Internet Engineering Task Force (IETF)                         R. Raszuk
Request for Comments: 6769                                       NTT MCL
Category: Informational                                         J. Heitz
ISSN: 2070-1721                                                 Ericsson
                                                                   A. Lo
                                                                L. Zhang
                                                                   X. Xu
                                                            October 2012

                   Simple Virtual Aggregation (S-VA)


   All BGP routers in the Default-Free Zone (DFZ) are required to carry
   all routes in the Default-Free Routing Table (DFRT).  This document
   describes a technique, Simple Virtual Aggregation (S-VA), that allows
   some BGP routers not to install all of those routes into the
   Forwarding Information Base (FIB).

   Some routers in an Autonomous System (AS) announce an aggregate (the
   VA prefix) in addition to the routes they already announce.  This
   enables other routers not to install the routes covered by the VA
   prefix into the FIB as long as those routes have the same next-hop as
   the VA prefix.

   The VA prefixes that are announced within an AS are not announced to
   any other AS.  The described functionality is of very low operational
   complexity, as it proposes a confined BGP speaker solution without
   any dependency on network-wide configuration or requirement for any
   form of intra-domain tunneling.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

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RFC 6769                          S-VA                      October 2012

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
      1.1. Scope of This Document .....................................3
      1.2. Requirements Notation ......................................3
      1.3. Terminology ................................................3
   2. Operation of S-VA ...............................................4
   3. Deployment Considerations .......................................6
   4. Security Considerations .........................................7
   5. Acknowledgements ................................................7
   6. Normative References ............................................7
   7. Informative References ..........................................7

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RFC 6769                          S-VA                      October 2012

1.  Introduction

   This document describes a technique called Simple Virtual Aggregation
   (S-VA).  It allows some routers not to store some routes in the
   Forwarding Information Base (FIB) while still advertising and
   receiving the full Default-Free Routing Table (DFRT) in BGP.

   A typical scenario is as follows.  Core routers in the ISP maintain
   the full DFRT in the FIB and Routing Information Base (RIB).  Edge
   routers maintain the full DFRT in the BGP Local RIB (Loc-RIB), but do
   not install certain routes in the RIB and FIB.  Edge routers may
   install a default route to core routers, to Area Border Routers (ABR)
   that are installed on the Point of Presence (POP), to core boundary
   routers, or to Autonomous System Border Routers (ASBRs).

   S-VA must be enabled on an edge router that needs to save its RIB and
   FIB space.  The core routers must announce a new prefix called
   Virtual Aggregate (VA prefix).

1.1.  Scope of This Document

   The VA prefix is not intended to be announced from one AS into
   another, only between routers of the same AS.

   S-VA can be used for both IPv4 unicast and multicast address families
   and IPv6 unicast and multicast address families.

   S-VA does not need to operate on every router in an AS.

1.2.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

1.3.  Terminology

   RIB/FIB-Installing Router (FIR):  A router that does not suppress any
      routes and announces the VA prefix.  Typically, a core router, a
      POP to core boundary router, or an ASBR would be configured as an

   RIB/FIB-Suppressing Router (FSR):  An S-VA router that installs the
      VA prefix, but does not install routes that are covered by and
      have the same next-hop as the VA prefix into its FIB.  Typically,
      an edge router would be configured as an FSR.

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RFC 6769                          S-VA                      October 2012

   Suppress:  Not to install a route that is covered by the VA prefix
      into the global RIB or FIB.

   Legacy Router:  A router that does not run S-VA and has no knowledge
      of S-VA.

   Global Routing Information Base (RIB):  All routing protocols in a
      router install their selected routes into the RIB.  The routes in
      the RIB are used to resolve next-hops for other routes, to be
      redistributed to other routing protocols, and to be installed into
      the FIB.

   Local/Protocol Routing Information Base (Loc-RIB):  The Loc-RIB
      contains the routes that have been selected by the local BGP
      speaker's Decision Process as in [RFC4271].

   NLRI:  Network Layer Reachability Information [RFC4271]

2.  Operation of S-VA

   There are three types of routers in S-VA: FIB-Installing routers
   (FIR), FIB-Suppressing routers (FSR), and, optionally, legacy
   routers.  While any router can be an FIR or an FSR, the simplest form
   of deployment is for AS border routers to be configured as FIRs and
   for customer facing edge routers to be configured as FSRs.

   When a FIR announces a VA prefix, it sets the path attributes as
   follows.  The ORIGIN MUST be set to INCOMPLETE (value 2).  The
   NEXT_HOP MUST be set to the same value as that of the routes that are
   intended to be covered by the VA prefix.  The ATOMIC_AGGREGATE and
   AGGREGATOR attributes SHOULD NOT be included.  The FIR MUST attach a
   NO_EXPORT community attribute [RFC1997].  The NLRI SHOULD be 0/0.

   A FIR SHOULD NOT FIB-suppress any routes.

   An FSR must detect the VA prefix or prefixes (including 0/0) and
   install them in all of Loc-RIB, RIB, and FIB.  The FSR MAY suppress
   any more-specific routes that carry the same next-hop as the VA

   Generally, any more-specific route that carries the same next-hop as
   the VA prefix is eligible for suppression.  However, provided that
   there is at least one less-specific prefix with a different next-hop
   between the VA prefix and the suppressed prefixes, then those
   suppressed prefixes must be reinstalled.

   An example with three prefixes can be considered where the VA-prefix
   (prefix 1) is the least specific and covers prefix 2 and prefix 3.

