How to configure GRE Tunnel on Cisco IOS Router

(Rene Molenaar) #1

This topic is to discuss the following lesson:

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(system) #2

Good stuff Rene.

Next steps would be configuration for VRF-lite aware GRE tunnel with IPsec?

:slight_smile:

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(Rene Molenaar) #3

That would be a good idea. I’ll write something soon.

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(system) #4

Thank you, very helpful!
i am going to read more from this website! :slight_smile:

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(system) #5

Excellent ! I am new to Tunnelling stuff. Helped me to understand well. Thanks Rene :smiley:

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(Rene Molenaar) #6

Glad I could help Satish.

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(system) #7

Great explanation Rene! I would be reading more IE stuff from your site!

Please keep the writing on.

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(Rene Molenaar) #8

Glad you like it Pradeep! I will keep on writing don’t worry :slight_smile:

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(system) #9

Whats the point of having a loopback interface? Why are you not using loopback interface as the source for the GRE tunnel. Overall a great explanation, top hit on Google.

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(Rene Molenaar) #10

The point of the loopback interface is to have some networks that we can advertise in EIGRP.

You could use the loopback interface as the source for the tunnel but for this tutorial it’s not necessary because:

- The branch and HQ router would each require an additional static route so they can reach each others loopback interface.
- The branch and HQ router each only have a single connection to the ISP router. If they were connected to a second ISP it would be nice to use a loopback interface for GRE because you could create some redundancy, this means you would have to advertise the loopbacks in a routing protocol though.

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(system) #11

*Jul 11 05:59:54.736: %DUAL-6-NBRINFO: EIGRP-IPv4 10: Neighbor 10.1.1.2 (Tunnel0) is blocked: not on common subnet (10.1.1.1/31)

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(Rene Molenaar) #12

You get this error when the two EIGRP neighbors are not in the same subnet. Check your IP addresses and subnet masks. In your example you are using a /31 subnet mask…

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(system) #13

Good lesson Rene, one question though, this might be very noobie(well I am!!),
Why did you advertise the tunnel network into EIGRP, I’m thinking since there is connectivity via the tunnel between the locations, only the absent networks need to be advertised. Please help me out.
Thanks a lot

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(Rene Molenaar) #14

Hi Kelvin,

It’s a good question, most beginners ask me the same question. The “network” command for OSPF or EIGRP does two things:

- It advertises the network.
- It will start sending hello packets on the interface.

So for this example, “network 172.16.1.0” and “network 172.16.3.0” are required because we want to advertise these networks.

And “network 192.168.13.0” is required so that the two routers will send hello packets to each other through the tunnel, we need this so they can become neighbors.

Does that make sense?

Rene

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(system) #15

Thanks a lot Rene, yes it does make a lot of sense.
Labbed it and saw it in action, was letting my head confuse me that the tunnel is a trunk!!!

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(Rene Molenaar) #16

Good :slight_smile:

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(system) #17

hi rene . please help to me for DMVPN configuration . thank you .

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(Rene Molenaar) #18

Here is an example:

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(system) #19

hi rene if we use ospf as place of eigrp protocol
in gre tunnel its possible how and why

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(ALFREDO V) #20

I have to be honest with you. I purchased all you CCNP ebooks and was kind of overwhelming to go through a lot of theory with the examples at the end, but the way you organized these chapters makes it easier, exciting and they are not long. You did a great job combining theory, configuration and practice and you have the talent to explain complicated material and make it easy to understand.
Thanks

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