Internet DRAFT - draft-ietf-bess-evpn-etree
draft-ietf-bess-evpn-etree
BESS Workgroup A. Sajassi, Ed.
INTERNET-DRAFT S. Salam
Intended Status: Standards Track Cisco
Updates: 7385 J. Drake
Juniper
J. Uttaro
ATT
S. Boutros
VMware
J. Rabadan
Nokia
Expires: April 28, 2018 October 28, 2017
E-TREE Support in EVPN & PBB-EVPN
draft-ietf-bess-evpn-etree-14
Abstract
The Metro Ethernet 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 RFC7387 ("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 RFC7432, BGP MPLS Based Ethernet VPN (EVPN), with
some extensions and how such a solution can offer a more efficient
implementation of these functions than that of RFC7796, E-Tree
Support in Virtual Private LAN Service (VPLS). This document makes
use of the most significant bit of the "Tunnel Type" field (in PMSI
Tunnel Attribute) governed by the IANA registry created by RFC7385,
and hence updates RFC7385 accordingly.
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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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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Copyright and License Notice
Copyright (c) 2017 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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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Specification of Requirements . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
2 E-Tree Scenarios . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Scenario 1: Leaf or Root Site(s) per PE . . . . . . . . . . 6
2.2 Scenario 2: Leaf or Root Site(s) per AC . . . . . . . . . . 6
2.3 Scenario 3: Leaf or Root Site(s) per MAC Address . . . . . . 8
3 Operation for EVPN . . . . . . . . . . . . . . . . . . . . . . . 9
3.1 Known Unicast Traffic . . . . . . . . . . . . . . . . . . . 9
3.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic . . . . . . 10
3.2.1 BUM Traffic Originated from a Single-homed Site on a
Leaf AC . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.2 BUM Traffic Originated from a Single-homed Site on a
Root AC . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.3 BUM Traffic Originated from a Multi-homed Site on a
Leaf AC . . . . . . . . . . . . . . . . . . . . . . . . 11
3.2.4 BUM Traffic Originated from a Multi-homed Site on a
Root AC . . . . . . . . . . . . . . . . . . . . . . . . 11
3.3 E-Tree Traffic Flows for EVPN . . . . . . . . . . . . . . . 12
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3.3.1 E-Tree with MAC Learning . . . . . . . . . . . . . . . . 12
3.3.2 E-Tree without MAC Learning . . . . . . . . . . . . . . 13
4 Operation for PBB-EVPN . . . . . . . . . . . . . . . . . . . . . 13
4.1 Known Unicast Traffic . . . . . . . . . . . . . . . . . . . 14
4.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic . . . . . . 14
4.3 E-Tree without MAC Learning . . . . . . . . . . . . . . . . 15
5 BGP Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1 E-Tree Extended Community . . . . . . . . . . . . . . . . . 15
5.2 PMSI Tunnel Attribute . . . . . . . . . . . . . . . . . . . 17
6 Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . 18
7 Security Considerations . . . . . . . . . . . . . . . . . . . . 18
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 18
8.1 Considerations for PMSI Tunnel Types . . . . . . . . . . . . 19
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1 Normative References . . . . . . . . . . . . . . . . . . . 19
9.2 Informative References . . . . . . . . . . . . . . . . . . 20
Appendix-A . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1 Introduction
The Metro Ethernet 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 Circuits (AC) (e.g., a 802.1Q VLAN tag but may also be
represented by a MAC address) is labeled as either a Root or a Leaf
site. 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 sits. 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. The document identifies the functional
components of an overall solution to emulate E-Tree services in MPLS
networks in addition to multipoint-to-multipoint Ethernet LAN (E-LAN)
services specified in [RFC7432] and [RFC7623].
