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Simple Virtual Aggregation (S-VA)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6769.
Authors Robert Raszuk , Jakob Heitz , Alton Lo , Lixia Zhang , Xiaohu Xu
Last updated 2012-06-21 (Latest revision 2012-06-05)
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GROW Working Group                                             R. Raszuk
Internet-Draft                                                   NTT MCL
Intended status: Informational                                  J. Heitz
Expires: December 7, 2012                                       Ericsson
                                                                   A. Lo
                                                                L. Zhang
                                                                   X. Xu
                                                            June 5, 2012

                   Simple Virtual Aggregation (S-VA)


   The continued growth in the Default Free Routing Table (DFRT)
   stresses the global routing system in a number of ways.  One of the
   most costly stresses is Forwarding Information Base (FIB) size: ISPs
   often must upgrade router hardware simply because the FIB has run out
   of space, and router vendors must design routers that have adequate

   FIB suppression is an approach to relieving stress on the FIB by NOT
   loading selected RIB entries into the FIB.  Simple Virtual
   Aggregation (S-VA) is a simple form of Virtual Aggregation (VA) that
   allows any and all edge routers to shrink their RIB and FIB
   requirements substantially and therefore increase their useful

   S-VA does not increase FIB requirements for core routers.  S-VA is
   extremely easy to configure considerably more so than the various
   tricks done today to extend the life of edge routers.  S-VA can be
   deployed autonomously by an ISP (cooperation between ISPs is not
   required), and can co-exist with legacy routers in the ISP.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 7, 2012.

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
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   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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Scope of this Document . . . . . . . . . . . . . . . . . .  5
     1.2.  Requirements notation  . . . . . . . . . . . . . . . . . .  5
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Operation of S-VA  . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Deployment considerations  . . . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   ISPs today manage constant Default Free Routing Table (DFRT) growth
   in a number of ways.  One way, of course, is for ISPs to upgrade
   their router hardware before DFRT growth outstrips the size of the
   FIB.  This is too expensive for many ISPs.  They would prefer to
   extend the lifetime of routers whose FIBs can no longer hold the full

   A common approach taken by lower-tier ISPs is to default route to
   their providers.  Routes to customers and peer ISPs are maintained,
   but everything else defaults to the provider.  This approach has
   several disadvantages.  First, packets to Internet destinations may
   take longer-than-necessary AS paths.

   This problem can be mitigated through careful configuration of
   partial defaults, but this can require substantial configuration
   overhead.  A second problem with defaulting to providers is that the
   ISP is no longer able to provide the full DFRT to its customers.
   Finally, provider defaults prevents the ISP from being able to detect
   martian packets.  As a result, the ISP transmits packets that could
   otherwise have been dropped over its expensive provider links.

   An alternative is for the ISP to maintain full routes in its core
   routers, but to filter routes from edge routers that do not require a
   full DFRT.  These edge routers can then default route to the core or
   exit routers.  This is often possible with edge routers that
   interface to customer networks.  The problem with this approach is
   that it cannot be used for all edge routers.  For instance, it cannot
   be used for routers that connect to transits.  It should also not be
   used for routers that connect to customers which wish to receive the
   full DFRT.

   This draft describes a very simple technique, called Simple Virtual
   Aggregation (S-VA), that allows any and all edge routers to have
   substantially reduced FIB requirements even while still advertising
   and receiving the full DFRT over BGP.  The basic idea is as follows.
   Core routers in the ISP maintain the full DFRT in the FIB and RIB.
   Edge routers maintain the full DFRT in the BGP protocol RIB, but
   suppress certain routes from being installed in RIB and FIB tables.
   Edge routers may install a default route to core routers, to ABRs
   which are installed on the Point of Presence (POP) to core boundary
   or to the ASBR routers.

   S-VA requires no changes to BGP and no changes to any choice of
   forwarding mechanisms in routers.  Configuration is extremely simple:
   S-VA must be enabled on the edge router which needs to save its RIB
   and FIB space.  In the same time operator must inject into his intra-

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   domain routing a new prefix further called virtual aggregate (VA-
   prefix) which will be used as the aggregate forwarding reference by
   the edge routers performing S-VA.  Everything else is automatic.
   ISPs can deploy FIB suppression autonomously and with no coordination
   with neighbor ASes.

