RE: [DNA] Question regarding prefix assignment

[dunmore@comp.lancs.ac.uk]

Tero Kauppinen wrote:

> 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?
>

I believe this scenario *could* be possible but I'm not sure any sane
administrator would do this. If X is a vlan switch and the switch allows
vlan tags to be switched to multiple ports, then you could have a scenario
such as

vlan x = prefix A
vlan y = prefix B

and

vlan x switched to AP2 and AP1
vlan y switched to AP1 (thus only AP1 gets RAs for prefix B)

The only reason I can think of why anyone would do this is if there is some
administrative reason why only AP1 should see prefix B. Perhaps this would
be a restricted prefix that equates to an SSID for a 'top secret' meeting
room.

Cheers,
Martin Dunmore