L2VPN Workgroup                                              Ali Sajassi
INTERNET-DRAFT                                               Samer Salam
Intended Status: Standards Track                                   Cisco

Expires: December 29, 2012                                 June 29, 2012


                        E-TREE Support in E-VPN
                   draft-sajassi-l2vpn-evpn-etree-00


Abstract

   The Metro Ethernet Forum (MEF) has defined a rooted-multipoint
   Ethernet service known as Ethernet Tree (E-Tree). [ETREE-FRAMEWORK]
   proposes a solution framework for supporting this service in MPLS
   networks. This document discusses how those functional requirements
   can be easily met with E-VPN.


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
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Copyright and License 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



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   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) 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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   described in the Simplified BSD License.



Table of Contents

   1  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
   2  E-Tree Scenarios and E-VPN Support  . . . . . . . . . . . . . .  3
     2.1 Scenario 1: Leaf OR Root site(s) per PE  . . . . . . . . . .  3
     2.2 Scenario 2: Leaf AND Root site(s) per PE . . . . . . . . . .  5
     2.3 Scenario 3: Leaf AND Root site(s) per Ethernet Segment . . .  6
   3 Operation  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1 E-Tree with MAC Learning . . . . . . . . . . . . . . . . . .  7
     3.2 E-Tree without MAC Learning  . . . . . . . . . . . . . . . .  8
   4  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . .  8
   5  Security Considerations . . . . . . . . . . . . . . . . . . . .  8
   6  IANA Considerations . . . . . . . . . . . . . . . . . . . . . .  8
   7  References  . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     7.1  Normative References  . . . . . . . . . . . . . . . . . . .  8
     7.2  Informative References  . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  9






















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1  Introduction

   The Metro Ethernet Forum (MEF) has defined a rooted-multipoint
   Ethernet service known as Ethernet Tree (E-Tree). In an E-Tree
   service, endpoints are labeled as either Root or Leaf sites. Root
   sites can communicate with all other sites. Leaf sites can
   communicate with Root sites but not with other Leaf sites.

   [ETREE-FRAMEWORK] proposes the solution framework for supporting E-
   Tree service in MPLS networks. The document identifies the functional
   components of the overall solution to emulate E-Tree services in
   addition to Ethernet LAN (E-LAN) services on an existing MPLS
   network.

   [EVPN] is a solution for multipoint L2VPN services, with advanced
   multi-homing capabilities, using BGP for distributing customer/client
   MAC address reach-ability information over the MPLS/IP network.

   This document discusses how the functional requirements for E-Tree
   service can be easily met with E-VPN.

1.1  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].


2  E-Tree Scenarios and E-VPN Support

   In this section, we will categorize support for E-Tree into three
   different scenarios, depending on the nature of the site association
   (Root/Leaf) per PE or per Ethernet Segment:

   - Leaf OR Root site(s) per PE

   - Leaf AND Root site(s) per PE

   - Leaf AND Root site(s) per Ethernet Segment

   For each scenario, we will describe the E-VPN mechanism for
   supporting the E-Tree service.

2.1 Scenario 1: Leaf OR Root site(s) per PE

   In this scenario, a PE may have Root sites OR Leaf sites for a given
   VPN instance, but not both concurrently. The PE may have both Root
   and Leaf sites albeit for different VPNs. Every Ethernet Segment



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   connected to the PE is uniquely identified as either a Root or a Leaf
   site.

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----ES1----+--+   |  |  | MPLS |  |  |   +--+----ES2-----+CE2|
    +---+    (Root)  |  | E |  |  |  /IP |  |  | E |  |   (Leaf)   +---+
                     |  | V |  |  |      |  |  | V |  |
                     |  | I |  |  |      |  |  | I |  |            +---+
                     |  |   |  |  |      |  |  |   +--+----ES3-----+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Leaf)   +---+
                     +---------+            +---------+

   Figure 1: Scenario 1

   One approach for addressing this scenario involves associating two
   BGP Route-Targets (RTs) with every E-VPN Instance (EVI): one RT is
   associated with the Root sites and the other is associated with the
   Leaf sites. On a per EVI basis, every PE exports the single RT
   associated with its type of site(s). Furthermore, a PE with Root
   site(s) imports both Root and Leaf RTs, whereas a PE with Leaf
   site(s) only imports the Root RT. This approach suffers from two
   shortcomings:

   - Additional configuration overhead, as it requires the network
   operator to configure two RTs per EVI.