   In configurations where BGP routes are used to resolve other routes
   or where BGP routes are redistributed to other protocols which both
   happen via RIB simple-va can rather then suppressing routes before
   they are installed in global RIB flag them as "suppress eligible".
   That will allow for seamless route resolution or redistribution while
   in the same time FIB size will continue to be limited as previously
   flagged routes will not be send from RIB to FIB.

1.1.  Scope of this Document

   The scope of this document is limited to Intra-domain S-VA operation.
   In other words, the case where a single ISP autonomously operates
   S-VA internally without any coordination with neighboring ISPs.

   Note that this document assumes that the S-VA "domain" (i.e. the unit
   of autonomy) is the AS (that is, different ASes run S-VA
   independently and without coordination).  For the remainder of this
   document, the terms ISP, AS, and domain are used interchangeably.

   This document applies equally to IPv4 and IPv6 both unicast and
   multicast address families.

   S-VA may operate with a mix of upgraded routers and legacy routers.
   There are no topological restrictions placed on the mix of routers.
   S-VA functionality is local to the router on which it is enabled and
   routing correctness is guaranteed.

   Note that S-VA is a greatly simplified variant of "full VA"
   [I-D.ietf-grow-va].  With full VA, all routers (core or otherwise)
   can have reduced FIBs.  However, full VA requires substantial new
   configuration and operational complexity compared to S-VA.  Full VA
   also requires the use of MPLS LSPs between all routers.  Note that
   S-VA was formerly specified in [I-D.ietf-grow-va].  It has been moved
   to this separate draft to simplify its understanding.

1.2.  Requirements notation

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

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1.3.  Terminology

   RIB/FIB-Installing Router (FIR):  An router that does not suppress
      any routes, and advertises itself as a default route for 0/0.
      Typically a core router, POP to core boundary router or an ASBR
      would be configured as an FIR.
   RIB/FIB-Suppressing Router (FSR):  An S-VA router that installs a
      route to 0/0, and may suppress other routes.  Typically an edge
      router would be configured as an FSR.
   Install and Suppress:  The terms "install" and "suppress" are used to
      describe whether a protocol local RIB entry has been loaded or not
      loaded into the global RIB and FIB.  In other words, the phrase
      "install a route" means "install a route into the global RIB and
      FIB", and the phrase "suppress a route" means "do not install a
      route from BGP into the global RIB and FIB".
   Legacy Router:  A router that does not run S-VA, and has no knowledge
      of S-VA.
   Global Routing Information Base (RIB):  The term global RIB is used
      to indicate the router's main routing information base.  That RIB
      is normally used to populate FIB tables of the router.  It needs
      to be highlighted that unless FIB compression is used global RIB
      and FIB tables are in sync.
   Local/Protocol Routing Information Base (loc-RIB):  The term local
      RIB is used to indicate the protocol's table where product of SPF
      or BGP best path selection is kept before being installed in
      global RIB.  For example, in some protocol implementations BGP
      loc-RIB can be further divided into Adj-RIBs-In, the Loc-RIB, and
      the Adj-RIBs-Out.

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 (there are no topology
   constraints), the most simple form of deployment is for AS border or
   POP border routers to be configured as FIRs, and for customer facing
   edge routers respectively in the AS or in the POP to be configured as

   There are two basic network deployment scenarios for S-VA - with and
   without presence of a default route.  In both cases simple VA
   operates in an identical way however when default route is found and
   is eligible to become a less specific prefix by router's
   configuration the S-VA may use it.  That should not prevent detection
   of any other potential prefix with different next hop as the next hop
   of default route.

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   In the event of FIRs originating a default BGP route to NLRI 0/0
   [RFC4271].  The ORIGIN is set to INCOMPLETE (value 2) and the BGP
   NEXT_HOP is set to match the other BGP routes which are also
   advertised by said FIR.  The ATOMIC_AGGREGATE and AGGREGATOR
   attributes are not included.  The FIR MUST attach a NO_EXPORT
   Community Attribute [RFC1997] to the default route.

   FIRs should not FIB-suppress any routes.  They may, however, still
   use some form of local FIB compression algorithm if deemed necessary.

