I have a series of routers (Cisco ISR 1811) that has defaulted to an MTU size of 17867. This only happens on tunnel interfaces, and seems to happen for all tunnel interfaces on that series router.  The MTU on my 2811 & 2821 all default to MTU 1514 bytes.

There is no MTU manully configured.  I have searched and can't find what would cause this.

Any thoughts?

Thank You

Tunnel1 is up, line protocol is up
  Hardware is Tunnel
  Description: Tunnel Holly
  Internet address is z.z.z.z/30
  MTU 17867 bytes, BW 1500 Kbit/sec, DLY 50000 usec,
     reliability 255/255, txload 1/255, rxload 1/255
  Encapsulation TUNNEL, loopback not set
  Keepalive not set
  Tunnel source y.y.y.y, destination x.x.x.x
  Tunnel protocol/transport IPSEC/IP
  Tunnel TTL 255
  Tunnel transport MTU 1427 bytes
  Tunnel transmit bandwidth 8000 (kbps)
  Tunnel receive bandwidth 8000 (kbps)
  Tunnel protection via IPSec (profile "Crytp_Profile_1")
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/0 (size/max)
  5 minute input rate 2000 bits/sec, 2 packets/sec
  5 minute output rate 2000 bits/sec, 2 packets/sec
     25658 packets input, 2909400 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     26567 packets output, 3632936 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 unknown protocol drops
     0 output buffer failures, 0 output buffers swapped out