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Is it possible to see a Teredo address (from 2001::/32 block) in an IPv6 neighbor cache? For example, in the case of Junos, in the output of show ipv6 neighbors. As much as I tested, this is not possible because traffic leaving the Teredo client is already encapsulated in UDP/IPv4.

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    I don't think you will, as they aren't actually neighbors. – Michael Hampton Oct 17 '18 at 15:02
  • @MichaelHampton Do you mean that Teredo addresses are never configured on a physical interfaces and thus they are never a neighbor? – Martin Oct 17 '18 at 19:52
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    Teredo is a tunnel and has no neighbors. Neighbors are other IPv6 speaking devices on the same link. Teredo has none, as the link is virtual and no other devices are on it. – Michael Hampton Oct 17 '18 at 19:56
  • @MichaelHampton Yes, I think I understand this the same way. At least in Linux using Miredo Teredo client, the teredo interface(which is actually a tun interface) gets the IPv6 address from 2001::/32 block and traffic destined to IPv6 addresses leaving the Teredo host are encapsulated to UDP/IPv4. So indeed, the interface with Teredo IPv6 address has no neighbors. Also, as much as I understand the Teredo relay part, then they announce the 2001::/32 prefix, but do not have any interfaces with addresses from 2001::/32 address block, i.e there should be no neighbors from 2001::/32. – Martin Oct 18 '18 at 9:13
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    I think you should consider rewriting your comments to an answer, @MichaelHampton, so it can be accepted. – Teun Vink Oct 19 '18 at 12:24
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You won't see Teredo addresses in the neighbor cache, because Teredo creates a virtual point to point link and therefore has no neighbors. A neighbor is another device (which, for this purpose, speaks IPv6) on the same Layer 2 link. But a Teredo connection has no such devices sharing its virtual link.

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If you are seeing teredo addresses in a neigbour cache then something is seriously misconfigured.

Teredo creates a virtual interface on the teredo client allowing it to communicate with other teredo clients and relays. This virtual interface is not an Ethernet-like interface, so it has no need for neighbour discovery.

Unlike with 6to4 teredo has no provision for running a network behind the teredo client. So teredo addresses should never be assigned to a real network segment and therefore should never appear in a neighbour cache.

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