GRE Tunnel MTU shows 17862 bytes

[PraveenKumar6756]

Can someone tell me the difference between the following config? Why the first MTU shows 17862 bytes besides the fact that actual MTU of an interface is 1422 bytes. 

MTU 17862 bytes, BW 100 Kbit/sec, DLY 50000 usec

Tunnel protocol/transport IPSEC/IP                                                     

  Tunnel TTL 255                                                                         

  Tunnel transport MTU 1422 bytes                                                        

  Tunnel transmit bandwidth 8000 (kbps)                                                  

  Tunnel receive bandwidth 8000 (kbps)

[Georg Pauwen]

Hello,

which router model and which IOS is this on ?

[PraveenKumar6756]

Hi,

This is cisco3925-chassis and running on C3900e Software (C3900e-UNIVERSALK9-M), Version 15.2(4)M6a

[Cristian Matei]

Hi,

    In the newer IOS versions, the GRE tunnel interface MTU is now visible under "Tunnel Transport MTU". The other, additional MTU value you're seeing has nothing to do with MTU, an unfortunate naming, it is based on the platform/physical interface buffers, thus the value you'll see there will vary between platforms. 

Regards,

Cristian Matei.