Loopback, unnumbered and two ifs.

Discussion in 'Cisco' started by spam.sc@gmail.com, Dec 22, 2008.

  1. Guest

    We're currently running a configuration that uses an IP address on the
    FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and
    OSPF. It works well but I am aware of the problems involved if we lose
    the FE connection so I am planning to move to a loopback interface.

    Which should be simple enough, but I'm not positive of what I can get
    away with. The obvious setup I came up with was:
    R1:
    FE0/0: 10.1.1.254/24
    Loopback0: 10.10.1.1/30
    Serial: ip unnumbered Loopback0
    R2:
    FE0/0: 10.1.2.254/24
    Loopback0: 10.10.1.5/30
    Serial: ip unnumbered Loopback0

    But the question that came up was whether, instead, I could use
    10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way,
    can the two be on the same subnet?


    Then I wondered, could I use a single address for the whole router, in
    other words:
    R1:
    Loopback0: 10.1.1.254/24
    Serial: ip unnumbered Loopback0
    FE0/0: ip unnumbered Loopback0
    R2:
    Loopback0: 10.1.2.254/24
    Serial: ip unnumbered Loopback0
    FE0/0: ip unnumbered Loopback0

    Or is that too simple?
     
    , Dec 22, 2008
    #1
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  2. In article
    <>,
    wrote:

    > We're currently running a configuration that uses an IP address on the
    > FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and
    > OSPF. It works well but I am aware of the problems involved if we lose
    > the FE connection so I am planning to move to a loopback interface.
    >
    > Which should be simple enough, but I'm not positive of what I can get
    > away with. The obvious setup I came up with was:
    > R1:
    > FE0/0: 10.1.1.254/24
    > Loopback0: 10.10.1.1/30
    > Serial: ip unnumbered Loopback0
    > R2:
    > FE0/0: 10.1.2.254/24
    > Loopback0: 10.10.1.5/30
    > Serial: ip unnumbered Loopback0
    >
    > But the question that came up was whether, instead, I could use
    > 10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way,
    > can the two be on the same subnet?


    No, because then it won't send traffic to the other machine's loopback
    IP over the network.

    You should usually use /32 for loopback IPs, so you don't have to waste
    any addresses in that block.

    > Then I wondered, could I use a single address for the whole router, in
    > other words:
    > R1:
    > Loopback0: 10.1.1.254/24
    > Serial: ip unnumbered Loopback0
    > FE0/0: ip unnumbered Loopback0
    > R2:
    > Loopback0: 10.1.2.254/24
    > Serial: ip unnumbered Loopback0
    > FE0/0: ip unnumbered Loopback0
    >
    > Or is that too simple?


    You can't use the loopback IP for Ethernet, because ARP needs a specific
    IP to respond to. Also, all the devices on an Ethernet need to be in
    the same subnet, and the mask has to be part of the Ethernet
    configuration so they know what subnet it is.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Dec 22, 2008
    #2
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  3. bod43 Guest

    On 22 Dec, 18:06, Barry Margolin <> wrote:
    > In article
    > <>,
    >
    >
    >
    >
    >
    >   wrote:
    > > We're currently running a configuration that uses an IP address on the
    > > FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and
    > > OSPF. It works well but I am aware of the problems involved if we lose
    > > the FE connection so I am planning to move to a loopback interface.

    >
    > > Which should be simple enough, but I'm not positive of what I can get
    > > away with. The obvious setup I came up with was:
    > > R1:
    > >   FE0/0: 10.1.1.254/24
    > >   Loopback0: 10.10.1.1/30
    > >   Serial: ip unnumbered Loopback0
    > > R2:
    > >   FE0/0: 10.1.2.254/24
    > >   Loopback0: 10.10.1.5/30
    > >   Serial: ip unnumbered Loopback0

    >
    > > But the question that came up was whether, instead, I could use
    > > 10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way,
    > > can the two be on the same subnet?

    >
    > No, because then it won't send traffic to the other machine's loopback
    > IP over the network.
    >
    > You should usually use /32 for loopback IPs, so you don't have to waste
    > any addresses in that block.
    >
    > > Then I wondered, could I use a single address for the whole router, in
    > > other words:
    > > R1:
    > >   Loopback0: 10.1.1.254/24
    > >   Serial: ip unnumbered Loopback0
    > >   FE0/0: ip unnumbered Loopback0
    > > R2:
    > >   Loopback0: 10.1.2.254/24
    > >   Serial: ip unnumbered Loopback0
    > >   FE0/0: ip unnumbered Loopback0

    >
    > > Or is that too simple?

    >
    > You can't use the loopback IP for Ethernet, because ARP needs a specific
    > IP to respond to.  Also, all the devices on an Ethernet need to be in
    > the same subnet, and the mask has to be part of the Ethernet
    > configuration so they know what subnet it is.


