Using /64 from Customer Prefix for the Inter-Router Link
The IPv6 Company
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La Navata - Galapagar
Madrid
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Spain
jordi.palet@theipv6company.com
http://www.theipv6company.com/
v6ops
This document describes the usage of a /64 from the customer
prefix for numbering IPv6 point-to-point links in
non-broadcast layer 2 media.
There are different alternatives for numbering IPv6 point-to-point links,
and from an operational perspective, they may have different advantages or
disadvantages that need to be taken in consideration under the scope of
each specific network architecture design.
describes using /127 prefixes for inter-router
point-to-point links, using two different address pools, one for numbering
the point-to-point links and another one for delegating the prefixes at the
end of the point-to-point link. However this doesn't exclude other choices.
This document describes an alternative the approach, using a /64 from
the customer prefix, which ensure compliance with standards, and consequently
facilitate interoperability, avoids possible future issues if more addresses
are needed (e.g., managed bridges) and simplifies the addressing plan.
The use of /64 also facilitates an easier way for routing the shorter
aggregated prefix into the point-to-point link. Consequently it simplifies
the "view" of a more unified addressing plan, providing an easier path for
following up any issue when operating IPv6 networks.
The proposed approach is suitable for those point-to-point links
connecting ISP to Customers and enterprise networks, but not limited to
those cases, and in fact, is being used by a relevant number of networks
worldwide, in several different scenarios.
This mechanism would not work in broadcast layer two media that
rely on ND (as it will try ND for all the addresses within the shorter
prefix being delegated thru the point-to-point link).
The IPv6 Addressing Architecture () specifies
that all the Interface Identifiers for all the unicast addresses
(except for 000/3) are required to be 64 bits long and to be
constructed in Modified EUI-64 format.
The same document also mandates the usage of the predefined
subnet-router anycast address, which has cleared to zero all the bits
that do not form the subnet prefix.
describes possible issues when using /64 for the
point-to-point linkes, however, it also states that they can be mitigated
by other means, and indeed, considering the publication date of that document,
those issues should not be any longer considered. The fact is that many
operators wordwide, today use /64 without any concerns, as vendors have
taken the necessary code updates.
Consequently, we shall conclude that /64 it is a valid approach
to use /64 prefixes for the point-to-point links.
Often, in point-to-point links, hardware tokens are not available,
or there is the need to keep certain bits (u, g) cleared, so the links
can be manually numbered sequentially with most of the bits cleared
to zero. This numbering makes as well easier to remember the interfaces,
which typically will become numbered as 1 (with 63 leading zero bits)
for the provider side and 2 (with 63 leading zero bits)
for the customer side.
Using interface identifiers as 1 and 2 is not only a very simple
approach, but also a very common practice. Other different choices
can as well be used as required in each case.
On the other hand, using the EUI-64, makes it more difficult to
remember and handle the interfaces, but provides an additional degree
of protection against port (actually address) scanning as described
at .
Following this approach and assuming that a shorter prefix is typically
delegated to a customer, for example a /48, it is possible to simplify
the routing aggregation of the point-to-point links. Towards this,
the point-to-point link may be numbered using the first /64 of the /48
delegated to the customer.
Let's see a practical example:
A service provider uses the prefix 2001:db8::/32 and is using
2001:db8:aaaa::/48 for a given customer.
Instead of allocating the point-to-point link from a different
addressing pool, it may use 2001:db8:aaaa::/64 (which is the first
/64 subnet from the 2001:db8:aaaa::/48) to number the link.
This means that, in the case the non-EUI-64 approach is used,
the point-to-point link may be numbered as 2001:db8:aaaa::1/64
for the provider side and 2001:db8:aaaa::2/64 for the customer side.
Note that using the first /64 and interface identifiers
1 and 2 is a very common practice. However other values may
be chosen according to each case specific needs.
In this way, as the same address pool is being used for both,
the prefix and the point-to-point link, one of the advantages of
this approach is to make very easy the recognition of the point-to-point
link that belongs to a given customer prefix, or in the other way around,
the recognition of the prefix that is linked by a given point-to-point link.
For example, making a trace-route to debug any issue to a given address
in the provider network, will show a straight view, and it becomes
unnecessary one extra step to check a database that correlate an
address pool for the point-to-point links and the customer prefixes,
as all they are the same.
Moreover, it is possible to use the shorter prefix as the provider
side numbering for the point-to-point link and keep the /64 for the
customer side. In our example, it will become:
Point-to-point link at provider side: 2001:db8:aaaa::1/48
Point-to-point link at customer side: 2001:db8:aaaa::2/64
This provides one additional advantage as in some platforms
the configuration may be easier saving one step for the route of the
delegated prefix (no need for two routes to be configured,
one for the delegated prefix, one for the point-to-point link).
It is possible because the longest-prefix-match rule.
The behavior of this type of configuration has been successfully
deployed in different operator and enterprise networks, using commonly
available implementations with different routing protocols,
including RIP, BGP, IS-IS, OSPF, along static routing,
and no failures or interoperability issues have been reported.
As stated in , "the requesting router
MUST NOT assign any delegated prefixes or subnets from the delegated
prefix(es) to the link through which is received the DHCP message
from the delegating router", however the approach described in
this document is still useful in other DHCPv6 scenarios or
non-DHCPv6 scenarios.
Furthermore, was updated by
Prefix Exclude Option for DHCPv6-based Prefix Delegation
(), precisely to define a new DHCPv6 option,
which covers the case described by this document.
Moreover, has no explicit requirement
that avoids the approach described in this document.
This approach is being used by operators in both, residential/SOHO
and enterprise networks, so the routers at the customer end for those
networks MUST support if DHCPv6-PD is used.
In the case of Customer Edge Routers there is a specific requirement
() WPD-8 (Prefix delegation Requirements), marked
as SHOULD for . However, in an scenario where
the approach described in this document is followed, together with
DHCPv6-PD, the CE Router MUST support .
This document does not have any new specific security considerations.
This document does not have any new specific IANA considerations.
The author would like to acknowledge the inputs of Mikael Abrahamsson ...
Acknowledgement to co-authors, Cesar Olvera and Miguel Angel Díaz,
of a previous related document (draft-palet-v6ops-point2point, 2006),
as well as inputs for that version from Alain Durand, Chip Popoviciu,
Daniel Roesen, Fred Baker, Gert Doering, Olaf Bonness, Ole Troan,
Pekka Savola and Vincent Jardin, are also granted.