Internet Engineering Task Force (IETF) A. Sajassi, Ed.
Request for Comments: 8317 S. Salam
Updates: 7385 Cisco
Category: Standards Track J. Drake
ISSN: 2070-1721 Juniper
January 2018 Ethernet-Tree (E-Tree) Support in Ethernet VPN (EVPN) and
Provider Backbone Bridging EVPN (PBB-EVPN)
The MEF Forum (MEF) has defined a rooted-multipoint Ethernet service
known as Ethernet-Tree (E-Tree). A solution framework for supporting
this service in MPLS networks is described in RFC 7387, "A Framework
for Ethernet-Tree (E-Tree) Service over a Multiprotocol Label
Switching (MPLS) Network". This document discusses how those
functional requirements can be met with a solution based on RFC 7432,
"BGP MPLS Based Ethernet VPN (EVPN)", with some extensions and a
description of how such a solution can offer a more efficient
implementation of these functions than that of RFC 7796,
"Ethernet-Tree (E-Tree) Support in Virtual Private LAN Service
(VPLS)". This document makes use of the most significant bit of the
Tunnel Type field (in the P-Multicast Service Interface (PMSI) Tunnel
attribute) governed by the IANA registry created by RFC 7385; hence,
it updates RFC 7385 accordingly.
Status of This Memo
This is an Internet Standards Track document.
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). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
Copyright (c) 2018 IETF Trust and the persons identified as the
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The MEF Forum (MEF) has defined a rooted-multipoint Ethernet service
known as Ethernet-Tree (E-Tree) [MEF6.1]. In an E-Tree service, a
customer site that is typically represented by an Attachment Circuit
(AC) (e.g., an 802.1Q VLAN tag [IEEE.802.1Q]), is labeled as either a
Root or a Leaf site. A customer site may also be represented by a
Media Access Control (MAC) address along with a VLAN tag. Root sites
can communicate with all other customer sites (both Root and Leaf
sites). However, Leaf sites can communicate with Root sites but not
with other Leaf sites. In this document, unless explicitly mentioned
otherwise, a site is always represented by an AC.
[RFC7387] describes a solution framework for supporting E-Tree
service in MPLS networks. This document identifies the functional
components of an overall solution to emulate E-Tree services in MPLS
networks and supplements the multipoint-to-multipoint Ethernet LAN
(E-LAN) services specified in [RFC7432] and [RFC7623].
[RFC7432] defines EVPN, a solution for multipoint Layer 2 Virtual
Private Network (L2VPN) services with advanced multihoming
capabilities that uses BGP for distributing customer/client MAC
address reachability information over the MPLS/IP network. [RFC7623]
combines the functionality of EVPN with [IEEE.802.1ah] Provider
Backbone Bridging (PBB) for MAC address scalability.
This document discusses how the functional requirements for E-Tree
service can be met with a solution based on EVPN [RFC7432] and
PBB-EVPN [RFC7623] with some extensions to their procedures and BGP
attributes. Such a solution based on PBB-EVPN or EVPN can offer a
more efficient implementation of these functions than that of
[RFC7796], "Ethernet-Tree (E-Tree) Support in Virtual Private LAN
Service (VPLS)". This efficiency is achieved by performing filtering
of unicast traffic at the ingress Provider Edge (PE) nodes as opposed
to egress filtering where the traffic is sent through the network and
gets filtered and discarded at the egress PE nodes. The details of
this ingress filtering are described in Section 4.1. Since this
document specifies a solution based on [RFC7432], the knowledge of
that document is a prerequisite. This document makes use of the most
significant bit of the Tunnel Type field (in the PMSI Tunnel
attribute) governed by the IANA registry created by [RFC7385]; hence,
it updates [RFC7385] accordingly. Section 3 discusses E-Tree
scenarios, Sections 4 and 5 describe E-Tree solutions for EVPN and
PBB-EVPN (respectively), and Section 6 covers BGP encoding for E-Tree
2.1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Terms and Abbreviations
Broadcast Domain: In a bridged network, the broadcast domain
corresponds to a Virtual LAN (VLAN), where a VLAN is typically
represented by a single VLAN ID (VID) but can be represented by
several VIDs where Shared VLAN Learning (SVL) is used per
Bridge Table: An instantiation of a broadcast domain on a MAC-VRF.
CE: A Customer Edge device, e.g., a host, router, or switch.
