1

In a networking course that I'm following, there was a mention of several ways of how IPv6 link local addresses can be used, such as

  1. Assigning the link local address of a router/L3 switch as the default gateway on the hosts

  2. Using it as next-hop addresses for static routes.

and some more.

I wonder, though, when it comes to these 2 uses.

For example, why would we want to configure a link-local address as the next hop address for static routes? I have the same question with configuring the link-local address of a router/L3 switch as the default gateway on hosts.

Why should we use a link-local address, instead of using the global unicast/unique local IPv6 addresses?

1 Answer 1

3

You do not manually assign gateways for IPv6. An IPv6 host gets its gateways from the RAs sent by the routers, and the routers send RAs from their link-local addresses.

Remember that IPv6 can have multiple routable addresses of either or both Global and ULA. Each IPv6 router on a link will send RAs telling the host for which networks it can be a gateway, and the RAs are sent from the router link-local address, so the next hop for the particular network of the host source address will be the router link-local address.

You can find out more about it in RFC 4861, Neighbor Discovery for IP version 6 (IPv6). It explains things like:

  Router Advertisements carry link-layer addresses; no additional
  packet exchange is needed to resolve the router's link-layer
  address.

  Router Advertisements carry prefixes for a link; there is no need
  to have a separate mechanism to configure the "netmask".

  Router Advertisements enable Address Autoconfiguration.

  Routers can advertise an MTU for hosts to use on the link,
  ensuring that all nodes use the same MTU value on links lacking a
  well-defined MTU.

-and-

  The use of link-local addresses to uniquely identify routers (for
  Router Advertisement and Redirect messages) makes it possible for
  hosts to maintain the router associations in the event of the site
  renumbering to use new global prefixes.
1
  • Not everyone follows the rules. There are RA's sent from non LLA's. And there are many instances of fully static configured IPv6. (Ever used an Alteon? It doesn't listen to RAs, send RAs, or allow a LLA anywhere.) [I hate IPv6. Even the nuts that wrote it don't do it correctly.]
    – Ricky
    Jan 11, 2023 at 1:17

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