[RFC7432] defines EVPN, 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. [RFC7623] combines the functionality of EVPN with
[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 (PBB-)EVPN (i.e.,
[RFC7432] and [RFC7623]) with some extensions to their procedures and
BGP attributes. Such (PBB-)EVPN based solution can offer a more
efficient implementation of these functions than that of RFC7796, E-
Tree Support in Virtual Private LAN Service (VPLS). This efficiency
is achieved by performing filtering of unicast traffic at the ingress
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 is described in section
3.1. Since this document specifies a solution based on [RFC7432], it
requires the readers to have the knowledge of [RFC7432] as
prerequisite. This document makes use of the most significant bit of
the "Tunnel Type" field (in PMSI Tunnel Attribute) governed by the
IANA registry created by RFC7385, and hence updates RFC7385
accordingly. Section 2 discusses E-Tree scenarios. Section 3 and 4
describe E-Tree solutions for EVPN and PBB-EVPN respectively, and
section 5 covers BGP encoding for E-Tree solutions.
1.1 Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
1.2 Terminology
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 [802.1Q].
Bridge Table: An instantiation of a broadcast domain on a MAC-VRF.
CE: 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.
Ethernet Segment (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'.
Ethernet Segment Identifier (ESI): A unique non-zero identifier that
identifies an Ethernet segment is called an 'Ethernet Segment
Identifier'.
Ethernet Tag: An Ethernet tag identifies a particular broadcast
domain, e.g., a VLAN. An EVPN instance consists of one or more
broadcast domains.
P2MP: Point to Multipoint.
PE: Provider Edge device.
2 E-Tree Scenarios
This document categorizes E-Tree scenarios into the following three
scenarios, depending on the nature of the Root/Leaf site association:
- Either Leaf or Root site(s) per PE
- Either Leaf or Root site(s) per Attachment Circuit (AC)
- Either Leaf or Root site(s) per MAC address
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2.1 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 the communications among Leaf PEs. To prevent the
communications 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 topology constrain in EVPN, two BGP
Route-Targets (RTs) are used for every EVPN Instance (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 Root site(s) imports both Root and Leaf RTs, whereas a PE with
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 the approach B in
scenario 2 (described below) needs to be used.
2.2 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).
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+---------+ +---------+
| PE1 | | PE2 |
+---+ | +---+ | +------+ | +---+ | +---+
|CE1+-----AC1----+--+ | | | | | | +--+---AC2--+CE2|
+---+ (Leaf) | |MAC| | | MPLS | | |MAC| | (Leaf) +---+
| |VRF| | | /IP | | |VRF| |
| | | | | | | | | | +---+
| | | | | | | | +--+---AC3--+CE3|
| +---+ | +------+ | +---+ | (Root) +---+
+---------+ +---------+
Figure 2: Scenario 2
In this scenario, just like the previous scenario (in section 2.1),
two Route Targets (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 and thus 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 apply the proper
ingress filtering.
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
Root or Leaf indication before advertisements 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)
B) To allow for a single RT 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 5.1.
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 [RFC7432] (section 6). 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 and thus 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
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such applications Leaf/Root RTs will be used to constrain MAC
advertisements and they 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 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.
2.3 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 and a PE may receive traffic from both Root AND Leaf
sites on a single Attachment Circuit (AC) of 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. This scenario is considered in this document for EVPN
service with only known unicast traffic because the Designated
Forwarding (DF) filtering per [RFC7432] would not be compatible with
the required egress filtering - i.e., Broadcast, Unknown, and
Multicast (BUM) traffic is not supported in this scenario and it is
dropped by the ingress PE.
For this scenario, the approach B in scenario 2 (described above) 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
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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.
3 Operation for EVPN
[RFC7432] defines the notion of 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 know unicast traffic, additional extensions to [RFC7432]
is needed (i.e., a new BGP Extended Community for Leaf indication
described in section 5.1) in order to enable ingress filtering as
described in detail in the following sections.
3.1 Known Unicast Traffic
Since in EVPN, MAC learning is performed in the control plane via
advertisement of BGP routes, the filtering needed by 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 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 flag indicates that the MAC address is associated with a Root
site. This scheme applies to all scenarios described in section 2.
