Tell me more ×
Server Fault is a question and answer site for professional system and network administrators. It's 100% free, no registration required.
up vote 1 down vote favorite

If my laptop or other computer is tied to a wired network, and it automatically joins a wifi router attached to that same network, can the two connections to the network cause a loop that starts a broadcast storm?

Assume STP is off... of course.

share|improve this question

3 Answers

up vote 1 down vote accepted

Probably not as each interface will have it's own MAC address (as well as it's own IP address).

Name resolution may be a problem, but that's all I would expect to see in such a scenario.

share|improve this answer
What about with a wireless bridge I have then? I have a few different model routers running DD-WRT serving various functions, but two of them are set up as a wireless bridge, and these two use the same MAC for wired and wireless connections. I do have some routers with different MAC addresses for each interface, but they're all running as standard routers/DHCP servers – mattpace Jul 15 '11 at 20:35
The fact that the router uses the same MAC address for each of it's own interfaces shouldn't have any bearing because a broadcast packet forwarded from one router interface (wired) to the other (wireless) (and vice versa) won't ever be forwarded back through the interface that it was receieved on. Think of the wireless interface as a switch port connected to a hub, because that's essentially what it is. In addition, any hosts connected wired or wirelessly won't forward broadcast traffic back out to the network because they don't perform packet forwarding because they're not switches. – joeqwerty Jul 15 '11 at 20:52
Thanks for everything! – mattpace Jul 15 '11 at 22:25
Glad to help... – joeqwerty Jul 15 '11 at 22:40
up vote 2 down vote

No, because your computer is probably not forwarding arbitrary packets it receives.

I used to run this way in the office. My computer has wireless enabled all the time, and when it is docked, it has a physical link to the same network.

(Now I have unplugged the wire so that when I undock, my laptop doesn't suddenly lose the IP address it was using to talk on the network.)

share|improve this answer
up vote 1 down vote

By "same network" you likely mean "same broadcast domain."

On a workstation with a wired interface (10/100/1000Base-T Ethernet NIC) and wireless interface (802.11a/b/g/n NIC) connected to the same broadcast domain -- the default paradigm for all mainstream operating systems that I know of will not create a broadcast storm.

These interfaces are considered layer 3, IP interfaces. Layer 3 interfaces in general segment broadcast domains. It is of special note that you can enable IP forwarding in your operating system with

Quick and dirty Linux:

echo 1 > /proc/sys/net/ipv4/ip_forward

Windows Registry

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\IPEnableRouter

REG_DWORD set to 1.

However, enabling IP fowarding will still not create broadcast storms in your example.

To do that you would have to "bridge" the two interfaces at layer 2 -- essentially turning your workstation into a two port bridge/switch. With your workstation/2 port switch "connected" upstream to the same broadcast domain on both workstation ports (the wired and wireless interfaces) you have created a layer 2 loop -- and a broadcast storm can result.

I say can because if your upstream switch is running spanning tree -- it is likely that one of the interfaces on the upstream switch with be placed in a blocking state as the workstation's "two port bridge/switch" software capability is oftentimes not STP aware and will forward on BPDU's.

-John

share|improve this answer
I'm not understanding your statement "Layer 3 interfaces in general segment broadcast domains". Broadcasts occur at layer 2 and at layer 3. Hosts connected to the same physical network are in the same layer 2 broadcast domain and hosts in the same layer 3 network are in the same layer 3 broadcast domain. – joeqwerty Jul 19 '11 at 12:39
In general, a layer 3 interface -- with an IP -- capable of forwarding IP or not -- that receives a layer 2 Ethernet frame destined for ffff.ffff.ffff -- will not flood the frame out all the other interfaces on the device. – Weaver Jul 20 '11 at 1:06
L3 broadcast addresses -- all 1's in the host portion of IP address, when dropped onto the local link at L2, are dropped on the link as L2 broadcasts -- ffff.ffff.ffff in Ethernet. L3 aware interfaces on the same broadcast domain will process the Ethernet frame passing the payload up to L3/IP but not flood the frame after ingressing at L2. If IP chooses to act and forward that is IP's decision. – Weaver Jul 20 '11 at 1:25

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.