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New Member

Multipoint GRE and Multicast

In the "Dynamic Layer-3 VPNs (RFC 2547) Support Using Multipoint GRE (mGRE) Tunnels" document, the following statement is made, "For multipoint GRE tunnels, a tunnel key must be configured. Unlike other tunnels, the tunnel destination is optional. However, if the tunnel destination is supplied, it must map to an IP multicast address." I've tried to implement a tunnel with a multicast address as the destination, but the IOS will not allow the assignment. I've tried different methods and approaches, but nothing seemed to work. I've gone through the various RFC documents for GRE and found nothing that would explicately restrict this. Has anyone actually been able to do this? Thanks in advance.

4 REPLIES
New Member

Re: Multipoint GRE and Multicast

Try this in your tunnel interface. You'll see that you don,t have to put the tunnel destination address

ip address ...

...

ip nhrp authentication ...

ip nhrp map [destination private IP1] [destination public IP1]

ip nhrp map multicast [destination public IP1]

ip nhrp map [destination private IP2] [destination public IP2]

ip nhrp map multicast [destination public IP2]

ip nhrp network-id ...

ip nhrp holdtime ...

ip nhrp nhs [destination private IP1]

ip nhrp nhs [destination private IP2]

...

tunnel source ...

tunnel mode gre multipoint

tunnel key ...

tunnel protection ...

Let me know if it's works

New Member

Re: Multipoint GRE and Multicast

I've already done something similar, but that's not providing the functionality/capability I need. I'm specifically looking for the method to actually use a multicast address for the multipoint tunnel destination.

New Member

Re: Multipoint GRE and Multicast

What's the functionality that you want to use that the multicast destination address will give you ?

It's not a real multicast destination address that you have to put. It's normal that your IOS doesn't permit this. You have to map a NBMA address to your tunnel destination address (mapping). This will allow your broadcast and multicast request to be send to the destination address of your mapping.

New Member

Re: Multipoint GRE and Multicast

The functionality is having an actually multicast address as the destination of the multipoint tunnel--whether mapped or otherwise. The document states "if the tunnel destination is supplied, it must map to an IP multicast address" and this is a Cisco document. The assumption is that I want to "allow ... broadcast and multicast requests (sic) to be sent (sic) to the destination address..." and that is not what I asked--there already exist plenty of examples and instructions on how to do so. The Cisco document states a multicast address could be a tunnel destination but the Cisco IOS is seemingly not allowing this to occur.

So, I ask again "Has anyone actually been able to do this?"

If so, how was this done?

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