Why do I need to manually adjust the MTU and MSS when using GRE tunnels if Cisco devices (and likely...

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Why do I need to manually adjust the MTU and MSS when using GRE tunnels if Cisco devices (and likely other TCP/IP devices) automatically use Path MTU Discovery (PMTUD)?

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I was reading a Reddit thread where the OP was advised to adjust the MTU and MSS when using GRE tunnels. This got me thinking because I know that Cisco devices (and likely other TCP/IP devices) use Path MTU Discovery (PMTUD). Since PMTUD is supposed to automatically adjust the MTU to prevent fragmentation, why would I still need to manually adjust the MTU and MSS? If the answer is that PMTUD is done before GRE encapsulation, why is that the case? Why isn’t PMTUD just redone after the interface is changed to a GRE tunnel?

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Posted: 5 months ago
Reddit URL: View post on reddit.com
External URL: reddit.com/r/networking/...