What is the recommended MTU size for GRE tunnel interfaces?

[mrmarie]

Hello 

We have two Cisco 800 router connected via 4G between two sites (GRE Tunnel), the MTU size configured in tunnel interface (1476), so is this value correct or should I change it as some as some protocols do not pass.

Appreciate your help

Thanks

[balaji.bandi]

here is the over heads (from my documents)

GRE  Only set MTU to 1500-24 = 1476 byte

Overhead associated with IPsec and GRE

==========================================

Encapsulation Overhead GRE only           |     24 bytes

IPsec (Transport Mode)                            |     36 bytes

IPsec (Tunnel Mode)                                |     52 bytes

IPsec (Transport Mode) + GRE                 |     60 bytes

IPsec (Tunnel Mode) + GRE                     |     76 bytes

Good reference :

https://www.cisco.com/c/en/us/support/docs/ip/generic-routing-encapsulation-gre/25885-pmtud-ipfrag.html

[Georg Pauwen · Accepted solution]

Hello,

1476 is the default. Try and also set the mss:

interface Tunnel0

--> ip tcp adjust-mss 1436

If that does not help, set the mtu to 1400 and the tcp adjust-mss to 1360.

Also, post the full running configuration of the router.

[MHM Cisco World]

GRE with IPSec total 1408 but it safe to be 1400

[Joseph W. Doherty]

As both Balaji and Georg have already noted, "standard" GRE headers consume 24 bytes, so typically you will, as you also note, set an interface's IP MTU (not interface's actual/physical MTU) to 1476.

Georg also makes a good suggestion about using the adjust-mss command.  However, the 1436 value assumes standard/usual/minimal sized IP and TCP headers are being used.  (NB: there's other good information in the document that Balaji provided the link to.)

BTW, have you tried pinging across this link, using a DF setting with various packet sizes?