   FSRs must detect the VA prefix or prefixes (including 0/0) and
   install it both in loc-RIB, RIB and FIB.  Following that FSR may
   suppress any more specific routes which carry the same next hop as
   the VA prefix.  To guarantee semantical correctness FSR by default
   should also be able to detect installation of not matching next hop
   route and reinstall all the more specifics which were previously
   eligible for suppression to maintain semantical forwarding

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

   Similarly when IBGP multipath is enabled and when multiple VA
   prefixes are detected which are multipath candidates under given
   network condition only those more specific prefixes are subject to
   suppression which have the identical set of next hops as multipath
   set of VA prefixes.

   We illustrate the expected behavior on the figure below.  This figure
   shows an autonomous system with a FIR FIR1 and an FSR FSR1.  FSR1 is
   an ASBR and is connected to two remote ASBRs, EP1 and EP2.

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

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   Suppose that FSR1 has been enabled to perform S-VA.  Originally it
   receives all routes from FIR1 (doing next hop self) as well as
   directly connected EBGP peers EP1 and EP2.  FIR1 now will advertise a
   VA prefix 0/0 with next hop set to himself.  That will trigger
   detection of such prefix on FSR1 and suppression all routes which
   have the same next hop as VA prefix and which otherwise would be
   installed in RIB and FIB.  However it needs to be observed that FSR1
   will not suppress any EBGP routes received from his peers EP1 and EP2
   due to next hop being different from the one assigned to VA-prefix.

3.  Deployment considerations

   The simplest deployment model of S-VA is its use within the POP.  In
   such model the POP to core boundary routers (usually RRs in the data
   path) would act as FIRs and would inject VA-prefix 0/0 to all of its
   clients within the POP.  In such model of operation an observation
   can be made that such ABRs do have full routing knowledge and client
   to ABR distance is negligible as compared with client to intra-domain
   exit distance.

   Therefore under the above intra POP S-VA deployment model clients can
   be configured that even in the event of lack of ABR to ABR
   advertisement symmetry there is still no need to monitor if more
   specific unsuppressed route would cover suppressed one.  Thus in this
   particular deployment model there is no need to detect and reinstall
   the previously suppressed ones.

   Another deployment consideration should be given to networks which
   may utilize route reflection.  In the event of enabling IBGP
   multipath a special care must be taken that both outbound prefixes as
   well as VA-prefixes would pass via said route reflectors to their

   In order to address the above aspects the following solutions could
   be considered:

   - Use of intra-POP S-VA
   - Full mesh  Small or medium side networks where S-VA can be deployed
      are normally fully meshed and do not use route reflection.  It
      also needs to pointed out that some large networks are also fully
      meshed today.
   - Use of add-paths  Use of add-paths new BGP encoding will allow to
      distribute more then one overall best path from RR to each client.

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   - Alternate advertisement of VA-prefix   S-VA prefix does not need to
      be advertised in BGP.  The BGP suppression will happen as long as
      we configure the S-VA with next hop(s) and implementation verifies
      that such VA-prefix is installed in the RIB and FIB.

   BGP routes may be used to resolve nexthops for static routes or other
   BGP routes.  Because the default route does not imply reachability of
   any destination, a router can be configured not to resolve nexthops
   using the default route.  In this case, simple-va should not suppress
   a route that may be used to resolve a nexthop for another route.

   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 redistribution policy is
   in place simple-va code should refrain from suppressing prefixes
   matching such policy.

   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 default route.
   There are several ways to achieve this.  One is to have the FIR
   aggregate the routes instead of the FSR.  Another is to install a
   blackhole route for the nonexistent addresses on the originating
   router.  This issue is not specific to simple-va, but applicable to
   the general use of default routes.

4.  IANA Considerations

   There are no IANA considerations.

5.  Security Considerations

   The authors are not aware of any new security considerations due to

6.  Acknowledgements

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

   Authors would like to thank Clarence Filsfils, Nick Hilliard, S.

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   Moonesamy and Tom Petch for their review and valuable input.

7.  References

7.1.  Normative References

   [RFC1997]  Chandrasekeran, 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.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

7.2.  Informative References

              Francis, P., Xu, X., Ballani, H., Jen, D., Raszuk, R., and
              L. Zhang, "FIB Suppression with Virtual Aggregation",
              draft-ietf-grow-va-06 (work in progress), December 2011.

Authors' Addresses

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


   Jakob Heitz
   300 Holger Way
   San Jose, CA  95135


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   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
   No.3 Xinxi Rd., Shang-Di Information Industry Base, Hai-Dian District
   Beijing, Beijing  100085

   Phone: +86 10 82836073

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