Tagging MAC addresses with a Leaf indication enables remote PEs to
perform ingress filtering for known unicast traffic - i.e., on the
ingress PE, the MAC destination address lookup yields, in addition to
the forwarding adjacency, a flag which indicates whether the target
MAC is associated with a Leaf site or not. 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 situation where MAC moves are allowed among Leaf and Root sites
(e.g., non-static MAC), PEs can receive multiple MAC/IP
advertisements routes for the same MAC address with different
Leaf/Root indications (and possibly different ESIs for multi-homing
scenarios). In such situations, MAC mobility procedures (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.
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To support the above ingress filtering functionality, a new E-Tree
Extended Community with a Leaf indication flag is introduced [section
5.1]. This new Extended Community MUST be advertised with MAC/IP
Advertisement routes learned from a Leaf site. Besides MAC/IP
Advertisement route, no other EVPN routes are required to carry this
new extended community.
3.2 Broadcast, Unkonwn, and Multicast (BUM) Traffic
This specification does not provide support for filtering BUM
(Broadcast, Unknown, and Multicast) traffic on the ingress PE; due to
the multi-destination nature of BUM traffic, is 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 when they originated from a Leaf AC - i.e., to be
tagged with a Leaf label as specified in section 5.1. This Leaf label
allows for disposition PE (e.g., egress PE) to perform the necessary
egress filtering function in data-plane similar to ESI label in
[RFC7432]. The allocation of the Leaf label is on a per PE basis
(e.g., independent of ESI and EVI) as descried in the following
sections.
The Leaf label can be upstream assigned for P2MP LSP or downstream
assigned for ingress replication tunnels. The main difference between
downstream and upstream assigned Leaf label is that in case of
downstream assigned not all egress PE devices need to receive the
label in MPLS encapsulated BUM packets just like 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). Other mechanisms
for identifying Root or Leaf sites such as the use of source MAC
address of the receiving frame are optional. The scenarios below are
described in context of Root/Leaf AC; however, they can be extended
to Root/Leaf MAC address if needed.
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3.2.1 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 (Section 5.1) indicating 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 ES's on that PE. This Leaf label is
advertised to other PE devices, using the E-Tree Extended Community
(section 5.1) along with an Ethernet Auto-discovery per ES (EAD-ES)
route with ESI of zero and a set of Route Targets (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
Route Targets (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 single-homed sites.
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.
3.2.2 BUM Traffic Originated from a Single-homed Site on a Root AC
In this scenario, the ingress PE does not add any ESI label or Leaf
label and it operates per [RFC7432] procedures.
3.2.3 BUM Traffic Originated from a Multi-homed Site on a Leaf AC
In this scenario, it is assumed that while different ACs (VLANs) on
the same ES could have 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 - ie, the service
is EVPN L2 and not EVPN IRB [EVPN-IRB]. IRB use cases described in
[EVPN-IRB] 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 Leaf label described in
section 3.2.1. 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 Ethernet Segment.
3.2.4 BUM Traffic Originated from a Multi-homed Site on a Root AC
In this scenario, both the ingress and egress PE devices follows the
procedure defined in [RFC7432] for adding and/or processing an ESI
MPLS label - i.e., existing procedures for BUM traffic in [RFC7432]
are sufficient and there is no need to add a Leaf label.
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3.3 E-Tree Traffic Flows for EVPN
Per [RFC7387], a generic E-Tree service supports all of the following
traffic flows:
- Known unicast traffic from Root to Roots & Leaf
- Known unicast traffic from Leaf to Root
- 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
learning function.
The following subsections will describe the operation of EVPN to
support E-Tree service with and without MAC learning.
