How is Tunnel Transport MTU Calculated?
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06-08-2010 02:00 PM - edited 03-04-2019 08:43 AM
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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06-08-2010 03:19 PM
You pose a very good question. Every time I have to deal with tunnels and MTU I consult this document.
https://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml
Christopher Gatlin
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06-14-2010 06:45 AM
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
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07-18-2012 04:20 AM
Hi,
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
Ayhan CELIK