EVI: An EVPN Instance spanning the Provider Edge (PE) devices
participating in that EVPN.
MAC-VRF: A Virtual Routing and Forwarding table for Media Access
Control (MAC) addresses on a PE.
ES: When a customer site (device or network) is connected to one or
more PEs via a set of Ethernet links, then that set of links is
referred to as an "Ethernet Segment".
ESI: An Ethernet Segment Identifier is a unique non-zero identifier
that identifies an ES.
Ethernet Tag: An Ethernet Tag identifies a particular broadcast
domain, e.g., a VLAN. An EVPN instance consists of one or more
PE: Provider Edge device.
3. E-Tree Scenarios
This document categorizes E-Tree scenarios into the following three
categories, depending on the nature of the Root/Leaf site
Scenario 1: either Leaf or Root site(s) per PE;
Scenario 2: either Leaf or Root site(s) per Attachment Circuit (AC);
Scenario 3: either Leaf or Root site(s) per MAC address.
3.1. Scenario 1: Leaf or Root Site(s) per PE
In this scenario, a PE may receive traffic from either Root ACs or
Leaf ACs for a given MAC-VRF/bridge table, but not both. In other
words, a given EVPN Instance (EVI) on a Provider Edge (PE) device is
either associated with Root(s) or Leaf(s). The PE may have both Root
and Leaf ACs, albeit for different EVIs.
| PE1 | | PE2 |
+---+ | +---+ | +------+ | +---+ | +---+
|CE1+---AC1----+--+ | | | MPLS | | | +--+----AC2-----+CE2|
+---+ (Root) | |MAC| | | /IP | | |MAC| | (Leaf) +---+
| |VRF| | | | | |VRF| |
| | | | | | | | | | +---+
| | | | | | | | +--+----AC3-----+CE3|
| +---+ | +------+ | +---+ | (Leaf) +---+
Figure 1: Scenario 1
In this scenario, tailored BGP Route Target (RT) import/export
policies among the PEs belonging to the same EVI can be used to
prevent communication among Leaf PEs. To prevent communication among
Leaf ACs connected to the same PE and belonging to the same EVI,
split-horizon filtering is used to block traffic from one Leaf AC to
another Leaf AC on a MAC-VRF for a given E-Tree EVI. The purpose of
this topology constraint is to avoid having PEs with only Leaf sites
importing and processing BGP MAC routes from each other. To support
such a topology constraint in EVPN, two BGP RTs are used for every
EVI: one RT is associated with the Root sites (Root ACs) and the
other is associated with the Leaf sites (Leaf ACs). On a per-EVI
basis, every PE exports the single RT associated with its type of
site(s). Furthermore, a PE with a Root site(s) imports both Root and
Leaf RTs, whereas a PE with a Leaf site(s) only imports the Root RT.
For this scenario, if it is desired to use only a single RT per EVI
(just like E-LAN services in [RFC7432]), then approach B in Scenario
2 (described below) needs to be used.
3.2. Scenario 2: Leaf or Root Site(s) per AC
In this scenario, a PE can receive traffic from both Root ACs and
Leaf ACs for a given EVI. In other words, a given EVI on a PE can be
associated with both Root(s) and Leaf(s).
| PE1 | | PE2 |
+---+ | +---+ | +------+ | +---+ | +---+
|CE1+-----AC1----+--+ | | | | | | +--+---AC2--+CE2|
+---+ (Leaf) | |MAC| | | MPLS | | |MAC| | (Leaf) +---+
| |VRF| | | /IP | | |VRF| |
| | | | | | | | | | +---+
| | | | | | | | +--+---AC3--+CE3|
| +---+ | +------+ | +---+ | (Root) +---+
Figure 2: Scenario 2
In this scenario, (as in Scenario 1 Section 3.1), two RTs (one for
Root and another for Leaf) can be used. However, the difference is
that on a PE with both Root and Leaf ACs, all remote MAC routes are
imported; thus, in order to apply the proper ingress filtering, there
needs to be a way to differentiate remote MAC routes associated with
Leaf ACs versus the ones associated with Root ACs.
In order to recognize the association of a destination MAC address to
a Leaf or Root AC and, thus, support ingress filtering on the ingress
PE with both Leaf and Root ACs, MAC addresses need to be colored with
a Root or Leaf-Indication before advertising to other PEs. There are
two approaches for such coloring:
(A) to always use two RTs (one to designate Leaf RT and another for
Root RT), or
(B) to allow for a single RT to be used per EVI, just like
[RFC7432], and, thus, color MAC addresses via a "color" flag in
a new extended community as detailed in Section 6.1.
