are the Metrics fixed? Protocol specific?
Hardware specific? O/S?

"__spc__" <(E-Mail Removed)> hath wroth:

>I have indeed pondered this, so thank you for this reserach.
>A question though; how are the Metrics fixed? Protocol specific?
>Hardware specific? O/S?

Metrics are set by the ROUTE command. Run:
route /?
for the full incantation. In Unix/Linux, it's the ifconfig command.

It can be set manually from the command line, or automagically by some
application such as Wireless Zero Config. You can also change the
METRIC value with the route command and force the traffic to go via
wireless instead of via ethernet by simply changing the METRIC.

There are also protocols such as RIP and RIP2 that will tweak a
different METRIC value in order to switch default distribution route
for connectivity fallback systems. Back in the stone age of
networking, I was using shell scripts to switch routes when the T1
died and we had to automagically switch to a backup ISDN or dialup
connection. Lowest cost METRIC is also the way mesh wireless networks
decide which way to send their traffic.

METRIC is TCP/IP specific. Note that the route table:
route print
or
netstat -r
is full of IP addresses, and not MAC addresses. There are similar
alogrithms for doing the same thing at the MAC layer, such as Spanning
Tree Algorithm, but that's not for individual PC's.

METRIC is totally hardware specific. In theory, the lowest cost route
is lowest METRIC and is set automagically by the route command.
However, most implimentations simply give every connection a default
value of 20 and leave it for you to untangle the route.

METRIC is not operating system specific, but this might be because
Microsoft has not found time to "extend" the route command and add
useless features. Please don't mention this to Microsoft.

--
Jeff Liebermann (E-Mail Removed)
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558