Your company is extending their network by opening a new Remote Office in a different city. In the HeadQuarters, you run EIGRP between buildings and the design team has decided to add the Remote Office into the EIGRP domain and to maintain the same level of protection/privacy over the Internet.
To achieve this you must configure an IPsec tunnel and run EIGRP over it - you've chosen to use the old method of crypto maps with a GRE tunnel that allows you to run dynamic routing protocols over IPsec.

You perform the configuration below:

Quiz-6

and you see that you have a successfull IPsec tunnel between HQ and Remote Office and the EIGRP peering is also UP: 

HQ-Router#ping 172.16.255.2
Sending 5, 100-byte ICMP Echos to 172.16.255.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 52/76/100 ms
HQ-Router#
HQ-Router#sh crypto isakmp sa
IPv4 Crypto ISAKMP SA
dst             src             state          conn-id slot status
155.1.1.1       155.2.2.2       QM_IDLE           1001    0 ACTIVE
HQ-Router#
HQ-Router#sh crypto ipsec sa
   local  ident (addr/mask/prot/port): (155.1.1.1/255.255.255.255/47/0)
   remote ident (addr/mask/prot/port): (155.2.2.2/255.255.255.255/47/0)
...
    #pkts encaps: 9, #pkts encrypt: 9, #pkts digest: 9
    #pkts decaps: 8, #pkts decrypt: 8, #pkts verify: 8
...
HQ-Router#
*Mar  1 01:35:27.435: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 172.16.255.2 (Tunnel1) is up: new adjacency
HQ-Router#
HQ-Router#sh ip ei nei
IP-EIGRP neighbors for process 100
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
3   172.16.255.2            Tu1               14 00:00:06   84  5000  0  29
2   172.16.3.2              Fa1/0             10 00:05:49   52   312  0  120
1   172.16.2.2              Fa0/1             13 00:05:51   53   318  0  119
0   172.16.1.2              Fa0/0             11 00:05:53   50   300  0  121
HQ-Router#

After a while, you are aproached by the NOC telling you that the new IPsec tunnel and the EIGRP neighborship keep flapping up and down, impacting the connectivity between HQ and the remote office.

What is the problem ?

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