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mrmarie
Beginner

What is the recommended MTU size for GRE tunnel interfaces?

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

1 ACCEPTED SOLUTION

Accepted Solutions
Georg Pauwen
VIP Expert

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.

View solution in original post

4 REPLIES 4
balaji.bandi
VIP Master

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

 

BB

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Georg Pauwen
VIP Expert

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.

View solution in original post

MHM Cisco World
Collaborator

GRE with IPSec total 1408 but it safe to be 1400

Joseph W. Doherty
Hall of Fame Expert

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?