Two Network cards

If one has two network cards connected to a computer (eg one ethernet
and one wifi) and both are connected to the internet is there a priority
for outgoing traffic or can this cause problems.

Geoff Lane
Followup-To: comp.os.linux.networking

In article <fs2rhb$qa3$1@news.datemas.de>, Geoff Lane
datemasde.t1m@gishpuppy.com says...
> If one has two network cards connected to a computer (eg one ethernet
> and one wifi) and both are connected to the internet is there a priority
> for outgoing traffic or can this cause problems.
>
With a normal configuration both NICs will have a default route set - if
you want to use just one for internet access then remove the default
route for the other one. With both default routes set I suspect the
order they are tried will depend on the order in which they were loaded
into the routing table, so you might try swapping them around to see if
it makes any difference. Otherwise I think you're into advanced routing
with multiple tables.
As long as both NICs are running on the same network I don't think it
will cause any problems anyway.

Rob Morley wrote:

>> If one has two network cards connected to a computer (eg one ethernet
>> and one wifi) and both are connected to the internet is there a priority
>> for outgoing traffic or can this cause problems.
>>
> With a normal configuration both NICs will have a default route set - if
> you want to use just one for internet access then remove the default
> route for the other one. With both default routes set I suspect the
> order they are tried will depend on the order in which they were loaded
> into the routing table, so you might try swapping them around to see if
> it makes any difference. Otherwise I think you're into advanced routing
> with multiple tables.
> As long as both NICs are running on the same network I don't think it
> will cause any problems anyway.

The query only occured as we were setting up a new wifi adsl router at
our club; used the wired connection to set up the security on the wifi
then forgot to disable the ethernet before inserting the wifi card.

All appeared to work fine but I was then curious as to which interface
would be used.

Geoff Lane

Geoff Lane wrote:
> Rob Morley wrote:
>
>>> If one has two network cards connected to a computer (eg one
>>> ethernet and one wifi) and both are connected to the internet is
>>> there a priority for outgoing traffic or can this cause problems.
>>>
>> With a normal configuration both NICs will have a default route set -
>> if you want to use just one for internet access then remove the
>> default route for the other one. With both default routes set I
>> suspect the order they are tried will depend on the order in which
>> they were loaded into the routing table, so you might try swapping
>> them around to see if it makes any difference. Otherwise I think
>> you're into advanced routing with multiple tables.
>> As long as both NICs are running on the same network I don't think it
>> will cause any problems anyway.
>
> The query only occured as we were setting up a new wifi adsl router at
> our club; used the wired connection to set up the security on the wifi
> then forgot to disable the ethernet before inserting the wifi card.
>
> All appeared to work fine but I was then curious as to which interface
> would be used.
>
> Geoff Lane
>
Although you posted to linux.networking you didn't actually specify
which operating system was being used. If windows you may find this of
interest:-
http://technet.microsoft.com/en-us/l.../bb878031.aspx

LR wrote:

>> The query only occured as we were setting up a new wifi adsl router at
>> our club; used the wired connection to set up the security on the wifi
>> then forgot to disable the ethernet before inserting the wifi card.
>>
>> All appeared to work fine but I was then curious as to which interface
>> would be used.

> Although you posted to linux.networking you didn't actually specify
> which operating system was being used. If windows you may find this of
> interest:-
> http://technet.microsoft.com/en-us/l.../bb878031.aspx

Thanks, I'll view it.

We were using XP at the time but do run Linux machines at the club but
being Easter it was a quiet night :-)

Geoff Lane

LR wrote:
> Geoff Lane wrote:
>> Rob Morley wrote:
>>
>>>> If one has two network cards connected to a computer (eg one
>>>> ethernet and one wifi) and both are connected to the internet is
>>>> there a priority for outgoing traffic or can this cause problems.
>>>>
>>> With a normal configuration both NICs will have a default route set -
>>> if you want to use just one for internet access then remove the
>>> default route for the other one. With both default routes set I
>>> suspect the order they are tried will depend on the order in which
>>> they were loaded into the routing table, so you might try swapping
>>> them around to see if it makes any difference. Otherwise I think
>>> you're into advanced routing with multiple tables.
>>> As long as both NICs are running on the same network I don't think it
>>> will cause any problems anyway.
>>
>> The query only occured as we were setting up a new wifi adsl router at
>> our club; used the wired connection to set up the security on the wifi
>> then forgot to disable the ethernet before inserting the wifi card.
>>
>> All appeared to work fine but I was then curious as to which interface
>> would be used.
>>
>> Geoff Lane
>>
> Although you posted to linux.networking you didn't actually specify
> which operating system was being used. If windows you may find this of
> interest:-
> http://technet.microsoft.com/en-us/l.../bb878031.aspx

Interesting, but does not match my experience with at least 15 laptops.
XP Pro SP2 seems to get confused with two connections, with network
traffic slowing to a crawl. Disable either connection and the remaining
connection returns to "normal" speeds. In the end, all laptops were set
to manual for wireless.

--
PeeGee

The reply address is a spam trap. All mail is reported as spam.
"Nothing should be able to load itself onto a computer without the
knowledge or consent of the computer user. Software should also be able
to be removed from a computer easily."
Peter Cullen, Microsoft Chief Privacy Strategist (Computing 18 Aug 05)

PeeGee wrote:
>>
>> http://technet.microsoft.com/en-us/l.../bb878031.aspx
>
> Interesting, but does not match my experience with at least 15
> laptops. XP Pro SP2 seems to get confused with two connections, with
> network traffic slowing to a crawl. Disable either connection and the
> remaining connection returns to "normal" speeds. In the end, all
> laptops were set to manual for wireless.
>
Since XP SP2 I haven't had any problems. The automatic setting for
"Routing Metric" has always given priority to the Ethernet connection. I
have just checked my dual boot laptops at home and with fully updated XP
SP2 they both give a "routing metric" of 20 for ethernet and 25 for
wireless and traffic goes via ethernet, routing metric is set to
automatic. If I change the wireless metric to 15 all traffic goes via
wireless and it only gets confusing if you set both to the same metric
and have the "automatic" setting on both cards disabled.
The metric setting can be manually configured in the TCP/IP advanced
settings.

On Mar 22, 11:45*am, Geoff Lane <datemasde....@gishpuppy.com> wrote:
> If one has two network cards connected to a computer (eg one ethernet
> and one wifi) and both are connected to the internet is there a priority
> for outgoing traffic or can this cause problems.

When you want to send an IP packet, your machine will check its
routing table to see how it should send it. If your routing tables
contains multiple entries for on route (e.g. multiple entries for
0.0.0.0), then it checks the "metric" associated with each route to
decide which one it will take.

You can have as many NIC's as you want without there being any
confusion, so long as your routing table is set right.

In windows, open up a console and type "route print". Under
"Destination" check for "0.0.0.0"; this is a route to the internet. If
there's more than one entry for 0.0.0.0, then the check the metric.
The lower metric wins.

I dunno if modern OS's allow you to give two routes the same metric,
but if so then it probably choose the route simply by putting the
gateway addresses in numerical order... or something like that.

It should depend on the default router settings

Tomás Ó hÉilidhe wrote:

> In windows, open up a console and type "route print". Under
> "Destination" check for "0.0.0.0"; this is a route to the internet. If
> there's more than one entry for 0.0.0.0, then the check the metric.
> The lower metric wins.

I'll check that out when we next meet at the club.

Thanks for pointers.

Geoff Lane