Command ip tunnel add has several additional options.
ttl N -- set fixed TTL N on tunneled packets.
N is number in the range 1-255. 0 is special value,
meaning that packets inherit TTL value.
Default value is: inherit.
tos T -- set fixed tos T on tunneled packets.
Default value is: inherit.
dev DEV -- bind tunnel to device DEV, so that
tunneled packets will be routed only via this device and will
not be able to escape to another device, when route to endpoint changes.
nopmtudisc -- disable Path MTU Discovery on this tunnel.
It is enabled by default. Note that fixed ttl is incompatible
with this option: tunnels with fixed ttl always make pmtu discovery.
ipip and sit tunnels have no more options. gre
tunnels are more complicated:
key K -- use keyed GRE with key K. K is
either number or IP address-like dotted quad.
csum -- checksum tunneled packets.
seq -- serialize packets.
1cm NB.
I think this option does not
work. At least, I did not test it, did not debug it and
even do not understand, how it is supposed to work and for what
purpose Cisco planned to use it.
Actually, these GRE options can be set separately for input and
output directions by prefixing corresponding keywords with letter
i or o. F.e. icsum orders to accept only
packets with correct checksum and ocsum means, that
our host will calculate and send checksum.
Command ip tunnel add is not the only operation,
which can be made with tunnels. Certainly, you may get short help page
with:
ip tunnel help
Besides that, you may view list of installed tunnels with the help of command:
ip tunnel ls
Also you may look at statistics:
ip -s tunnel ls Cisco
where Cisco is name of tunnel device. Command
ip tunnel del Cisco
destroys tunnel Cisco. And, finally,
ip tunnel change Cisco mode sit local ME remote HE ttl 32
changes its parameters.