   - Introduces a scalability limitation where only 32K E-Tree EVIs can
   be supported (due to 2 bytes RT value, and the fact that two RTs are
   required per EVI).

   To alleviate both of these issues, we propose a new BGP Extended
   Community attribute encoded as follows:

       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Type       |   Sub-Type    |            Value                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Value                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where,

   Type = To be assigned by IANA

   Sub-Type = 1 byte, value TBA1 denotes Root, value TBA2 denotes Leaf




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   Value = 6 bytes uniquely identifying an EVI.

   This extended community is a new transitive extended community, and
   will be referred to as the EVI-Import Extended Community. This
   extended community is used in lieu of the RT on the following E-VPN
   routes:

   - MAC Advertisement Routes
   - Ethernet A-D Routes
   - Inclusive Multicast Routes

   On a per EVI basis, every PE exports routes with the single EVI-
   Import extended community associated with its type of site(s).
   Furthermore, a PE with Root site(s) imports routes with both Root and
   Leaf EVI-Import extended community. Whereas, a PE with Leaf site(s)
   only imports the Root EVI-Import extended community.

2.2 Scenario 2: Leaf AND Root site(s) per PE

   In this scenario, a PE may have a set of one or more Root sites AND a
   set of one or more Leaf sites for a given VPN instance. Every
   Ethernet Segment connected to the PE is uniquely identified as either
   a Root or a Leaf site.

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----ES1----+--+   |  |  |      |  |  |   +--+----ES2-----+CE2|
    +---+    (Leaf)  |  | E |  |  | MPLS |  |  | E |  |   (Leaf)   +---+
                     |  | V |  |  |  /IP |  |  | V |  |
                     |  | I |  |  |      |  |  | I |  |            +---+
                     |  |   |  |  |      |  |  |   +--+----ES3-----+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Root)   +---+
                     +---------+            +---------+

   Figure 2: Scenario 2

   This scenario requires that the MPLS-encapsulated frames be tagged
   with an indication of whether they originated from a Root or a Leaf
   Ethernet Segment, so that the proper connectivity constraints can be
   enforced. This can be achieved in E-VPN through the use of the ESI
   MPLS label, since this label identifies the Ethernet Segment of
   origin of a given frame. For E-Tree service, the ESI MPLS label must
   be used to encapsulate not only multi-destination frames (i.e.
   broadcast, multicast & unknown unicast), but also known unicast
   frames. The egress PE determines whether or not to forward a
   particular frame to an Ethernet Segment depending on a combination of
   the split-horizon rule defined in [EVPN] and on the E-Tree



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   connectivity constraints:

   - If the ESI Label indicates that the source Ethernet Segment is a
   Root, then the frame can be forwarded on a segment granted that it
   passes the split-horizon check.

   - If the ESI Label indicates that the source Ethernet Segment is a
   Leaf, then the frame can be forwarded only on a Root segment, granted
   that it passes the split-horizon check.

   When advertising the ESI MPLS label for a given Ethernet Segment, a
   PE must indicate whether the corresponding ESI is a Root or a Leaf
   site. This can be done by re-purposing one of the Reserved bits in
   the Flags field of the ESI MPLS label Extended Community attribute
   ([EVPN] Section 8) to indicate Root/Leaf status.

2.3 Scenario 3: Leaf AND Root site(s) per Ethernet Segment

   In this scenario, a PE may have a set of one or more Root sites AND a
   set of one or more Leaf sites for a given VPN instance. An Ethernet
   Segment connected to the PE may be identified as both a Root and a
   Leaf site concurrently.

                     +---------+            +---------+
                     |   PE1   |            |   PE2   |
    +---+            |  +---+  |  +------+  |  +---+  |            +---+
    |CE1+-----ES1----+--+   |  |  |      |  |  |   +--+----ES2-----+CE2|
    +---+ (Leaf/Root)|  | E |  |  | MPLS |  |  | E |  | (Leaf/Root)+---+
                     |  | V |  |  |  /IP |  |  | V |  |
                     |  | I |  |  |      |  |  | I |  |            +---+
                     |  |   |  |  |      |  |  |   +--+----ES3-----+CE3|
                     |  +---+  |  +------+  |  +---+  |   (Leaf)   +---+
                     +---------+            +---------+

   Figure 3: Scenario 3

   This scenario can be addressed by extending the use of the ESI MPLS
   label, as described in the previous section, so that for an Ethernet
   Segment that has both Root and Leaf sites attached, two ESI MPLS
   labels are allocated and advertised: one ESI MPLS label denotes Root
   and the other denotes Leaf. The ingress PE imposes the right ESI MPLS
   label depending on whether the Ethernet frame originated from the
   Root or Leaf site on that Ethernet Segment. The mechanism by which
   the PE identifies whether a given frame originated from a Root or
   Leaf site on the segment is outside the scope of this document.