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   Prefix 2 is less specific than prefix 3 and covers the latter.  If
   all three have the same next-hop, then only the bigger one, i.e.,
   VA-Prefix, is announced.  However, if prefix 2 has a different
   next-hop, then it will need to be announced separately.  In this
   case, it is important to also announce prefix 3 separately.

   Similarly, when Internal BGP (IBGP) multipath is enabled, and when
   multiple VA prefixes form a multipath, only those more-specific
   prefixes of which the set of next-hops are identical to the set of
   next-hops of the VA prefix multipath are subject to suppression.

   The expected behavior is illustrated in Figure 1.  This figure shows
   an AS with a FIR, FIR1, and an FSR, FSR1.  FSR1 is an ASBR and is
   connected to two external ASBRs, EP1 and EP2.

        |      Autonomous System                   |   +----+
        |                                          |   |EP1 |
        |                                      /---+---|    |
        |   To   ----\ +----+          +----+ /    |   +----+
        | Other       \|FIR1|----------|FSR1|/     |
        |Routers      /|    |          |    |\     |
        |        ----/ +----+          +----+ \    |   +----+
        |                                      \---+---|EP2 |
        |                                          |   |    |
        |                                          |   +----+

                            Figure 1

   Suppose that FSR1 has been enabled to perform S-VA.  Originally, it
   receives all routes from FIR1 (doing next-hop-self) as well as from
   EP1 and EP2.  FIR1 now will advertise a VA prefix 0/0 with the next-
   hop set to itself.  This will cause FSR1 to suppress all routes with
   the same next-hop as the VA prefix.  However, FSR1 will not suppress
   any routes received from EP1 and EP2, because their next-hops are
   different from that of the VA prefix.

   Several FIRs may announce different S-VA prefixes.  For example, in a
   POP, each edge router can announce into the POP an S-VA prefix that
   covers the addresses of the customers it services.

   Several FIRs may announce the same S-VA prefix.  In this case, an FSR
   must choose to install only one of them.  For example, two redundant
   ASBRs, both of which announce the complete DFRT, may each also
   announce the default route as an S-VA prefix into the AS.

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RFC 6769                          S-VA                      October 2012

   S-VA may be used to split traffic among redundant exit routers.  For
   example, suppose in Figure 1 that EP1 and EP2 are two redundant ASBRs
   that announce the complete DFRT.  Each may also announce two S-VA
   prefixes into the AS: 0/1 and 128/1.  EP1 might announce 0/1 with
   higher preference and EP2 might announce 128/1 with higher
   preference.  FIR1 will now install into its FIB 0/1 pointing to EP1
   and 128/1 pointing to EP2.  If either EP1 or EP2 were to fail, then
   FSR1 would switch the traffic to the other exit router with a single
   FIB installation of one S-VA prefix.

3.  Deployment Considerations

   BGP routes may be used to resolve next-hops for static routes or
   other BGP routes.  Because the default route does not imply
   reachability of any destination, a router can be configured to not
   resolve next-hops using the default route.  In this case, S-VA should
   not suppress a route that may be used to resolve a next-hop for
   another route from installation into the RIB.  It may still suppress
   it from installation into the FIB.

   Selected BGP routes in the RIB may be redistributed to other
   protocols.  If they no longer exist in the RIB, they will not be
   redistributed.  This is especially important when the conditional
   redistribution is taking place based on the length of the prefix,
   community value, etc.  In those cases where a redistribution policy
   is in place, S-VA implementation should refrain from suppressing
   installation into the RIB routes matching such policy.  It may still
   suppress them from installation into the FIB.

   A router may originate a network route or an aggregate route into
   BGP.  Some addresses covered by such a route may not exist.  If this
   router were to receive a packet for an unreachable address within an
   originated route, it must not send that packet to the VA prefix
   route.  There are several ways to achieve this.  One way is to have
   the FIR aggregate the routes instead of the FSR.  Another way is to
   install a black hole route for the nonexistent addresses on the
   originating router.  This issue is not specific to S-VA, but
   applicable to the general use of default routes.

   Like any aggregate, an S-VA prefix may include more address space
   than the sum of the prefixes it covers.  As such, the S-VA prefix may
   provide a route for a packet for which no real destination exists.
   An FSR will forward such a packet to the FIR.

   If an S-VA prefix changes its next-hop or is removed, then many
   routes may need to be downloaded into the FIB to achieve convergence.

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4.  Security Considerations

   The authors are not aware of any new security considerations due to
   S-VA.  The local nature of the proposed optimization eliminates any
   external exposure of the functionality.  The presence of more
   specifics that are used as VA prefixes is also a normal BGP behavior
   in current networks.

5.  Acknowledgements

   The concept for Virtual Aggregation comes from Paul Francis.  In this
   document, the authors only simplified some aspects of its behavior to
   allow simpler adoption by some operators.

   The authors would like to thank Clarence Filsfils, Nick Hilliard, S.
   Moonesamy, and Tom Petch for their review and valuable input.

6.  Normative References

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, October 2007.

7.  Informative References

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271, January

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RFC 6769                          S-VA                      October 2012

Authors' Addresses

   Robert Raszuk
   101 S Ellsworth Avenue Suite 350
   San Mateo, CA 94401


   Jakob Heitz
   300 Holger Way
   San Jose, CA 95134


   Alton Lo
   Arista Networks
   5470 Great America Parkway
   Santa Clara, CA 95054


   Lixia Zhang
   3713 Boelter Hall
   Los Angeles, CA 90095


   Xiaohu Xu
   Huawei Technologies
   Huawei Building, No.3 Xinxi Rd.,
   Shang-Di Information Industry Base, Hai-Dian District
   Beijing 100085
   P.R. China

   Phone: +86 10 82836073

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