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.


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

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


| =========================================================================== | 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 | 30 | 20 | 1 | 30 | 20 | 30 | 20 | 30 | 20 | 30 | 20 | 1 | 1 | Default Gateway: | =========================================================================== | 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.

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

Thanks, Jeff!

Reply to
William P.N. Smith

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?

Reply to

"__spc__" hath wroth:

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.

Reply to
Jeff Liebermann

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