RIPE Routing Working Group Recommendations on IPv6 Route Aggregation
====================================================================

Rob Evans
Philip Smith


Introduction
============

Recent discussion has shown there is a limited requirement to be able
to advertise more specific prefixes from an IPv6 Provider Aggregatable
(PA) allocation where a Local Internet Registry (LIR) contains several
networks which are not interconnected, or for traffic engineering
purposes.  This document recommends such advertisements are limited
in both length and scope.  It is intended to supplement the working
group's Recommendations on Route Aggregation [RIPE-399].


Background
==========

The IPv6 address allocation and assignment policy for the RIPE NCC
Service Region [V6-ALLOC] only allows LIRs to obtain more than the
minimum PA allocation if they can demonstrate address utilisation that
requires it.  This fits with the address space management principle of
conservation.  However, as understood in the RIPE Routing Working
Group's Recommendations on Route Aggregation [RIPE-399], there are
occasionally requirements for the advertisement of more specific
routes from within an allocation.  With a few ISPs currently filtering
at the minimum PA allocation (/32) within the relevant address ranges,
this can cause significant difficulties for some networks wishing to
deploy IPv6.

Some reasons for wanting to advertise multiple prefixes from a PA
allocation could be:

  -  The LIR has several networks that are not interconnected.
  -  Traffic engineering: A single prefix that covers an LIR's
     entire customer base may attract too much traffic over a
     single peering link

This document is only concerned with IPv6 Provider Aggregatable (PA)
allocations, and does not discuss Provider Independent (PI) prefixes
which are unlikely to be divisible beyond the default assignment of
/48.


Recommendation
==============

It is suggested that prefix filters allow for prudent subdivision of
an IPv6 allocation.  The operator community will ultimately decide
what degree of subdivision is supportable, but the majority of ISPs
accept prefixes up to a length of /48 within PA space.

Advertisement of more specific prefixes should not be used unless
absolutely necessary and, where sensible, a covering aggregate
should also be advertised.  Further, LIRs should use BGP methods
such as NO_EXPORT [RFC-1997] and NOPEER [RFC-3765] or provider-specific
communities, as described in [RIPE-399] to limit the propagation of
more specific prefixes in the routing table.

Operators should register appropriate "route6" objects in their
preferred routing registry, or ROAs in the RPKI, to reflect any more
specific advertisements.


Discussion
==========

There is a valid need for some LIRs to advertise more than one IPv6
PA prefix.  As either obtaining more address space and advertising
more /32 prefixes, or advertising more specific prefixes within an
already allocated /32 have the same impact on the routing table,
it is suggested that the latter approach is taken to prevent address
space wastage.

It is understood that this may not cover all possibilities.  There may
be circumstances where sites will have to consider the suitability of
Provider Independent addresses, or LIRs may have to consider
mechanisms of obtaining more than a /32 of Provider Aggregatable
space.


References
==========

[V6-ALLOC] http://www.ripe.net/ripe/docs/ipv6policy.html
[RIPE-399] http://www.ripe.net/ripe/docs/ripe-399
[RFC-1997] http://www.rfc-editor.org/rfc/rfc1997.txt
[RFC-3765] http://www.rfc-editor.org/rfc/rfc3765.txt