[c-nsp] GRE tunnel (inside ICMP fails after two pings) - Wits End

[David Deutsch]

Hoping the list can help with this one.

I have a basic GRE tunnel between my Cisco ASR1006 and a Linux box.

On the Cisco side:

interface Tunnel100
 description Tun 100 - BPT
 ip address 172.16.100.1 255.255.255.0
 tunnel source x.x.136.1
 tunnel destination x.x.x.234

I have several of these basic GRE tunnels from this router, however this is
the only one giving me problems.

The tunnel source is my loopback, I can ping the local 172.168.100.1,
however when I try to ping the other inside:

#ping 172.16.100.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.100.2, timeout is 2 seconds:
!!...
Success rate is 40 percent (2/5), round-trip min/avg/max = 97/105/113 ms

It always dies after two pings, every time. Additionally, pinging from the
other side has the exact same results:

ping 172.16.100.1
PING 172.16.100.1 (172.16.100.1): 56 data bytes
64 bytes from 172.16.100.1: seq=0 ttl=255 time=83.430 ms
64 bytes from 172.16.100.1: seq=1 ttl=255 time=88.326 ms
... then nothing.

I've gone as far as to completely rebuild the Linux side with no luck and
I'm starting to feel that I've missed something basic on the Cisco side,
except I've used these tunnels for years.

Any advice/ideas?

Thanks,
David