How to configure GRE tunnel between 2 Cisco Routers

I have two routers connected back to back. I am running OSPF between them.
I would like to know how to configure GRE tunnel between these 2 routers.
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SrikantRajeevAsked:
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Istvan KalmarHead of IT Security Division Commented:
Please show us botth routers config
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Don JohnstonInstructorCommented:
Here's the basic idea:

And a link:

http://www.expertsupport.net/en/US/tech/tk86/tk89/technologies_configuration_example09186a008011520d.shtml



Router 1:

interface tunnel 0
 ip address 192.168.1.1 255.255.255.252
 tunnel source fa0/0
 tunnel destination 12.1.2.3

Router 2:

interface tunnel 0
 ip address 192.168.1.2 255.255.255.252
 tunnel source fa0/0
 tunnel destination 160.160.1.19

Open in new window

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SrikantRajeevAuthor Commented:
the link is not opening.
Just specifying source & destination IP will established the GRE tunnel
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lanboyoCommented:
Yes. You would need to put the network command for the GRE interface's network address in the OSPF configuration to make OSPF work on the interface however.
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Don JohnstonInstructorCommented:
>the link is not opening.

Works fine for me.

>Just specifying source & destination IP will established the GRE  tunnel

On each router, correct.


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Istvan KalmarHead of IT Security Division Commented:
Please provide us:

sh ip int brief
sh ip route
sh ip mtu..


Don config is good, if it isn't working you must to use lower MTU on GRE tunnel interface, becouse GRE has 24 byte overhead, so you need this on tunnel interfaces:

int tun x
 ip mtu 1476

and also on fasteth interfaces
int tun x
 ip mtu 1476
 ip tcp adjust-mss 1428
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tirabytesCommented:
On the source interface (Fa0) do you have an active ACL enabled? And can you provide us your Running/Startup config, Route Table as mentioned above?

First I would take off OSPF and replace it with a simple static route, and see how that goes.  
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SrikantRajeevAuthor Commented:
MY GRE tunnel is working fine by just specifying the source & destination tunnel.
Let me know if any one has configured IPSec tunnel between Cisco Router & NEC IX router.
If any one has sample configuration to configure IPSec tunnel between Cisco router & NEC IX router please share the same with me
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Don JohnstonInstructorCommented:
You should open a new question for this.
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