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BESS WorkGroup                                                S. Mohanty
Internet-Draft                                                  M. Ghosh
Intended status: Standards Track                           Cisco Systems
Expires: September 6, 2018                                     S. Breeze
                                                               J. Uttaro
                                                           March 5, 2018

                  BGP EVPN Flood Traffic Optimization


   In EVPN, the Broadcast, Unknown Unicast and Multicast (BUM) traffic
   is sent to all the routers participating in the EVPN instance.  In a
   multi-homing scenario, when more than one PEs share the same Ethernet
   Segment, i.e. there are more than one PEs in a redundancy group, only
   the PE that is the Designated-Forwarder (DF) for the ES will forward
   that packet on the access interface whereas all non-DF PEs will drop
   the packet.  From the perspective of the network, this is quite
   wasteful.  This is especially true if there are significantly more
   PEs on the Ethernet Segment.  This draft explores this problem and
   provides a solution for the same.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on September 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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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.

Table of Contents

   1.  Requirements Language and Terminology . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Problem Description . . . . . . . . . . . . . . . . . . . . .   4
   4.  Solution 1. Suppress the advertisement of the IMET route  . .   5
   5.  Solution 2. Advertisement of the IMET route from the BDF  . .   6
   6.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Requirements Language and Terminology

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

   o  ES: Ethernet Segment

   o  EVI: Ethernet virtual Instance, this is a mac-vrf.

   o  IMET: Inclusive Multicast Route

   o  DF: Designated Forwarder

   o  BDF: Backup Designated Forwarder

2.  Introduction

   BGP [RFC7432] describes a solution for disseminating mac addresses
   over an mpls core via the Border Gateway Protocol.  In EVPN, data
   plane learning is confined to the access, and the control plane

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   learning happens via BGP in the core.  This prevents unnecessary
   flooding in the data plane as the traffic is directed to where the
   destination is learnt from.  However, in case of Broadcast, Unknown
   Unicast and Multicast (BUM) traffic, the PE needs to do a flooding to
   all the other PEs in the domain.

   PEs elect a Designated Forwarder (DF) amongst themselves, for a given
   ES, by exchanging type-4 routes via BGP.  The role of a DF is to
   forward BUM traffic received from the core, towards its access facing
   interface.  A PE in a non-DF role will drop flood traffic received on
   its core-facing interface.  Note that the DF election process is only
   confined to the set of PEs who host the same Ethernet Segment.
   Remote PEs are not interested in type-4 routes for Ethernet Segments
   that they do not host.  Hence remote PEs are ignorant of the DFs for
   segments which is not local to them.  Consequently, when the remote
   PE needs to do a BUM flooding using ingress replication, it will
   flood the frames to all participating PEs, irrespective of whether
   DFs or not.  The key to creating a list of PEs with which to flood
   to, is the Inclusive multicast ethernet tag route which is described

   The IMET route (type-3) in EVPN advertises the BUM label for the EVI
   to all the other PEs who are interested in the same EVI.  For ingress
   replication the label is encapsulated in the PMSI attribute.  The
   label is used to encapsulate the BUM traffic at the ingress entity.
   This label is inserted just above the split-horizon label in the BUM
   frame.  When the BUM packet is received by a PE that is multi-homed
   to the same Ethernet segment as the PE that originated the BUM
   packet, and, is the DF for that (EVI, ES) pair, after popping the
   transport label, the receiving PE is going to check if the split-
   horizon label is its own.  If so, it will drop the packet if no other
   ES is configured.  Otherwise it will forward the frame on all other
   Segments that are part of the same EVI. if the PE is not the DF, it
   will straightaway drop the packet immediately.

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            |- -PE1- - -|
            |           |
      CE1---|- -PE2 - --|-RR - - PE5- -|-CE2
            |           |
            |- -PE3- - -+
       CE2- - - PE4- - -+

   An EVPN Network

                                 Figure 1

3.  Problem Description

   In the Figure 1. above, PE1, P2 and PE3 are all multi-homed to CE1 on
   the same Ethernet Segment, say ES1.  PE4 has a single host which is
   not multi-homed.  The same EVPN instance (Bridge-Domain) exists on
   all the PEs.  For this EVPN instance, PE1 is the Designated Forwarder
   on ES1.  Also, PE3 is the backup DF [I-D.ietf-bess-evpn-df-election].
   When PE5 sends the BUM traffic, the flooded frames are received by
   PE1, PE2, PE3 and PE4.  PE1 is going to forward the flood traffic on
   its access link towards CE1.  PE2 and PE3 will drop the flooded
   frames that they receive from the core.  PE4 will forward it as it
   has a single-homed host on the same EVPN instance.

   Here it is wasteful for PE2 and PE3 to receive the flooded frames.
   Whilst the majority of deployments usually have two PEs as part of
   the redundancy group, in some cases, there may be more than two PEs
   on the same ES.  An example being when capacity demands of the PE are
   close to the hardware limits of the PE.  In this scenario, operators
   may chose to protect their investments and increase their resilience
   by installing additional PEs, instead of replacing them or further
   segmenting the access network.  Further,increasing the number of PEs
   results in efficient load-balancing across vlans.

   We can now formally describe the issue.  In general, consider an EVPN
   instance, EVIi, that exists in a PE, say PEk.  As per existing EVPN
   behavior, even If PEk is not the DF for any of its Ethernet Segments
   (that are multi-homed to other PEs) and also there are no other
   single-homed Ethernet Segments that are part of EVIi in PEk , PEk
   will still receive BUM traffic meant for EVIi from a remote PE, PEj.
   This traffic is simply dropped as PEk is not a DF for any of these
   Ethernet Segments.

