How is Tunnel Transport MTU Calculated?

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Jun 8th, 2010
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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
  Description: xxxxxxx
  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?

Thanks, Mike

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michael.perry Mon, 06/14/2010 - 06:45
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Thanks! That document does have good information. I don't know what all of those outputs mean but I will ask a Cisco SE the next time they come around and see if they know. Everything is working and I'm out of time to look into it.

What got us into reviewing this is a problem with an AP talking over a WAN link to a wireless controller. The packet was being fragmented causing the wireless controller to not process the conversation. Not sure why that would be the case but allowing the mtu size to automatically lower based on mtu size along the way resolved the issue.

The following command was added the tunnel interface

tunnel path-mtu-discovery

I hope that helps someome. Thanks, Mike

ayhan.celik Wed, 07/18/2012 - 04:20
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Your IP MTU is set to 1472 Bytes,

- When Using GRE, it normally has 4 bytes GRE header, but when a key is used, it is 8 Bytes.

-New IP header for GRE Encapsulation is again 20 Bytes



Total= 28 Bytes GRE Overhead,

This 28 Bytes overhead must be deducted from IP MTU

Transport MTU= 1472 - 28 = 1444 Bytes



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