    There is as far as I know no reason to use ip unnumbered
    other than to conserve IP addresses. Have you considered
    dispensing with them entirely?
     
    bod43, Dec 22, 2008
    #3
  4. S. Checker Guest

    Barry Margolin <> wrote:
    > In article
    > <>,
    > wrote:
    >
    >> We're currently running a configuration that uses an IP address on the
    >> FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and
    >> OSPF. It works well but I am aware of the problems involved if we lose
    >> the FE connection so I am planning to move to a loopback interface.
    >>
    >> Which should be simple enough, but I'm not positive of what I can get
    >> away with. The obvious setup I came up with was:
    >> R1:
    >> FE0/0: 10.1.1.254/24
    >> Loopback0: 10.10.1.1/30
    >> Serial: ip unnumbered Loopback0
    >> R2:
    >> FE0/0: 10.1.2.254/24
    >> Loopback0: 10.10.1.5/30
    >> Serial: ip unnumbered Loopback0
    >>
    >> But the question that came up was whether, instead, I could use
    >> 10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way,
    >> can the two be on the same subnet?

    >
    > No, because then it won't send traffic to the other machine's loopback
    > IP over the network.
    >
    > You should usually use /32 for loopback IPs, so you don't have to waste
    > any addresses in that block.


    Not totally sure I follow you here.

    After all, if I used two subnets on the two ends of the serial link, as
    I initially wrote up there, you're going to need a route on R1 to get
    to 10.1.2.0/24. So you'd have a route (dynamic or static) on R1 like
    10.1.2.0/24 via 10.10.1.5, Serial0/0/0.

    But the problem there is that it's not on the same subnet with your own
    serial port and therefore by IP rules you'd have to have a gateway.
    What I'd expect to work would be a route like
    10.1.2.0/24 via 10.10.1.2, Serial0/0/0
    10.10.1.2/32 is directly connected, Serial0/0/0

    (With 10.10.1.2 being the loopback/serial address on R2)

    Am I just completely confused? I think I'm overthinking.

    >
    >> Then I wondered, could I use a single address for the whole router, in
    >> other words:
    >> R1:
    >> Loopback0: 10.1.1.254/24
    >> Serial: ip unnumbered Loopback0
    >> FE0/0: ip unnumbered Loopback0
    >> R2:
    >> Loopback0: 10.1.2.254/24
    >> Serial: ip unnumbered Loopback0
    >> FE0/0: ip unnumbered Loopback0
    >>
    >> Or is that too simple?

    >
    > You can't use the loopback IP for Ethernet, because ARP needs a specific
    > IP to respond to. Also, all the devices on an Ethernet need to be in
    > the same subnet, and the mask has to be part of the Ethernet
    > configuration so they know what subnet it is.


    Yeah, that was kind of dopey of me.
    --
    MW: And let's not get carried away. It's really amazingly hard to pack a
    parachute so it won't open.
    RS: I assume the use of cable ties is a no-no?
    -- in the sdm
     
    S. Checker, Jan 9, 2009
    #4
  5. In article <>,
    (S. Checker) wrote:

    > Barry Margolin <> wrote:
    > > In article
    > > <>,
    > > wrote:
    > >
    > >> We're currently running a configuration that uses an IP address on the
    > >> FastEthernet port, ip unnumbered on the Serial pointing to FE0/0, and
    > >> OSPF. It works well but I am aware of the problems involved if we lose
    > >> the FE connection so I am planning to move to a loopback interface.
    > >>
    > >> Which should be simple enough, but I'm not positive of what I can get
    > >> away with. The obvious setup I came up with was:
    > >> R1:
    > >> FE0/0: 10.1.1.254/24
    > >> Loopback0: 10.10.1.1/30
    > >> Serial: ip unnumbered Loopback0
    > >> R2:
    > >> FE0/0: 10.1.2.254/24
    > >> Loopback0: 10.10.1.5/30
    > >> Serial: ip unnumbered Loopback0
    > >>
    > >> But the question that came up was whether, instead, I could use
    > >> 10.10.1.1/30 and 10.10.1.2/30 on Loopback0, or to put it another way,
    > >> can the two be on the same subnet?

    > >
    > > No, because then it won't send traffic to the other machine's loopback
    > > IP over the network.
    > >
    > > You should usually use /32 for loopback IPs, so you don't have to waste
    > > any addresses in that block.

    >
    > Not totally sure I follow you here.
    >
    > After all, if I used two subnets on the two ends of the serial link, as
    > I initially wrote up there, you're going to need a route on R1 to get
    > to 10.1.2.0/24. So you'd have a route (dynamic or static) on R1 like
    > 10.1.2.0/24 via 10.10.1.5, Serial0/0/0.
    >
    > But the problem there is that it's not on the same subnet with your own
    > serial port and therefore by IP rules you'd have to have a gateway.
    > What I'd expect to work would be a route like
    > 10.1.2.0/24 via 10.10.1.2, Serial0/0/0
    > 10.10.1.2/32 is directly connected, Serial0/0/0


    That's why you should configure them with /32 masks, not /30. If you
    configure them as /30, your routing protocol will not create that second
    route on R1. It will receive the /30 route from R2, but not put it into
    the routing table because its directly connected route on Loopback0
    takes precedence.

    --
    Barry Margolin,
    Arlington, MA
    *** PLEASE post questions in newsgroups, not directly to me ***
    *** PLEASE don't copy me on replies, I'll read them in the group ***
     
    Barry Margolin, Jan 10, 2009
    #5
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