Approach A would require the same data-plane enhancements as approach
B if MAC-VRF and bridge tables used per VLAN are to remain consistent
with Section 6 of [RFC7432]. In order to avoid data-plane
enhancements for approach A, multiple bridge tables per VLAN may be
considered; however, this has major drawbacks (as described in
Appendix A); thus, it is not recommended.
Given that both approaches A and B would require the same data-plane
enhancements, approach B is chosen here in order to allow for RT
usage consistent with baseline EVPN [RFC7432] and for better
generality. It should be noted that if one wants to use RT
constraints in order to avoid MAC advertisements associated with a
Leaf AC to PEs with only Leaf ACs, then two RTs (one for Root and
another for Leaf) can still be used with approach B; however, in such
applications, Leaf/Root RTs will be used to constrain MAC
advertisements and are not used to color the MAC routes for ingress
filtering (i.e., in approach B, the coloring is always done via the
new extended community).
If, for a given EVI, a significant number of PEs have both Leaf and
Root sites attached (even though they may start as Root-only or Leaf-
only PEs), then a single RT per EVI should be used. The reason for
such a recommendation is to alleviate the configuration overhead
associated with using two RTs per EVI at the expense of having some
unwanted MAC addresses on the Leaf-only PEs.
3.3. Scenario 3: Leaf or Root Site(s) per MAC Address
In this scenario, a customer Root or Leaf site is represented by a
MAC address on an AC and a PE may receive traffic from both Root and
Leaf sites on that AC for an EVI. This scenario is not covered in
either [RFC7387] or [MEF6.1]; however, it is covered in this document
for the sake of completeness. In this scenario, since an AC carries
traffic from both Root and Leaf sites, the granularity at which Root
or Leaf sites are identified is on a per-MAC-address basis. This
scenario is considered in this document for EVPN service with only
known unicast traffic because the Designated Forwarder (DF) filtering
per [RFC7432] would not be compatible with the required egress
filtering; that is, Broadcast, Unknown Unicast, and Multicast (BUM)
traffic is not supported in this scenario; it is dropped by the
For this scenario, the approach B in Scenario 2 is used in order to
allow for single RT usage by service providers.
| PE1 | | PE2 |
+---+ | +---+ | +------+ | +---+ | +---+
|CE1+-----AC1----+--+ | | | | | | +--+-----AC2----+CE2|
+---+ (Root) | | E | | | MPLS | | | E | | (Leaf/Root)+---+
| | V | | | /IP | | | V | |
| | I | | | | | | I | | +---+
| | | | | | | | +--+-----AC3----+CE3|
| +---+ | +------+ | +---+ | (Leaf) +---+
Figure 3: Scenario 3
In conclusion, the approach B in scenario 2 is the recommended
approach across all the above three scenarios, and the corresponding
solution is detailed in the following sections.
4. Operation for EVPN
[RFC7432] defines the notion of the Ethernet Segment Identifier (ESI)
MPLS label used for split-horizon filtering of BUM traffic at the
egress PE. Such egress filtering capabilities can be leveraged in
provision of E-Tree services, as it will be seen shortly for BUM
traffic. For known unicast traffic, additional extensions to
[RFC7432] are needed (i.e., a new BGP extended community for Leaf-
Indication described in Section 6.1) in order to enable ingress
filtering as described in detail in the following sections.
4.1. Known Unicast Traffic
In EVPN, MAC learning is performed in the control plane via
advertisement of BGP routes. Because of this, the filtering needed
by an E-Tree service for known unicast traffic can be performed at
the ingress PE, thus providing very efficient filtering and avoiding
sending known unicast traffic over the MPLS/IP core to be filtered at
the egress PE, as is done in traditional E-Tree solutions (i.e.,
E-Tree for VPLS [RFC7796]).
To provide such ingress filtering for known unicast traffic, a PE
MUST indicate to other PEs what kind of sites (Root or Leaf) its MAC
addresses are associated with. This is done by advertising a Leaf-
Indication flag (via an extended community) along with each of its
MAC/IP Advertisement routes learned from a Leaf site. The lack of
such a flag indicates that the MAC address is associated with a Root
site. This scheme applies to all scenarios described in Section 3.
