Two Network cards - Networking

This is a discussion on Two Network cards - Networking ; 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...

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Thread: Two Network cards

  1. 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

  2. Re: Two Network cards

    Geoff Lane writes:

    > 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.
    >

    Yes, this can cause problems unless you can assign a distinct
    i.p. address to each network adapter. Use the one which is faster.

    --
    Allan

  3. Re: Two Network cards

    In article , 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.

  4. Re: Two Network cards

    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


  5. Re: Two Network cards

    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

  6. Re: Two Network cards

    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


  7. Re: Two Network cards

    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)

  8. Re: Two Network cards

    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.

  9. Re: Two Network cards

    On Mar 22, 11:45*am, Geoff Lane 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.

  10. Re: Two Network cards

    It should depend on the default router settings



  11. Re: Two Network cards

    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

  12. Re: Two Network cards

    On Mar 22, 4:45 am, Geoff Lane 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.


    This will cause problems.

    If the two cards are both connected to the same network and you are
    not bridging, you have an illegal configuration -- you cannot route
    between two interfaces on the same network. If the two cards are
    connected to different networks, you may send a response packet to a
    gateway that considers the source address to be possibly spoofed and
    drops the packet.

    Outbound connections will likely work, as the operating system can
    choose the source address and will typically choose the address
    assigned to the interface it transmits the packets on. Inbound
    connections will be hit or miss.

    DS

  13. Re: Two Network cards

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


    Just as long as you make sure you have only 1 default route, everything
    will be fine. Though having two nics in the same net is generally not
    advised. (Think about broadcasts and stuff like that).

    Is one adapter used as a failover for the other? If so, you might want
    to try setting up a failover setup that activates the other adapter upon
    failure of the first one.

    -R-

  14. Re: Two Network cards

    Jurgen Haan wrote:

    > Is one adapter used as a failover for the other? If so, you might want
    > to try setting up a failover setup that activates the other adapter upon
    > failure of the first one.


    How does one do this?

    --
    Timothy Murphy
    e-mail (<80k only): tim /at/ birdsnest.maths.tcd.ie
    tel: +353-86-2336090, +353-1-2842366
    s-mail: School of Mathematics, Trinity College, Dublin 2, Ireland

  15. Re: Two Network cards

    Timothy Murphy wrote:
    > Jurgen Haan wrote:
    >
    >> Is one adapter used as a failover for the other? If so, you might want
    >> to try setting up a failover setup that activates the other adapter upon
    >> failure of the first one.

    >
    > How does one do this?
    >


    Uhm.. I would do it with custom scripting or something like that.
    Perhaps a heartbeat package or something. Generally used for
    loadbalancer failover.

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