[DNA] Question regarding prefix assignment

["Tero Kauppinen (JO/LMF)" <tero.kauppinen@ericsson.com>]

During the yesterday meeting it was brought up that using prefixes to 
conclude whether a link has changed or not can be problematic in certain 
cases. Can someone confirm whether the following scenario is possible or 
not?

                Router(s) advertising
                  prefixes A and B
                         |
                         |__________X_______ AP2
                         |                  (may use only prefix A)
                         |
                         |
                        AP1
               (may use prefixes A and B)

X marks a box-in-the-middle e.g. a bridge (as mentioned yesterday).

If this is possible, the problem lies here in the fact that if the node 
is first connected to the AP1 it can configure either prefix A or B, and 
if it then moves to AP2 it may only use prefix A even though still 
connected to the same link. Let's assume that a node uses prefix B and 
is connected to AP1. It also knows that the complete prefix list for the 
current link is A and B. It then moves to AP2 and receives an RA, 
notices that the advertised prefix(es) match and thus link has not 
changed. However, it's now using prefix B which is not valid under this 
section of the link.

If this is the case, I'm also curious how you actually prevent a node 
connected to AP2 from not configuring prefix B. Do the box X hassle with 
router advertisements?

Tero