I searched this support community and Cisco in general and can't find and the answer. Here is my scenario.
A dialer 1 interface set to mtu 1472
A fastethernet interface set to mtu 1472 in the dialer 1 pool.
A GRE IPSec tunnel 1 that uses dialer 1 as its source and no mtu showing in the config, but the following mtu statements showing in the output. It all works fine, but I'm curious, when I show the tunnel1 I get the following:
show int tunnel 1
Tunnel1 is up, line protocol is up
Hardware is Tunnel
Internet address is x.x.x.x/xx
MTU 17912 bytes, BW 100 Kbit/sec, DLY 10000 usec,
reliability 255/255, txload 139/255, rxload 209/255
Encapsulation TUNNEL, loopback not set
Keepalive not set
Tunnel source x.x.x.x (Dialer1), destination x.x.x.x
Tunnel protocol/transport GRE/IP
Tunnel TTL 255
Fast tunneling enabled
Path MTU Discovery, ager 10 mins, min MTU 92
Tunnel transport MTU 1444 bytes
I assume that the line that begins with MTU 17912 and Tunnel transport MTU 1444 bytes are calculated based on the MTU of the associated dialer and Fastethernet interfaces. Does anyone know how this works?