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This Question is Not Answered 1 Correct Answer available (4 pts) 2 Helpful Answers available (2 pts)
1872 Views 4 Replies Latest reply: Aug 22, 2011 12:54 AM by Cuong RSS

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GRE tunnels HELP?

Aug 20, 2011 10:50 PM

BHARAT 34 posts since
Apr 12, 2011



  • Martin 13,892 posts since
    Jan 16, 2009
    Currently Being Moderated
    1. Aug 20, 2011 11:13 PM (in response to BHARAT)
    Re: GRE tunnels HELP?

    Yes, either or VPN or GRE tunnels (generic routing encapsulation or GRE) will provide an end-to-end, segregated path across the network.


    GRE is a Cisco tunneling protocol capable of encapsulating a wide variety of network layer protocols packets inside special IP tunnels. The goal is to create a virtual point-to-point link between two remote locations, sort of VPN link.


    GRE is not encryption protocol; it is just encapsulation protocol. S, you still need add encryption for security;


    GRE ccan replaced OSPF Virtual links;


    GRE are easy to create;


    One side is

    1. create interface tunnel 0

    2. add ip address

    3. specify tunnel source

    4. tunnel destination


    the other side will have


    interface Tunnel0

    ip address

    tunnel source

    tunnel destination

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  • Kumar Mantri 311 posts since
    Jan 6, 2011
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    2. Aug 21, 2011 12:13 AM (in response to Martin)
    Re: GRE tunnels HELP?

    Ipsec is used with GRE encapsulation because GRE sends data in plain text format



    Find the link which describes about IPSEC over GRE tunnels

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  • Naren 221 posts since
    Feb 3, 2009
    Currently Being Moderated
    3. Aug 21, 2011 11:15 PM (in response to BHARAT)
    Re: GRE tunnels HELP?

    Hi  Bharat,


    Adding to the excellent answers above, after configuring GRE tunnel, your IGP will now see those routes learned via the external VPN network reachable via a point to point interface which is your GRE tunnel interface. As the name applies, point to point = hop count one, which makes it easier for you IGP for metric calculations too.


    Any discontinous network can be made to look as if it is single hop away (connected via the point to point tunnel interface).


    When you understand the fact that it can connect any discontinuos network, you can understand Brian's statement on how it can relace the virtual link in OSPF. As you progress in your studies and understand Stub areas, you will come to know that It can even be applied in places where a virtual link cannot be applied.


    Area 0 -- > Area 1 -- > Area 2 -- > Area 3


    In above network, if area 2 is a Stub, but you still want to have area 3 in your OSPF domain, you use a GRE tunnel between the Area12 ABR and AREA 23 ABR to extend area 0 to Area 3. And you will use a Virtual link between Area01 ABR and Area 12 ABR to extend area 0 out to Area 2.


    I  learned this concept for God Scott. Thanks to him

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  • Cuong 24 posts since
    Mar 7, 2011
    Currently Being Moderated
    4. Aug 22, 2011 12:54 AM (in response to BHARAT)
    Re: GRE tunnels HELP?

    GRE is layer 3 IP protocol. It can encapsulate following protocols: IP, IPX, apple talk ....

    GRE has two main disadvantages:

    +GRE works only on cisco routers

    +GRE lacks protection capacities: authentication, encryption, integrity checking


    It can be combined with other solution, such as IP SEC, to create a more robust VPN deloyment

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