Multiple public IPs statically NAT'd to a single inside host

Discussion in 'Cisco' started by Lone, May 16, 2006.

  1. Lone

    Lone Guest

    I have a PIX (Version 6.3) 515E configured with an inside and outside
    interfaces. I want to transition the existing T1 connection to a new
    T1 connection. In doing so, I'd like to have the existing static NAT
    definitions to work across both connections until the DNS is
    transferred over and then the original T1 connection is
    de-commissioned. So say I had:


    Essentially what I would like to do is:

    global (outside) 10 interface
    global (outside2) 20 interface

    nat (inside) 10 0 0

    static (inside,outside) netmask
    static (inside,outside2) netmask

    The PIX translates connections into properly. It receives
    ping requests on, but no responses. I believe this is
    because of the default NAT route is back through outside. I could of
    course change the default NAT route to outside2, but then it should
    break the existing static NATs. How do I go about implementing this as
    I believe it is possible?
    Lone, May 16, 2006
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  2. That's fine provided that there is an unambiguous way to determine
    which traffic should head to outside and which should head to
    outside2. As long as the range of source IPs that can read one of the
    two interfaces is fixed, you can use static "ip route" statements.
    The step above that would to allow dynamic routing configuration
    via OSPF, but unless you already have OSPF set up, that's probably
    not the best way to proceed.

    What you cannot do is have two default routes and expect that
    because a packet came in via one interface, that the PIX will
    "somehow" remember the interface and reply back through that interface.
    The PIX cannot do that. The PIX figures out which interface to
    go to via the routing tables -- packet by packet.

    There is no such thing as a "NAT route". If you are thinking of

    global (outside) 10 interface
    global (outside2) 20 interface
    nat (inside) 10

    then you need to know that the "" of the nat statement
    is NOT a route: it is a source IP selector on outgoing traffic. For
    example, you could have

    nat (inside) 10
    nat (inside) 20
    nat (inside) 30

    and that would mean to apply global policy 10 to packets sourced from
    host, global policy 20 for packets sourced from
    and global policy 30 for all other inside source packets.
    But those aren't routes: they do not control which interface the
    traffic heads towards. Routes are handled via "ip route" or
    rip or ospf. Having different policies for the same inside interface
    allows you to select which IP address different hosts will
    present themselves as when going out through the -same- interface.
    If arbitrary packets might enter through either interface, then
    you cannot do what you want through anything like the technique
    you are working with.... but under some conditions, you can
    do it via "reverse nat". I've written the constraints and
    configuration instructions in a few earlier postings in this newsgroup.
    Walter Roberson, May 16, 2006
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