Tagging MAC addresses with a Leaf-Indication enables remote PEs to
perform ingress filtering for known unicast traffic; that is, on the
ingress PE, the MAC destination address lookup yields (in addition to
the forwarding adjacency) a flag that indicates whether or not the
target MAC is associated with a Leaf site. The ingress PE cross-
checks this flag with the status of the originating AC, and if both
are Leafs, then the packet is not forwarded.
In a situation where MAC moves are allowed among Leaf and Root sites
(e.g., non-static MAC), PEs can receive multiple MAC/IP Advertisement
routes for the same MAC address with different Root or Leaf-
Indications (and possibly different ESIs for multihoming scenarios).
In such situations, MAC mobility procedures (see Section 15 of
[RFC7432]) take precedence to first identify the location of the MAC
before associating that MAC with a Root or a Leaf site.
To support the above ingress filtering functionality, a new E-Tree
extended community with a Leaf-Indication flag is introduced (see
Section 6.1). This new extended community MUST be advertised with
MAC/IP Advertisement routes learned from a Leaf site. Besides MAC/IP
Advertisement routes, no other EVPN routes are required to carry this
new extended community for the purpose of known unicast traffic.
4.2. BUM Traffic
This specification does not provide support for filtering Broadcast,
Unknown Unicast, and Multicast (BUM) traffic on the ingress PE; due
to the multidestination nature of BUM traffic, it is not possible to
perform filtering of the same on the ingress PE. As such, the
solution relies on egress filtering. In order to apply the proper
egress filtering, which varies based on whether a packet is sent from
a Leaf AC or a Root AC, the MPLS-encapsulated frames MUST be tagged
with an indication of when they originated from a Leaf AC (i.e., to
be tagged with a Leaf label as specified in Section 6.1). This Leaf
label allows for disposition PE (e.g., egress PE) to perform the
necessary egress filtering function in a data plane similar to the
ESI label in [RFC7432]. The allocation of the Leaf label is on a
per-PE basis (e.g., independent of ESI and EVI) as described in the
The Leaf label can be upstream assigned for Point-to-Multipoint
(P2MP) Label Switched Path (LSP) or downstream assigned for Ingress
Replication tunnels. The main difference between a downstream- and
upstream-assigned Leaf label is that, in the case of downstream-
assigned Leaf labels, not all egress PE devices need to receive the
label in MPLS-encapsulated BUM packets, just like the ESI label for
Ingress Replication procedures defined in [RFC7432].
On the ingress PE, the PE needs to place all its Leaf ACs for a given
bridge domain in a single split-horizon group in order to prevent
intra-PE forwarding among its Leaf ACs. This intra-PE split-horizon
filtering applies to BUM traffic as well as known unicast traffic.
There are four scenarios to consider as follows. In all these
scenarios, the ingress PE imposes the right MPLS label associated
with the originated Ethernet Segment (ES) depending on whether the
Ethernet frame originated from a Root or a Leaf site on that Ethernet
Segment (ESI label or Leaf label). The mechanism by which the PE
identifies whether a given frame originated from a Root or a Leaf
site on the segment is based on the AC identifier for that segment
(e.g., Ethernet Tag of the frame for 802.1Q frames [IEEE.802.1Q]).
Other mechanisms for identifying Root or Leaf sites, such as the use
of the source MAC address of the receiving frame, are optional. The
scenarios below are described in context of a Root/Leaf AC, however,
they can be extended to the Root/Leaf MAC address if needed.
4.2.1. BUM Traffic Originated from a Single-Homed Site on a Leaf AC
In this scenario, the ingress PE adds a Leaf label advertised using
the E-Tree extended community (see Section 6.1), which indicates a
Leaf site. This Leaf label, used for single-homing scenarios, is not
on a per-ES basis but rather on a per PE basis (i.e., a single Leaf
MPLS label is used for all single-homed ESs on that PE). This Leaf
label is advertised to other PE devices using the E-Tree extended
community (see Section 6.1) along with an Ethernet Auto-Discovery per
ES (EAD-ES) route with an ESI of zero and a set of RTs corresponding
to all EVIs on the PE where each EVI has at least one Leaf site.
