Victor Sudakov wrote: > Patrick M. Hausen wrote: > > > > > Am 23.03.2020 um 06:00 schrieb Victor Sudakov <[email protected]>: > > > I've noticed that a newly created gre0 interface has the expected "mtu > > > 1476" > > > value, but a newly created gif0 interface has "mtu 1280", why would the > > > default be so low? > > > > gif is frequently used as the innermost encapsulation like in gif tunnel > > across host mode IPsec. Then there might be PPPoE, too. Possibly a > > VLAN tag ... > > Please correct me if I'm wrong: > > - ESP overhead - 40 bytes > - UDP encapsulation of ESP (udp/4500): 8 bytes > - PPPoE overhead - 8 bytes (?) > - A VLAN tag just increases the max frame size, it does not reduce the IP MTU. > > So we could keep the safe default for gif(4) at 1500-40-8-8=1444 bytes. > OK, at 1400 as for if_ipsec. But not at 1280!
I should probably have counted the 20 bytes of the additional IP header which results in 1500-40-8-8-20=1424. So 1400 is really safe. -- Victor Sudakov, VAS4-RIPE, VAS47-RIPN 2:5005/49@fidonet http://vas.tomsk.ru/
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