3.3.1 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 Ethernet Segments, 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 Ethernet Segments, and advertise
reachability in EVPN MAC/IP Advertisement Routes. For scenarios where
two different RTs are used per EVI (one to designate Root site and
another to designate 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 multi-homed 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 (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 describes
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when each of these tunneling schemes can be used and how to signal
them.
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
transmit direction from itself to Leaf PEs and at the same time it
wants to support ingress-replication tunnels in receive direction,
the Root PE can signal it efficiently by using a new composite tunnel
type defined in section 5.2. This new composite tunnel type is
advertised by the Root PE to simultaneously indicate a P2MP tunnel in
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., mLDP or RSVP-
TE) originated at the Leaf PEs may be used and thus there will be no
need to use the modified PMSI tunnel attribute and the composite
tunnel type values defined in section 5.2.
3.3.2 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 5.2 should be used.
4 Operation for PBB-EVPN
In PBB-EVPN, the PE advertises a Root/Leaf indication along with each
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 [5.1] is
advertised with each EVPN MAC/IP Advertisement route.
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In the case where a multi-homed Ethernet Segment 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 denoting
Root, and the other B-MAC address is per PE and explicitly denoting
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 "Leaf-indication" flag
is set. In multi-homing scenarios where an Ethernet Segment has both
Root and Leaf ACs, it is assumed that while different ACs (VLANs) on
the same ES could have 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 - ie, the service
is L2 and not IRB. 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 Ethernet Segment. 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 this document.
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 multi-homed segments.
4.1 Known Unicast Traffic
For known unicast traffic, the PEs perform ingress filtering: On the
ingress PE, the C-MAC [RFC7623] destination address lookup yields, in
addition to the target B-MAC address and forwarding adjacency, a flag
which 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, and if both are a Leaf, then the packet is not forwarded.
4.2 Broadcast, Unkonwn, and Multicast (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:
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- If the EVPN MAC/IP Advertisement route indicates that the
advertised B-MAC is a Leaf, and the local Ethernet Segment 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 Ethernet Segment is a Leaf, then the source B-MAC
address is removed from the B-MAC list corresponding to the local
Ethernet Segment used for egress filtering - i.e., to unblock traffic
from that B-MAC address.
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 baseline [RFC7623] for an
ESI and does not require any additional flags in the data-plane.
Just as in section 3.2, the PE places all Leaf Ethernet Segments 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.
4.3 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 4.2 without the need for C-MAC learning (in data-plane) in I-
component (C-bridge table) of PBB-EVPN PEs (at both ingress and
egress PEs).
5 BGP Encoding
This document defines a new BGP Extended Community for EVPN.
5.1 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.
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The E-Tree Extended Community is encoded as an 8-octet value as
follows:
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 flag bits are
reserved and should be set to zero.
When this Extended Community (EC) is advertised along with MAC/IP
Advertisement route (for known unicast traffic) per section 3.1, the
Leaf-Indication flag MUST be set to one and Leaf Label SHOULD be set
to zero. The receiving PE MUST ignore Leaf Label and only processes
Leaf-Indication flag. A value of zero for Leaf-Indication flag is
invalid when sent along with MAC/IP advertisement route and an error
should be logged.
When this EC is advertised along with EAD-ES route (with ESI of zero)
for BUM traffic to enable egress filtering on disposition PEs per
sections 3.2.1 and 3.2.3, the Leaf Label MUST be set to a valid MPLS
label (i.e., 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 error.
The reserved bits SHOULD be set to zero by the transmitter and MUST
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be ignored by the receiver.
5.2 PMSI Tunnel Attribute
[RFC6514] defines 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) |
+---------------------------------+
Figure 5: 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 a MPLS
label to the Tunnel Identifier field of 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
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 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, and a
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PE that receives a PMSI Tunnel attribute with such information,
considers it as malformed and it SHOULD treat this Update as though
all the routes contained in this Update had been withdrawn per
section 5 of [RFC6514].
6 Acknowledgement
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
valuable comments.
