GRE, importance of IP address for tunnel interface

Discussion in 'Cisco' started by Prasanna, Sep 1, 2006.

  1. Prasanna

    Prasanna Guest

    Hi All,

    Setup is as follows

    Net1 Router Our glob network Router Net2 |Router1| |Router2|

    I was trying to establish a tunnel between Net1 to Net2
    In Router2(Cisco) I need to give totally 3 addresses
    Tunnel src= Tunnel.dst= and
    If I give the tunnel Ip mentioned it gives error saying it overlaps
    with the interface connected to Net2 network, obviously it overlaps,
    but how to get out of this error? what IP I need to configure? What
    sense it makes when we have already tunnel source and destination
    addresses(because GRE needs only those two addresses)?

    Earlier reply would really help.

    Thanks and Regards,
    Prasanna, Sep 1, 2006
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  2. Prasanna

    Bod43 Guest

    A tunnel /creates/ a new interface and it needs an address.
    You also need to specify the tunnel soure and destination
    addresses so that the tunnel (GRE) traffic can flow.

    You can typically use /any/ addresses for the tunnel
    interfaces themselves as long as there is no conflict
    with existing addresses in your net.

    interface Tunnel200
    ip address
    tunnel source FastEthernet0/0
    tunnel destination

    interface FastEthernet0/0
    description Internet Facing Link
    ip address

    interface Tunnel2
    ip address
    tunnel source BVI1
    tunnel destination

    interface BVI1
    description Internet Facing Link
    ip address

    I find this quite confusing, draw pictures.
    Bod43, Sep 1, 2006
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  3. Prasanna

    Prasanna Guest

    Thanks Bod,

    I have found the answer in other way

    interface Tunnel1
    Source: destination:
    Route entry: tunnel1

    Interface1 ethernet
    ip address

    ip address

    interface Tunnel1
    ip address
    tunnel source FastEthernet0/0
    tunnel destination

    interface FastEthernet0/0
    description Internet Facing Link
    ip address

    interface FastEthernet0/1
    ip address

    In Cisco whatever the address we configure that subnet packets are
    directed to that tunnel. Eg. If we configure tunnel ip address as then route entry
    tunnel1, will be created in the routing table dynamically.
    Thats why it was not allowing to assign IP address falling in any
    subnet of the existing interfaces.
    After configuring IP address which falls in destination network, it
    started working.

    Prasanna, Sep 4, 2006
  4. Prasanna


    Jan 31, 2009
    Likes Received:
    Hi Prasanna.
    I am trying to understand how tunnel works.
    Let me ask few questions.

    1. Why in your successful setup you don't
    assign IP adress to tunnel interface?

    2. Route entry: tunnel

    Where this route entry came from.?
    When it showed up in the routing table for the first time?
    Could you please post what isexact format of your NEW routing
    tables entries on both routers.

    slyubski, Jan 31, 2009
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