Multiple EAD-ES routes will need to be advertised if the number of
RTs that need to be carried exceed the limit on a single route per
[RFC7432]. The ESI for the EAD-ES route is set to zero to indicate
When a PE receives this special Leaf label in the data path, it
blocks the packet if the destination AC is of type Leaf; otherwise,
it forwards the packet.
4.2.2. BUM Traffic Originated from a Single-Homed Site on a Root AC
In this scenario, the ingress PE does not add any ESI or Leaf labels
and it operates per the procedures in [RFC7432].
4.2.3. BUM Traffic Originated from a Multihomed Site on a Leaf AC
In this scenario, it is assumed that while different ACs (VLANs) on
the same ES could have a different Root/Leaf designation (some being
Roots and some being Leafs), the same VLAN does have the same Root/
Leaf designation on all PEs on the same ES. Furthermore, it is
assumed that there is no forwarding among subnets (i.e., the service
is EVPN L2 and not EVPN Integrated Routing and Bridging (IRB)
[EVPN-INTEGRATED]). IRB use cases described in [EVPN-INTEGRATED] are
outside the scope of this document.
In this scenario, if a multicast or broadcast packet is originated
from a Leaf AC, then it only needs to carry a Leaf label as described
in Section 4.2.1. This label is sufficient in providing the
necessary egress filtering of BUM traffic from getting sent to Leaf
ACs, including the Leaf AC on the same ES.
4.2.4. BUM Traffic Originated from a Multihomed Site on a Root AC
In this scenario, both the ingress and egress PE devices follow the
procedure defined in [RFC7432] for adding and/or processing an ESI
MPLS label; that is, existing procedures for BUM traffic in [RFC7432]
are sufficient and there is no need to add a Leaf label.
4.3. E-Tree Traffic Flows for EVPN
Per [RFC7387], a generic E-Tree service supports all of the following
- known unicast traffic from Root to Roots & Leafs
- known unicast traffic from Leaf to Roots
- BUM traffic from Root to Roots & Leafs
- BUM traffic 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 only multicast and broadcast flows
need to be supported, the L2VPN PEs can avoid performing any MAC
The following subsections will describe the operation of EVPN to
support E-Tree service with and without MAC learning.
4.3.1. E-Tree with MAC Learning
The PEs implementing an E-Tree service must perform MAC learning when
unicast traffic flows must be supported among Root and Leaf sites.
In this case, the PE(s) with Root sites performs MAC learning in the
data path over the ESs and advertises reachability in EVPN MAC/IP
Advertisement routes. These routes will be imported by all PEs for
that EVI (i.e., PEs that have Leaf sites as well as PEs that have
Root sites). Similarly, the PEs with Leaf sites perform MAC learning
in the data path over their ESs and advertise reachability in EVPN
MAC/IP Advertisement routes. For scenarios where two different RTs
are used per EVI (one to designate a Root site and another to
designate a Leaf site), the MAC/IP Advertisement routes are imported
only by PEs with at least one Root site in the EVI (i.e., a PE with
only Leaf sites will not import these routes). PEs with Root and/or
Leaf sites may use the Ethernet Auto-Discovery per EVI (EAD-EVI)
routes for aliasing (in the case of multihomed segments) and EAD-ES
routes for mass MAC withdrawal per [RFC7432].
To support multicast/broadcast from Root to Leaf sites, either a P2MP
tree rooted at the PE(s) with the Root site(s) (e.g., Root PEs) or
Ingress Replication can be used (see Section 16 of [RFC7432]). The
multicast tunnels are set up through the exchange of the EVPN
Inclusive Multicast route, as defined in [RFC7432].
To support multicast/broadcast from Leaf to Root sites, either
Ingress Replication tunnels from each Leaf PE or a P2MP tree rooted
at each Leaf PE can be used. The following two paragraphs describe
when each of these tunneling schemes can be used and how to signal
When there are only a few Root PEs with small amount of multicast/
broadcast traffic from Leaf PEs toward Root PEs, then Ingress
Replication tunnels from Leaf PEs toward Root PEs should be
sufficient. Therefore, if a Root PE needs to support a P2MP tunnel
in the transmit direction from itself to Leaf PEs, and, at the same
time, it wants to support Ingress Replication tunnels in the receive
direction, the Root PE can signal it efficiently by using a new
composite tunnel type defined in Section 6.2. This new composite
tunnel type is advertised by the Root PE to simultaneously indicate a
P2MP tunnel in the transmit direction and an Ingress Replication
tunnel in the receive direction for the BUM traffic.
