static int
prelist_update(struct nd_prefixctl *new, struct nd_defrouter *dr,
    struct mbuf *m, int mcast)
{
[...]
	if (ia6_match == NULL && new->ndpr_vltime) {
		int ifidlen;

		/*
		 * 5.5.3 (d) (continued)
		 * No address matched and the valid lifetime is non-zero.
		 * Create a new address.
		 */

		/*
		 * Prefix Length check:
		 * If the sum of the prefix length and interface identifier
		 * length does not equal 128 bits, the Prefix Information
		 * option MUST be ignored.  The length of the interface
		 * identifier is defined in a separate link-type specific
		 * document.
		 */
		ifidlen = in6_if2idlen(ifp);
		if (ifidlen < 0) {
			/* this should not happen, so we always log it. */
			log(LOG_ERR, "prelist_update: IFID undefined (%s)\n",
			    if_name(ifp));
			goto end;
		}
		if (ifidlen + pr->ndpr_plen != 128) {
			nd6log((LOG_INFO,
			    "%s: invalid prefixlen %d for %s, ignored\n",
			    __func__, pr->ndpr_plen, if_name(ifp)));
			goto end;
		}

/*
 * Provide the length of interface identifiers to be used for the link attached
 * to the given interface.  The length should be defined in "IPv6 over
 * xxx-link" document.  Note that address architecture might also define
 * the length for a particular set of address prefixes, regardless of the
 * link type.  As clarified in rfc2462bis, those two definitions should be
 * consistent, and those really are as of August 2004.
 */
int
in6_if2idlen(struct ifnet *ifp)
{
	switch (ifp->if_type) {
	case IFT_ETHER:		/* RFC2464 */
	case IFT_PROPVIRTUAL:	/* XXX: no RFC. treat it as ether */
	case IFT_L2VLAN:	/* ditto */
	case IFT_BRIDGE:	/* bridge(4) only does Ethernet-like links */
	case IFT_INFINIBAND:
		return (64);
	case IFT_PPP:		/* RFC2472 */
		return (64);
	case IFT_FRELAY:	/* RFC2590 */
		return (64);
	case IFT_IEEE1394:	/* RFC3146 */
		return (64);
	case IFT_GIF:
		return (64);	/* draft-ietf-v6ops-mech-v2-07 */
	case IFT_LOOP:
		return (64);	/* XXX: is this really correct? */
	default:
		/*
		 * Unknown link type:
		 * It might be controversial to use the today's common constant
		 * of 64 for these cases unconditionally.  For full compliance,
		 * we should return an error in this case.  On the other hand,
		 * if we simply miss the standard for the link type or a new
		 * standard is defined for a new link type, the IFID length
		 * is very likely to be the common constant.  As a compromise,
		 * we always use the constant, but make an explicit notice
		 * indicating the "unknown" case.
		 */
		printf("in6_if2idlen: unknown link type (%d)\n", ifp->if_type);
		return (64);
	}
}

   For all unicast addresses, except those that start with the binary
   value 000, Interface IDs are required to be 64 bits long and to be
   constructed in Modified EUI-64 format.

gifconfig_gif1="198.51.100.1 203.0.113.1"
ifconfig_gif1_ipv6="inet6 2001:db8:55aa::1 2001:db8:55aa::2 prefixlen 128"

gifconfig_gif0="203.0.113.1 198.51.100.1"
ifconfig_gif0_ipv6="inet6 2001:db8:55aa::2 2001:db8:55aa::1 prefixlen 128"

ipv6_static_routes="home"
ipv6_route_home="2001:db8:aa55::/48 2001:db8:55aa::2"

ipv6_defaultrouter="2001:db8:55aa::1"

flush;
spdflush;
spdadd 2001:db8:aa55::/48 2000::/3 any -P out ipsec esp/tunnel/203.0.113.1-198.51.100.1/require;
spdadd 2000::/3 2001:db8:aa55::/48 any -P in ipsec esp/tunnel/198.51.100.1-203.0.113.1/require;

flush;
spdflush;
spdadd 2000::/3 2001:db8:aa55::/48 any -P out ipsec esp/tunnel/198.51.100.1-203.0.113.1/require;
spdadd 2001:db8:aa55::/48 2000::/3 any -P in ipsec esp/tunnel/203.0.113.1-198.51.100.1/require;