Re: Broadcast question

Discussion in 'Cisco' started by Walter Roberson, Jul 13, 2003.

  1. In article <3f0c6775$>,
    sPiDEr <spider@bgp5_.net_REMOVE_> wrote:
    :Senario:

    :I have a server haing multiple IP addresses, example:

    :primary: 192.168.1.10/24
    :alias: 192.168.2.10/24
    :alias: 192.168.3.10/24

    :If this machine is going to send out a broadcast packet, which broadcast IP
    :will it use? The primary broadcast IP or all the broadcast IPs?

    That's going to depend upon the operating system of the server
    and on how you configured in the extra addresses.

    For example, in SGI IRIX, there are no virtual interfaces, so
    IP addresses are always associated with physical interfaces. IRIX only
    allows one primary IP address per physical interface, and it only
    ever transmits packets using that primary IP address, never using
    one of the ifconfig -alias addresses.

    In some of the newer version of Solaris, I have heard that you can
    configure virtual interfaces; broadcast destination IPs on those
    virtual interfaces should be according to the primary subnet for
    each virtual interface.

    It is worth keeping in mind in this regard that in Unix, broadcasting
    requires a privileged process, and privileged processes usually have
    rights to construct new packets with any destination IP they want.
    Much of the time, the destination IP that is used is the subnet
    broadcast address associated with the primary interface.... but it is
    up to the program doing the broadcasting.

    When it comes to ARP, the destination IP is known, and the routing table
    is consulted to find the interface for that destination, and the
    subnet broadcast address for that interface will be used if the destination
    is considered direct. A good OS should be able to detect the alias and
    use the broadcast address associated with the alias... but seeing as
    IP aliases are not standardized, you cannot count on that being true
    over all operating systems that support IP aliasing.
    --
    And the wind keeps blowing the angel / Backwards into the future /
    And this wind, this wind / Is called / Progress.
    -- Laurie Anderson
     
    Walter Roberson, Jul 13, 2003
    #1
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