If the number of Root PEs is large, P2MP tunnels (e.g., Multipoint
LDP (mLDP) or RSVP-TE) originated at the Leaf PEs may be used; thus,
there will be no need to use the modified PMSI Tunnel attribute and
the composite tunnel type values defined in Section 6.2.
4.3.2. 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 mainly
multicast or broadcast. In this case, the PEs do not exchange EVPN
MAC/IP Advertisement routes. Instead, the Inclusive Multicast
Ethernet Tag route is used to support BUM traffic. In such
scenarios, the small amount of unicast traffic (if any) is sent as
part of BUM traffic.
The fields of this route are populated per the procedures defined in
[RFC7432], and the multicast tunnel setup criteria are as described
in the previous section.
Just as in the previous section, if the number of Root PEs are only a
few and, thus, Ingress Replication is desired from Leaf PEs to these
Root PEs, then the modified PMSI attribute and the composite tunnel
type values defined in Section 6.2 should be used.
5. Operation for PBB-EVPN
In PBB-EVPN, the PE advertises a Root or Leaf-Indication along with
each Backbone MAC (B-MAC) Advertisement route to indicate whether the
associated B-MAC address corresponds to a Root or a Leaf site. Just
like the EVPN case, the new E-Tree extended community defined in
Section 6.1 is advertised with each EVPN MAC/IP Advertisement route.
In the case where a multihomed ES has both Root and Leaf sites
attached, two B-MAC addresses are advertised: one B-MAC address is
per ES (as specified in [RFC7623]) and implicitly denotes Root, and
the other B-MAC address is per PE and explicitly denotes Leaf. The
former B-MAC address is not advertised with the E-Tree extended
community, but the latter B-MAC denoting Leaf is advertised with the
new E-Tree extended community where a "Leaf-indication" flag is set.
In multihoming scenarios where an ES has both Root and Leaf ACs, it
is assumed that while different ACs (VLANs) on the same ES could have
a different Root/Leaf designation (some being Roots and some being
Leafs), the same VLAN does have the same Root/Leaf designation on all
PEs on the same ES. Furthermore, it is assumed that there is no
forwarding among subnets (i.e., the service is L2 and not IRB). An
IRB use case is outside the scope of this document.
The ingress PE uses the right B-MAC source address depending on
whether the Ethernet frame originated from the Root or Leaf AC on
that ES. The mechanism by which the PE identifies whether a given
frame originated from a Root or Leaf site on the segment is based on
the Ethernet Tag associated with the frame. Other mechanisms of
identification, beyond the Ethernet Tag, are outside the scope of
Furthermore, a PE advertises two special global B-MAC addresses, one
for Root and another for Leaf, and tags the Leaf one as such in the
MAC Advertisement route. These B-MAC addresses are used as source
addresses for traffic originating from single-homed segments. The
B-MAC address used for indicating Leaf sites can be the same for both
single-homed and multihomed segments.
5.1. Known Unicast Traffic
For known unicast traffic, the PEs perform ingress filtering: on the
ingress PE, the Customer/Client MAC (C-MAC) [RFC7623] destination
address lookup yields, in addition to the target B-MAC address and
forwarding adjacency, a flag that indicates whether the target B-MAC
is associated with a Root or a Leaf site. The ingress PE also checks
the status of the originating site; if both are Leafs, then the
packet is not forwarded.
5.2. BUM Traffic
For BUM traffic, the PEs must perform egress filtering. When a PE
receives an EVPN MAC/IP Advertisement route (which will be used as a
source B-MAC for BUM traffic), it updates its egress filtering (based
on the source B-MAC address) as follows:
- If the EVPN MAC/IP Advertisement route indicates that the
advertised B-MAC is a Leaf, and the local ES is a Leaf as well,
then the source B-MAC address is added to its B-MAC list used for
egress filtering (i.e., to block traffic from that B-MAC address).
Otherwise, the B-MAC filtering list is not updated.
- If the EVPN MAC/IP Advertisement route indicates that the
advertised B-MAC has changed its designation from a Leaf to a
Root, and the local ES is a Leaf, then the source B-MAC address is
removed from the B-MAC list corresponding to the local ES used for
egress filtering (i.e., to unblock traffic from that B-MAC
When the egress PE receives the packet, it examines the B-MAC source
address to check whether it should filter or forward the frame. Note
that this uses the same filtering logic as the split-horizon
filtering described in Section 18.104.22.168 of [RFC7623] and does not
require any additional flags in the data plane.
