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I am asking this because my ISP uses SLAAC/DHCPv6 to assign addresses to clients. Firewalls, no matter what brand/name/type get a link local IPv6 address complete with a link local gateway AND clients on the intranet actually get a real global IPv6 address from a /64 block--the upstream router is actually doing delegation.

What's not very clear to me though is why the link local addresses? I know that when doing DHCP delegation routers don't need an address for themselves, so are these link local addresses actually in use or were they just negotiated like a client would negotiate a link local anywhere else (even when there's no IPv6 deployed).

Can a link local addresses/networks be used as transit networks and still be routable up until the end (as long as the host address is a global IP address)?

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    The IPv6 RFC requires that every IPv6 interface get a link-local address. There really is no such thing as a link-local gateway because link-local addressing cannot be routed, hence the name link-local. – Ron Maupin Mar 25 at 12:22
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RFC 4291, IP Version 6 Addressing Architecture explains Link-Local addressing:

2.5.6. Link-Local IPv6 Unicast Addresses

Link-Local addresses are for use on a single link. Link-Local
addresses have the following format:

|   10     |
|  bits    |         54 bits         |          64 bits           |
+----------+-------------------------+----------------------------+
|1111111010|           0             |       interface ID         |
+----------+-------------------------+----------------------------+

Link-Local addresses are designed to be used for addressing on a
single link for purposes such as automatic address configuration,
neighbor discovery, or when no routers are present.

Routers must not forward any packets with Link-Local source or
destination addresses to other links.

IPv6 uses Link-Local addressing on every interface, and it will communicate on-link using Link-Local addressing. For example, IPv6 DHCP is different than IPv4 DHCP, and it uses Link-Local addressing as the source address. Also, the required DAD (Duplicate Address Detection) uses Link-Local addressing, as does NDP (Neighbor Discovery Protocol), including RAs (Router Advertisements).

As the RFC above explains, you cannot send any packets with Link-Local source or destination addresses to a different link, so, no, they are not routable addresses.


RFC 7404, Using Only Link-Local Addressing inside an IPv6 Network Explains about using only Link-Local addresses between routers:

Abstract

In an IPv6 network, it is possible to use only link-local addresses on infrastructure links between routers. This document discusses the advantages and disadvantages of this approach to facilitate the decision process for a given network.

A big disadvantage is that the router interfaces with only Link-Local addresses are not reachable from outside the routers, and this can be a problem when troubleshooting.

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  • I can't believe you just summed up two RFCs in the bits that matter. Awesome. I was thinking about site local addresses before, which until just now I didn't get the real difference from site local to link local other than being "a smaller link." Now I do. Really awesome--thanks! – Vita Mar 25 at 13:29
  • Actually, Site-Local addressing has been deprecated in favor of ULA (Unique Local Addressing), which has some rules around addressing. IPv6 allow multiple addresses of each type (Link-Local, Global, and ULA) addressing to exist on a single interface. It is much harder to do that with IPv4, and there is an IPv6 RFC that explains the order in which an address will be used as a source address for outgoing packets. For example, if you have one of each kind of address on the interface, and you send a packet to a ULA address, then the ULA address on the interface will be used as the source address. – Ron Maupin Mar 25 at 13:34
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Yes a link-local address can be and often is used as a gateway address.

The purpose served by a gateway address depends on the underlying network type. On some network types (generally point to point ones) the gateway address is ignored, on others (generally multi-point ones) the gateway address is resolved to a L2 address to allow the packet to be sent to the correct L2 destination. Either way there is no reason the gateway address needs to be a routable address.

Link local addresses are also perfectly adequate for auto-configuration mechanisms like route advertisements and DHCPv6 prefix delegation.

Routers should have at least one global address so that they can source ICMP errors, but they don't need to have a global address for every interface. In the case of a home/small business gateway router it suffices to only have a global address on the "LAN side".

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