Comment 9 for bug 1896448

The test script fails at the following part:

        # test external mode
        ip -netns "$testns" link add dev "$DEV_NS" type ip6gretap external
        check_err $?

Which executes:

# ip -netns testns link add dev ip6gretap00 type ip6gretap external

Looking at ip-link(8) manpage, the ip6gretap type accepts the following options:

ip link add DEVICE type { ip6gre | ip6gretap } remote ADDR local ADDR [ [i|o]seq ]
              [ [i|o]key KEY ] [ [i|o]csum ] [ hoplimit TTL ] [ encaplimit ELIM ] [ tclass TCLASS
              ] [ flowlabel FLOWLABEL ] [ dscp inherit ] [ dev PHYS_DEV ]

So "external" is not supported yet in this iproute2 version (4.15.0-2ubuntu).