Just as in Section 4.2, the PE places all Leaf ESs of a given bridge
domain in a single split-horizon group in order to prevent intra-PE
forwarding among Leaf segments. This split-horizon function applies
to BUM traffic as well as known unicast traffic.
5.3. E-Tree without MAC Learning
In scenarios where the traffic of interest is only multicast and/or
broadcast, the PEs implementing an E-Tree service do not need to do
any MAC learning. In such scenarios, the filtering must be performed
on egress PEs. For PBB-EVPN, the handling of such traffic is per
Section 5.2 without the need for C-MAC learning (in the data plane)
in the I-component (C-bridge table) of PBB-EVPN PEs (at both ingress
and egress PEs).
6. BGP Encoding
This document defines a new BGP extended community for EVPN.
6.1. E-Tree Extended Community
This extended community is a new transitive extended community
[RFC4360] having a Type field value of 0x06 (EVPN) and the Sub-Type
0x05. It is used for Leaf-Indication of known unicast and BUM
traffic. It indicates that the frame is originated from a Leaf site.
The E-Tree extended community is encoded as an 8-octet value as
0 1 2 3
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=0x06 | Sub-Type=0x05 | Flags(1 Octet)| Reserved=0 |
| Reserved=0 | Leaf Label |
Figure 4: E-Tree Extended Community
The Flags field has the following format:
0 1 2 3 4 5 6 7
| MBZ |L| (MBZ = MUST Be Zero)
This document defines the following flags:
+ Leaf-Indication (L)
A value of one indicates a Leaf AC/site. The rest of the flag bits
are reserved and should be set to zero.
When this extended community is advertised along with the MAC/IP
Advertisement route (for known unicast traffic) per Section 4.1, the
Leaf-Indication flag MUST be set to one and the Leaf label SHOULD be
set to zero. The receiving PE MUST ignore Leaf label and only
process the Leaf-Indication flag. A value of zero for the Leaf-
Indication flag is invalid when sent along with a MAC/IP
Advertisement route, and an error should be logged.
When this extended community is advertised along with the EAD-ES
route (with an ESI of zero) for BUM traffic to enable egress
filtering on disposition PEs per Sections 4.2.1 and 4.2.3, the Leaf
label MUST be set to a valid MPLS label (i.e., a non-reserved,
assigned MPLS label [RFC3032]) and the Leaf-Indication flag SHOULD be
set to zero. The value of the 20-bit MPLS label is encoded in the
high-order 20 bits of the Leaf label field. The receiving PE MUST
ignore the Leaf-Indication flag. A non-valid MPLS label, when sent
along with the EAD-ES route, should be ignored and logged as an
The reserved bits SHOULD be set to zero by the transmitter and MUST
be ignored by the receiver.
6.2. PMSI Tunnel Attribute
[RFC6514] defines the PMSI Tunnel attribute, which is an optional
transitive attribute with the following format:
| Flags (1 octet) |
| Tunnel Type (1 octet) |
| Ingress Replication MPLS Label (3 octets) |
| Tunnel Identifier (variable) |
Table 1: PMSI Tunnel Attribute
This document defines a new composite tunnel type by introducing a
new 'composite tunnel' bit in the Tunnel Type field and adding an
MPLS label to the Tunnel Identifier field of the PMSI Tunnel
attribute, as detailed below. All other fields remain as defined in
[RFC6514]. Composite tunnel type is advertised by the Root PE to
simultaneously indicate a non-Ingress-Replication tunnel (e.g., P2MP
tunnel) in the transmit direction and an Ingress Replication tunnel
in the receive direction for the BUM traffic.
When receiver Ingress Replication labels are needed, the high-order
bit of the Tunnel Type field (composite tunnel bit) is set while the
remaining low-order seven bits indicate the Tunnel Type as before
(for the existing Tunnel Types). When this composite tunnel bit is
set, the "tunnel identifier" field begins with a three-octet label,
followed by the actual tunnel identifier for the transmit tunnel.
PEs that don't understand the new meaning of the high-order bit treat
the Tunnel Type as an undefined Tunnel Type and treat the PMSI Tunnel
attribute as a malformed attribute [RFC6514]. That is why the
composite tunnel bit is allocated in the Tunnel Type field rather
than the Flags field. For the PEs that do understand the new meaning
of the high-order, if Ingress Replication is desired when sending BUM
traffic, the PE will use the label in the Tunnel Identifier field
when sending its BUM traffic.