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 "Root" or "Leaf" by the network
operator/ service provider and thus preventing 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 as 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
the AC.
8 IANA Considerations
IANA has allocated value 5 in the "EVPN Extended Community Sub-Types"
registry defined in [RFC7153] as follow:
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:
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bit Name Reference
0-6 Unassigned
7 Leaf-Indication This document
8.1 Considerations for PMSI Tunnel Types
The "P-Multicast Service Interface Tunnel (PMSI Tunnel) Tunnel Types"
registry in the "Border Gateway Protocol (BGP) Parameters" registry
needs to be updated to reflect the use of the most significant bit as
"Composite Tunnel" bit (section 5.2).
For this purpose, this document updates [RFC7385] by changing the
previously unassigned values (i.e., 0x08 - 0xFA) as follow:
Value Meaning Reference
0x08-0x7A Unassigned
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 value in these ranges are not to be assigned. The value
0x7F which is the mirror image of (0xFF) is reserved in this
document.
9 References
9.1 Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC8126] Cotton et al, "Guidelines for Writing an IANA
Considerations Section in RFCs", June, 2017.
[RFC7387] Key et al., "A Framework for E-Tree Service over MPLS
Network", October 2014.
[MEF6.1] Metro Ethernet Forum, "Ethernet Services Definitions - Phase
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2", MEF 6.1, April 2008, https://mef.net/PDF_Documents/technical-
specifications/MEF6-1.pdf
[RFC7432] Sajassi et al., "BGP MPLS Based Ethernet VPN", February,
2015.
[RFC7623] Sajassi et al., "Provider Backbone Bridging Combined with
Ethernet VPN (PBB-EVPN)", September, 2015.
[RFC7385] Andersson et al., "IANA Registry for P-Multicast Service
Interface (PMSI) Tunnel Type Code Points", October, 2014.
[RFC7153] Rosen et al., "IANA Registries for BGP Extended
Communities", March, 2014.
[RFC6514] Aggarwal et al., "BGP Encodings and Procedures for
Multicast in MPLS/BGP IP VPNs", February, 2012.
[RFC4360] Sangli et al., "BGP Extended Communities Attribute",
February, 2006.
9.2 Informative References
[RFC4360] S. Sangli et al, "BGP Extended Communities Attribute",
February, 2006.
[RFC3032] E. Rosen et al, "MPLS Label Stack Encoding", January 2001.
[RFC7796] Y. Jiang et al, "Ethernet-Tree (E-Tree) Support in Virtual
Private LAN Service (VPLS)", March 2016.
[EVPN-IRB] A. Sajassi et al, "Integrated Routing and Bridging in
EVPN", draft-ietf-bess-evpn-inter-subnet-forwarding-03, February 8,
2017.
[802.1ah] IEEE, "IEEE Standard for Local and metropolitan area
networks - Media Access Control (MAC) Bridges and Virtual Bridged
Local Area Networks", Clauses 25 and 26, IEEE Std 802.1Q, DOI
10.1109/IEEESTD.2011.6009146.
Appendix-A
When two MAC-VRFs (two bridge tables per VLANs) 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 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 either require two lookups
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be performed per MAC address in each direction in case of a miss, or
duplicating many MAC addresses between the two bridge tables
belonging to the same VLAN (same E-Tree instance). 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 Root bridge
table and locally learned MAC addresses from Root ACs need to be
duplicated onto 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
dataplane performance inefficiencies in some platforms and thus this
document introduces the coloring as described in section 2.2 and
detailed in section 3.1.
Authors' Addresses
Ali Sajassi
Cisco
Email: sajassi@cisco.com
Samer Salam
Cisco
Email: ssalam@cisco.com
John Drake
Juniper
Email: jdrake@juniper.net
Jim Uttaro
AT&T
Email: ju1738@att.com
Sami Boutros
VMware
Email: sboutros@vmware.com
Jorge Rabadan
Nokia
Email: jorge.rabadan@nokia.com
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