Arguments:

• name NAME (default)

  -- select the tunnel device name.

• mode MODE

  -- set the tunnel mode. Three modes are currently available: ipip, sit and gre.

• remote ADDRESS

  -- set the remote endpoint of the tunnel.

• local ADDRESS

  -- set the fixed local address for tunneled packets. It must be an address on another interface of this host.

• ttl N

  -- set a fixed TTL N on tunneled packets. N is a number in the range 1-255. 0 is a special value meaning that packets inherit the TTL value. The default value is: inherit.

• tos T or dsfield T

  -- set a fixed TOS T on tunneled packets. The default value is: inherit.

• dev NAME

  -- bind the tunnel to the device NAME so that tunneled packets will only be routed via this device and will not be able to escape to another device when the route to endpoint changes.

• nopmtudisc

  -- disable Path MTU Discovery on this tunnel. It is enabled by default. Note that a fixed ttl is incompatible with this option: tunnelling with a fixed ttl always makes pmtu discovery.

• key K, ikey K, okey K

  -- (only GRE tunnels) use keyed GRE with key K. K is either a number or an IP address-like dotted quad. The key parameter sets the key to use in both directions. The ikey and okey parameters set different keys for input and output.

• csum, icsum, ocsum

  -- (only GRE tunnels) generate/require checksums for tunneled packets. The ocsum flag calculates checksums for outgoing packets. The icsum flag requires that all input packets have the correct checksum. The csum flag is equivalent to the combination ``icsum ocsum''.

• seq, iseq, oseq

  -- (only GRE tunnels) serialize packets. The oseq flag enables sequencing of outgoing packets. The iseq flag requires that all input packets are serialized. The seq flag is equivalent to the combination ``iseq oseq''.

  1cm NB. I think this option does not work. At least, I did not test it, did not debug it and do not even understand how it is supposed to work or for what purpose Cisco planned to use it. Do not use it.