Subject: [pim] Q on IPv6 BSR scoped zones

From: Nidhi Bhaskar <[email-masked]>
Date: Thu, 21 Aug 2003 16:13:52 -0700 (PDT)

Hi,

I've been looking into the new BSR draft and its description of how to
advertise scoped zones & do BSR election per zone. Have a question
from the perspective of IPv6.

In IPv6, configuring a boundary of scope 5 implies that it applies to
the following 'routable' ranges ff[0-f]5::/16, ff[0-f]4::/16 &
ff[0-f]3::/16. That is all group ranges of scope <=5.

So lets say in my PIM domain I configure a zone of scope 5 & a
5C-BSR/5C-RP within this zone & a 'global' gC-BSR/gC-RP outside it.
All routers in my PIM domain hear mappings for ff00::/8 pointing to
the gC-RP. In order for things to work correctly the mapping
distributed within the scoped zone should advertise the 5C-RP for all
the ranges (of scope <=5) listed above.

I.e. the BSM will carry 16*3 48 ranges, each with one RP. What this
means is that for IPv6 the BSR packet for a scoped zone will likely be
fragmented. And this will get worse as the configured zone is of a
larger scope! While this does work, why do we require fragmentation in
the most common network configuration I can think of ? What am I
missing ?

If this is indeed a problem then I propose the following :

IPv6 routers know how to derive a group range given a scope
identifier, so all the information that is being carried around in
this large BSM is really un-necessary.

Ideally, the IPv6 architecture would have used a multicast address
format such that contiguous scopes could be represented by a single
group/mask! Given that this is not the case, I think it may be a good
idea to come up with a format to represent ffX5::/16 in the
BSM. I.e. a range which routers know is to be interpreted as
ff[0-f]5::/16. So that the above zone can be represented in a BSM with
3 ranges instead of 48!

Thanks,
Nidhi.
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