   In addition to advertising two ESI MPLS labels per Ethernet Segment
   (for segments that have both Root and Leaf attached), a PE advertises



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   two special ESI MPLS labels: one for Root and another for Leaf. These
   are used by remote PEs for traffic originating from single-homed
   segments and for multi-homed segments that are not connected to the
   advertising PE.

3 Operation

   Per [ETREE-FRAMEWORK], a generic E-Tree service supports all of the
   following traffic flows:

        - Ethernet Unicast from Root to Leaf
        - Ethernet Unicast from Leaf to Root
        - Ethernet Unicast from Root to Root
        - Ethernet Broadcast/Multicast from Root to Roots & Leafs
        - Ethernet Broadcast/Multicast from Leaf to Roots

   A particular E-Tree service may need to support all of the above
   types of flows or only a select subset, depending on the target
   application. In the case where unicast flows need not be supported,
   the L2VPN PEs can avoid performing any MAC learning function.

   In the subsections that follow, we will describe the operation of E-
   VPN to support E-Tree service with and without MAC learning.

3.1 E-Tree with MAC Learning

   The PEs implementing an E-Tree service must perform MAC learning when
   unicast traffic flows must be supported from Root to Leaf or from
   Leaf to Root sites. In this case, the PE with Root sites performs MAC
   learning in the data-path over the Ethernet Segments, and advertises
   reachability in E-VPN MAC Advertisement routes. These routes will be
   imported by PEs that have Leaf sites as well as by PEs that have Root
   sites, in a given EVI. Similarly, the PEs with Leaf sites perform MAC
   learning in the data-path over their Ethernet Segments, and advertise
   reachability in E-VPN MAC Advertisement routes which are imported
   only by PEs with at least one Root site in the EVI. A PE with only
   Leaf sites will not import these routes. PEs with Root and/or Leaf
   sites may use the Ethernet A-D routes for aliasing (in the case of
   multi-homed segments) and for mass MAC withdrawal.

   To support multicast/broadcast from Root to Leaf sites, either a P2MP
   tree rooted at the PE(s) with the Root site(s) or ingress replication
   can be used. The multicast tunnels are set up through the exchange of
   the E-VPN Inclusive Multicast route, as defined in [E-VPN].

   To support multicast/broadcast from Leaf to Root sites, ingress
   replication should be sufficient for most scenarios where there is a
   single Root or few Roots. If the number of Roots is large, a P2MP



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   tree rooted at the PEs with Leaf sites may be used.

3.2 E-Tree without MAC Learning

   The PEs implementing an E-Tree service need not perform MAC learning
   when the traffic flows between Root and Leaf sites are multicast or
   broadcast. In this case, the PEs do not exchange E-VPN MAC
   Advertisement routes. Instead, the Ethernet A-D routes are used to
   exchange the E-VPN labels.

   The fields of the Ethernet A-D route are populated per the procedures
   defined in [E-VPN], and the route import rules are as described in
   previous sections.

4  Acknowledgement

   We would like to thank Sami Boutros for his comments.

5  Security Considerations

   Same security considerations as [E-VPN].

6  IANA Considerations

   Allocation of Extended Community Type and Sub-Type for E-VPN.

7  References

7.1  Normative References

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


   [ETREE-FRAMEWORK] Key et al., "A Framework for E-Tree Service over
              MPLS Network", draft-ietf-l2vpn-etree-frwk-00, work in
              progress, January 2012.

7.2  Informative References

   [EVPN] Aggarwal et al., "BGP MPLS Based Ethernet VPN", draft-ietf-
   l2vpn-evpn-00.txt, work in progress, February, 2012.


   [ETREE-REQ] Key et al., "Requirements for MEF E-Tree Support in
   VPLS", draft-ietf-l2vpn-etree-reqt-01.txt, work in progress, April,
   2012.




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Authors' Addresses


   Ali Sajassi
   Cisco
   170 West Tasman Drive
   San Jose, CA  95134, US
   Email: sajassi@cisco.com


   Samer Salam
   Cisco
   595 Burrard Street, Suite 2123
   Vancouver, BC V7X 1J1, Canada
   Email: ssalam@cisco.com




































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