Using the composite tunnel bit for Tunnel Types 0x00 'no tunnel
information present' and 0x06 'Ingress Replication' is invalid. A PE
that receives a PMSI Tunnel attribute with such information considers
it malformed, and it SHOULD treat this Update as though all the
routes contained in this Update had been withdrawn per Section 6 of
7. Security Considerations
Since this document uses the EVPN constructs of [RFC7432] and
[RFC7623], the same security considerations in these documents are
also applicable here. Furthermore, this document provides an
additional security check by allowing sites (or ACs) of an EVPN
instance to be designated as a "Root" or "Leaf" by the network
operator / service provider and thus prevent any traffic exchange
among "Leaf" sites of that VPN through ingress filtering for known
unicast traffic and egress filtering for BUM traffic. Since (by
default and for the purpose of backward compatibility) an AC that
doesn't have a Leaf designation is considered a Root AC, in order to
avoid any traffic exchange among Leaf ACs, the operator SHOULD
configure the AC with a proper role (Leaf or Root) before activating
8. IANA Considerations
IANA has allocated sub-type value 5 in the "EVPN Extended Community
Sub-Types" registry defined in [RFC7153] as follows:
SUB-TYPE VALUE NAME Reference
-------------- ------------------------- -------------
0x05 E-Tree Extended Community This document
This document creates a one-octet registry called "E-Tree Flags".
New registrations will be made through the "RFC Required" procedure
defined in [RFC8126]. Initial registrations are as follows:
Bit Name Reference
---- -------------- -------------
7 Leaf-Indication This document
8.1. Considerations for PMSI Tunnel Types
The "P-Multicast Service Interface (PMSI) Tunnel Types" registry in
the "Border Gateway Protocol (BGP) Parameters" registry has been
updated to reflect the use of the most significant bit as the
"composite tunnel" bit (see Section 6.2).
For this purpose, this document updates [RFC7385] by changing the
previously unassigned values (i.e., 0x08 - 0xFA) as follows:
Value Meaning Reference
--------- ----------------------------- --------------
0x7B-0x7E Experimental This Document
0x7F Reserved This Document
0x80-0xFA Reserved for Composite Tunnel This Document
0xFB-0xFE Experimental [RFC7385]
0xFF Reserved [RFC7385]
The allocation policy for values 0x08-0x7A is per IETF Review
[RFC8126]. The range for "Experimental" has been expanded to include
the previously assigned range of 0xFB-0xFE and the new range of
0x7B-0x7E. The values in these ranges are not to be assigned. The
value 0x7F, which is the mirror image of (0xFF), is reserved in this
Appendix A. Multiple Bridge Tables per E-Tree Service Instance
When two MAC-VRFs (two bridge tables per VLAN) are used for an E-Tree
service (one for Root ACs and another for Leaf ACs) on a given PE,
then the following complications in a data-plane path can result.
Maintaining two MAC-VRFs (two bridge tables) per VLAN (when both Leaf
and Root ACs exists for that VLAN) would require either that two
lookups be performed per MAC address in each direction in case of a
miss or that the duplication of many MAC addresses between the two
bridge tables belonging to the same VLAN (same E-Tree instance) be
made. Unless two lookups are made, duplication of MAC addresses
would be needed for both locally learned and remotely learned MAC
addresses. Locally learned MAC addresses from Leaf ACs need to be
duplicated onto a Root bridge table, and locally learned MAC
addresses from Root ACs need to be duplicated onto a Leaf bridge
table. Remotely learned MAC addresses from Root ACs need to be
copied onto both Root and Leaf bridge tables. Because of potential
inefficiencies associated with data-plane implementation of
additional MAC lookup or duplication of MAC entries, this option is
not believed to be implementable without data-plane performance
inefficiencies in some platforms; thus, this document introduces the
coloring as described in Section 3.2 and detailed in Section 4.1.
We would like to thank Eric Rosen, Jeffrey Zhang, Wen Lin, Aldrin
Issac, Wim Henderickx, Dennis Cai, and Antoni Przygienda for their
valuable comments and contributions. The authors would also like to
thank Thomas Morin for shepherding this document and providing
Ali Sajassi (editor)