   1.  This is an unnecessary usage of bandwidth in the EVPN Core.

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   2.  PEk receives traffic which it drops which is non-optimal usage of
       the L2 Forwarding engine.

   3.  PEj replicates a copy of the Ethernet Frame to PEk which is
       anyway to be dropped.  This consumes cycles at PEj.

   In this draft we address the above problem and give two simple
   solutions for the same.  These solutions do not mandate any protocol
   changes and are backwards compatible.

4.  Solution 1.  Suppress the advertisement of the IMET route

   The first solution is for a PE not to advertise the IMET route if the
   outcome is to drop the flooded traffic

   o  PEk only needs to advertise "Inclusive Multicast Ethernet Tag
      route" (Type-3 route) for an EVPN Instance, EVIi if and only if
      EVIi is configured on at least one Ethernet Segment (which also
      has a presence in another PEj, i.e Multihomed) and PEk is the DF
      for that specific Ethernet Segment.

   o  The Type-3 SHOULD also be advertised if there is a "Single-Home"
      Ethernet Segment on an EVI.

   o  Where a PE is the first DF for an ES on an EVPN Instance, the IMET
      should be advertised, whereas on the Last DF to Non-DF transition,
      it should be withdrawn.

   In the Figure 2 the same EVPN instance exists in PE1, PE2, PE3, PE4
   and pE5.  But only PE1 and PE4 advertise the IMET route.  So PE5
   sends the flood traffic to PE1 and PE4 only.

               - - ->
            |- -PE1- - -|
            |           |
      CE1---|- -PE2 - --|-RR - - PE5- -|-CE2
            |           |
            |- -PE3- - -+
       CE2- - - PE4- - -+
               - - ->

   An EVPN Network

                                 Figure 2

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   With this approach, on a DF PE (PE1) failure, BUM traffic will be
   dropped until the IMET from the next elected DF [PE2 or PE3], is
   received at PE5.  Note, present behaviour is that BUM is also dropped
   based on route type 4 withdraw in the peering PEs.  In comparison of
   this proposal with the existing methods, convergence delay will be
   MAX[Type 4, Type 3 Propagation delays] after the New DF is elected.
   This leads to our next solution extension, where convergence cannot
   be traded off over bandwidth optimization.

5.  Solution 2.  Advertisement of the IMET route from the BDF

   1.  Multihomed PEs can easily compute the Backup DF, based on the DF
       election mode in operation.

   2.  Extending Solution 1, we are proposing that a PE should only
       advertise Type-3 for an EVI if and only if one of the conditions

       *  It has an Single Home Ethernet Segment, in the EVI

       *  It is DF for at least one Ethernet-Segment, for that EVI

       *  It is BDF for at least one Ethernet-Segment, for that EVI

   This would mean that, in Fig. 2, in addition to the IMET routes that
   are being advertised from PE1 and PE4, PE3 also advertises the IMET
   route since it is the BDF.  It can be seen from the above example
   that with increasing number of multi-homed PEs sharing the same
   Ethernet-Segment and Vlans, only two PEs will advertise IMET on
   behalf of an EVI.  Of course, if there are some single-homed hosts,
   there may be some additional IMET advertisements.  But the real
   benefits are in the data plane since this results in no BUM traffic
   for PEs that do not need it; but would have, nevertheless, got it, as
   per the existing EVPN procedures.

6.  Protocol Considerations

   This idea conforms to existing EVPN drafts that deal with BUM
   handling [RFC7432], and [I-D.ietf-bess-evpn-igmp-mld-proxy].
   Additionally, to take DF Type 4 as explained in
   [I-D.sajassi-bess-evpn-per-mcast-flow-df-election] into
   consideration, along the other conditions specified in Sections 4 and
   5, the PE should advertise IMET if and only if there is at least one
   (S,G) for which it is DF.  For all other DF Types, no additional
   considerations are required.

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7.  Operational Considerations


8.  Security Considerations

   This document raises no new security issues for EVPN.

9.  Acknowledgements

   The authors would like to thank Ali Sajassi for his feedback and
   insight into the deployments that can benefit from this proposal.

10.  Contributors

   Samir Thoria
   Cisco Systems

   Email: sthoria@cisco.com

   Sameer Gulrajani
   Cisco Systems

   Email: sameerg@cisco.com

11.  References

11.1.  Normative References

              satyamoh@cisco.com, s., Patel, K., Sajassi, A., Drake, J.,
              and T. Przygienda, "A new Designated Forwarder Election
              for the EVPN", draft-ietf-bess-evpn-df-election-03 (work
              in progress), October 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,

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   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

11.2.  Informative References

              Sajassi, A., Thoria, S., Patel, K., Yeung, D., Drake, J.,
              and W. Lin, "IGMP and MLD Proxy for EVPN", draft-ietf-
              bess-evpn-igmp-mld-proxy-00 (work in progress), March

              Sajassi, A., mishra, m., Thoria, S., Rabadan, J., and J.
              Drake, "Per multicast flow Designated Forwarder Election
              for EVPN", draft-sajassi-bess-evpn-per-mcast-flow-df-
              election-00 (work in progress), March 2018.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

Authors' Addresses

   Satya Ranjan Mohanty
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134

   Email: satyamoh@cisco.com

   Mrinmoy Ghosh
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134

   Email: mrghosh@cisco.com

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   Sandy Breeze
   University of Warwick
   United Kingdom

   Email: sandy.breeze@eu.clara.net

   Jim Uttaro
   200 S. Laurel Avenue
   Middletown, CA  07748

   Email: uttaro@att.com

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