GRE Question

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Jun 29th, 2010
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Hi,


I noticed that some configurations of GRE tunnels define an IP address for the actual tunnel interface and that some configurations do not.  Can someone please explain in what situations an ip address should be assigned to a GRE tunnel interface and in what situations you would not?


Thank you,


David

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Richard Burts Wed, 06/30/2010 - 00:46
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David


It would be interesting to know where these examples came from and how old they are. In (very) old usage the GRE tunnel might not have an IP address because it was transporting Appletalk or IPX traffic. In more modern usage I do not see many GRE tunnels that do not have IP addressing configured>


HTH


Rick

yuchenglai Wed, 06/30/2010 - 09:09
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Rick,


Ultimately, what is the point of assigning an IP address to a GRE tunnel interface? What additional functionality is gained from that?


David Lai

Richard Burts Wed, 06/30/2010 - 09:21
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David


The functionality is that if you assign an IP address to the tunnel then you can pass IP traffic over the tunnel. If you do not assign an IP address to the tunnel then you can not pass IP traffic over the tunnel.


What kind of traffic would you want to go through the tunnel? You need to assign an address that matches that kind of traffic.


HTH


Rick

yuchenglai Wed, 06/30/2010 - 09:50
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Rick,


I do need to tunnel IP traffic within the GRE tunnel. So, are you saying that I can't simply define just a tunnel source and destination, but I also have to assign IP addresses to the tunnel interface as well?


David Lai

Richard Burts Wed, 06/30/2010 - 10:01
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David


Yes that is what I am saying.


If you configure a GRE tunnel with just a tunnel source and a tunnel destination but no IP address assigned to the tunnel, then the tunnel will not be processing IP and the tunnel will not transport any IP traffic.


It is the same concept as why we assign an IP address to a FastEthernet interface of the router. If your router has interface FastEthernet0/0 and if you do not configure an IP address on the interface and then you connect a PC to that interface you will find that the PC can not communicate because FastEthernet0/0 will not pass any IP traffic since it does not have an IP address.


It is a fundamental concept of routers - if you want the router interface to process IP traffic then the interface must be configured with an IP address.


HTH


Rick

yuchenglai Wed, 06/30/2010 - 10:50
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That certainly helps Rick. Thanks for your insight.


David Lai

Richard Burts Wed, 06/30/2010 - 11:01
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David


I am glad that the information that I provided has helped you to understand this better. I encourage you to continue your participation in the forum.


HTH


Rick

manish arora Wed, 06/30/2010 - 12:40
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Hi David ,

yes , you configue tunnel interfaces without ip address in condition when you want to use that tunnel as more of a private routing pipe . cannot give you exmaples as it is against my company policies . anyway, to understand the concept more try reading articals on honey d implementation for remote subnets on linux forums.


or give me a idea of what are you trying to do , so that i can explain you using your senario rather than giving my own example to you.


Thanks

Manish

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