01-18-2021 09:52 AM
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
Solved! Go to Solution.
01-18-2021 10:16 AM - edited 01-18-2021 10:18 AM
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.
01-18-2021 09:59 AM - edited 01-18-2021 10:01 AM
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 :
01-18-2021 10:16 AM - edited 01-18-2021 10:18 AM
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.
01-18-2021 11:11 AM
GRE with IPSec total 1408 but it safe to be 1400
01-18-2021 11:13 AM - edited 01-18-2021 11:14 AM
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?
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide