Tag Archives: mtu

MTU 1524 on GRE Tunnels (13 of 30)

When I first came across this configuration I was confused as to how it was handling either UDP packets of 1500 bytes or TCP packets where PMTUD failed, as either an ICMP issue, or an issue where TCP did not correctly address PMTUD issues.

interface Tunnel10
ip address
ip mtu 1524
ip tcp adjust-mss 1352
tunnel source Loopback1
tunnel destination

The traditional method for fixing this issue would be to clear the df bit, set the MSS as you would normally, and lower the MTU.… Read more