Can link local addresses be used for transit networks without breaking routability?

Question

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)?

Answer 1

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.

Answer 2

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".