Hi all Usually we configure ip mtu 1400 for DMVPN tunnel interface and there is a standard calculation for it. I did it long time ago and trying to see, if I am having the right understanding now.
Ethernet MTU - 1500 IPSec IP header - 20 bytes GRE IP header - 20 bytes Payload IP header - 20 bytes TCP header - 20 bytes Total of 80 bytes. 1500 - 80 = 1420 Including others like ESP header & trailer, GRE header etc, we round it to 1400. Hence, we add ip mtu of 1400 to DMVPN tunnel interface, to avoid fragmentation.in between. Correct me, if I am wrong. TCP MSS TCP MSS => IP MTU - TCP header size which is 1400 - 20 = 1380 bytes We usually configure "tcp adjust-mss 1360". Any idea why it is 1360 instead of 1380? With regards Kings
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