IP MTU 1416

[bgrove2913]

Can someone pleaes explain to me where the number 1416 comes from when it's needed to be configured on the tunnel(GRE over IPSec) interface?

Thanks

[ehirsel]

This link: http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

discusses path mtu discovery (PMTU) as well as implications for ptmt, ipsec, gre, and gre with ipsec.

I noted a 1420 gre mtu when gre is used with ipsec tunnel mode. I noted a 1476 (not 1416) value when gre is used, as 24 bytes are used for the gre header (1500 - 24 = 1476).

Let me know if you need any more help.

[bgrove2913]

Thanks, it's very good doc.

But if you look at the config examples in the UniverCD, they always use MTU 1416. Just wondering why.

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t13/ftgreips.htm#wp1062127