Say I have two IP's assigned to an interface (either a physical interface or an 802.1q sub-interface) and I create two gre interface, using each of the IP addresses assigned above as the local endpoint.  I can send GRE keepalives to either gre interface and a response is received.

Now if I bring one of the gre interfaces down and then send a keepalive to it, a response is still received.  Not until I bring down the other interface also do I stop getting responses.

Prior to the GRE/tunnel refactoring introduced in 3.10.0 (commit c5441932), the keepalives to the downed gre interface would not elicit a response.  I have tested kernels from 3.10.0 up through 3.13.0.

Steps to reproduce:
- Assign 192.168.56.1/24 to eth0 on a machine to be used to send the keepalives (SENDER).

- Assign 192.168.56.3/24 and 192.168.56.4/24 to eth0 on the device under test (DUT).
ip addr add 192.168.56.3/24 dev eth0
ip addr add 192.168.56.4/24 dev eth0

- Bring up the 2 GRE interfaces on DUT.
ip tunnel add gre1 mode gre remote 192.168.56.1 local 192.168.56.3 ttl 255
ip link set gre1 up
sysctl -w net.ipv4.conf.gre1.accept_local=1
sysctl -w net.ipv4.conf.gre1.forwarding=1
sysctl -w net.ipv6.conf.gre1.forwarding=1
ip tunnel add gre2 mode gre remote 192.168.56.1 local 192.168.56.4 ttl 255
ip link set gre2 up
sysctl -w net.ipv4.conf.gre2.accept_local=1
sysctl -w net.ipv4.conf.gre2.forwarding=1
sysctl -w net.ipv6.conf.gre2.forwarding=1

- I used scapy on SENDER to generate a GRE keepalive packet to 192.168.56.3 (08:00:27:53:aa:c1 is the MAC address of eth0 on DUT):
>>> ip = IP(src='192.168.56.1', dst='192.168.56.3')
>>> pkt = Ether(dst='08:00:27:53:aa:c1')/ip/GRE()/IP(src=ip.dst,
>>> dst=ip.src)/GRE()
>>> sendp(pkt, iface='eth0')

- When issuing the sendp command, run "tcpdump -nvi eth0 proto gre" on SENDER.  You should see the following request and reply:

22:24:09.608401 IP (tos 0x0, ttl 64, id 1, offset 0, flags [none], proto GRE (47), length 48)
    192.168.56.1 > 192.168.56.3: GREv0, Flags [none], length 28
        IP (tos 0x0, ttl 64, id 1, offset 0, flags [none], proto GRE (47), length 24)
    192.168.56.3 > 192.168.56.1: GREv0, Flags [none], length 4
        gre-proto-0x0
22:24:09.608879 IP (tos 0x0, ttl 63, id 1, offset 0, flags [none], proto GRE (47), length 24)
    192.168.56.3 > 192.168.56.1: GREv0, Flags [none], length 4
        gre-proto-0x0

- On DUT, run "ip link set gre1 down" and do the sendp command again from SENDER, watching the tcpdump output as well.  With kernels 3.10.0+, you'll still see a response packet.  With earlier kernels, no response is sent, which is the appropriate behavior.

Note that, with 3.10.0+, if you now run "ip link set gre2 down" and do the sendp again, no response is sent.