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Notice that although Router A indicates that the interface is up, because Router D does not yet have a tunnel interface, nothing can be sent across the link. Be careful of this, as you may get confused under pressure. The tunnel network 172.16.0.0/24 is even active in Router A’s routing table (it will not be found on Router D at this time):

From

Cover of Network Warrior, 2nd Edition

Note

Loopholes that may trick you in GRE