[nsp] gre tunnel using loopback for source

[Gert Doering]

Hi,

On Mon, Jun 02, 2003 at 04:29:11PM -0500, Pete Templin wrote:
> On IOS 12.2(16), I did notice (but didn't investigate further; didn't
> have an easy way to build a test network due to overlapping VLAN
> identifiers) that the tunnel would show a status of up/up as soon as a
> valid route was created to the destination, even if the far end router
> was non-existent.  Might have been a random goof on my part; but might
> be more important in other topologies.

No, that wasn't a goof on your part, that's just the way a GRE tunnel
works.  As there is nothing negotiated and no keepalives exchanged (by
default), the router has no way of knowing whether the remote end is 
alive or not - and thus the interface goes up as soon as source+
destination are configured and a route to the destination exists.

In the "interface tunnel X" configuration, there *is* a way to configure
keepalive - but I haven't yet tried it to see what the router will do
if this is set.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert.doering at physik.tu-muenchen.de