1

RFC 7084 describes Basic Requirements for IPv6 Customer Edge Routers. One of those requirements on the WAN-side is the following:

   WPD-4:  By default, the IPv6 CE router MUST initiate DHCPv6 prefix
           delegation when either the M or O flags are set to 1 in a
           received Router Advertisement (RA) message.  Behavior of the
           CE router to use DHCPv6 prefix delegation when the CE router
           has not received any RA or received an RA with the M and the
           O bits set to zero is out of scope for this document.

Of course i would expect a CE router to require a global prefix, but the prefix, at least as far as i understand, is already included in a Neighbor Discover Router Advertisement (RA) Message Prefix Information Option.

As far as i understand, the RA would have been received in the process of creating a link-local address with SLAAC. The RA has to be evaluated to decide that a global address should be created with the RA prefix.

W-2:  The IPv6 CE router MUST generate a link-local address and
      finish Duplicate Address Detection according to [RFC4862] prior
      to sending any Router Solicitations on the interface.  The
      source address used in the subsequent Router Solicitation MUST
      be the link-local address on the WAN interface.

Of course additional prefixes can be requested with DHCPv6 but WPD-4 just broadly specifies, that prefix delegation should be initiated...

So why can the CE router NOT just use the prefix of the Router Advertisement on the WAN-side?

Community
  • 1
goulashsoup
  • 155

1 Answers1

1

The prefix the CE router sees advertised by an upstream router on its WAN side is already in use for the network on its WAN link.

If the CE router used the same prefix to create the IPv6 network on its LAN side, it would have to act as a bridge, not a router. A router forwards traffic between two (or more) separate IPv6 networks that each have their own prefix.

Since RFC 7084 is specifically for IPv6 routers, the device must in some way receive a separate prefix for it to use to create its LAN-side IPv6 network. Otherwise it wouldn't be able to actually route IPv6, so it wouldn't truly be an IPv6 router.

If it only knew the prefix for its WAN side link, and just did SLAAC or something to simply give itself an IPv6 address on its WAN port, without creating a separate IPv6 network on its LAN side and routing between the two IPv6 networks, then it would simply be an IPv6 host, not an IPv6 router.

Spiff
  • 110,156