Internet Engineering Task Force (IETF) J. Rabadan, Ed.
Request for Comments: 9856 J. Kotalwar
Category: Standards Track S. Sathappan
ISSN: 2070-1721 Nokia
Z. Zhang
W. Lin
Juniper
August 2025
Multicast Source Redundancy in EVPNs
Abstract
In Ethernet Virtual Private Networks (EVPNs), IP multicast traffic
replication and delivery play a crucial role in enabling efficient
and scalable Layer 2 and Layer 3 services. A common deployment
scenario involves redundant multicast sources that ensure high
availability and resiliency. However, the presence of redundant
sources can lead to duplicate IP multicast traffic in the network,
causing inefficiencies and increased overhead. This document
specifies extensions to the EVPN multicast procedures that allow for
the suppression of duplicate IP multicast traffic from redundant
sources. The proposed mechanisms enhance the EVPN's capability to
deliver multicast traffic efficiently while maintaining high
availability. These extensions are applicable to various EVPN
deployment scenarios and provide guidelines to ensure consistent and
predictable behavior across diverse network topologies.
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
https://www.rfc-editor.org/info/rfc9856.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction
1.1. Terminology
1.2. Background on IP Multicast Delivery in EVPN Networks
1.2.1. Intra-Subnet IP Multicast Forwarding
1.2.2. Inter-Subnet IP Multicast Forwarding
1.3. Multi-Homed IP Multicast Sources in EVPN
1.4. The Need for Redundant IP Multicast Sources in EVPN
2. Solution Overview
2.1. Warm Standby Solution
2.2. Hot Standby Solution
2.3. Solution Selection
2.4. System Support
3. BGP EVPN Extensions
3.1. Single Flow Group Flag in the Multicast Flags Extended
Community
3.2. ESI Label Extended Community in S-PMSI A-D Routes
4. Warm Standby (WS) Solution for Redundant G-Sources
4.1. Specification
4.2. Warm Standby Example in an OISM Network
4.3. Warm Standby Example in a Single-BD Tenant Network
5. Hot Standby Solution for Redundant G-Sources
5.1. Specification
5.2. Extensions for the Advertisement of DCB Labels
5.3. Use of BFD in the HS Solution
5.4. Hot Standby Example in an OISM Network
5.4.1. Multi-Homed Redundant G-Sources
5.4.2. Single-Homed Redundant G-Sources
5.5. Hot Standby Example in a Single-BD Tenant Network
6. Security Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
Ethernet Virtual Private Networks (EVPNs) [RFC7432] support both
intra-subnet and inter-subnet IP multicast forwarding. [RFC9251]
outlines the procedures required to optimize the delivery of IP
multicast flows when both sources and receivers are connected to the
same EVPN Broadcast Domain. [RFC9625], on the other hand, defines
the procedures for supporting inter-subnet IP multicast within a
tenant network, where the IP multicast source and receivers of the
same multicast flow are connected to different Broadcast Domains
within the same tenant.
However, [RFC9251], [RFC9625], and conventional IP multicast
techniques do not provide a solution for scenarios where:
1. A given multicast group carries multiple flows (i.e., multiple
sources are active), and
2. Each receiver should receive only from one source.
Existing multicast solutions typically assume that there are no
redundant sources sending identical flows to the same IP multicast
group. In cases where redundant sources do exist, the receiver
application is expected to handle duplicate packets.
In conventional IP multicast networks, such as those running Protocol
Independent Multicasts (PIMs) Multicast (PIM) [RFC7761] or Multicast Virtual Private
Networks (MVPNs)
Network (MVPN) [RFC6513], a workaround is to configure all redundant
sources with the same IP address. This approach ensures that each
receiver gets only one flow because:
* The Rendezvous Point (RP) in the multicast network always creates
the (S,G) state for each source.
* The Last Hop Router (LHR) may also create the (S,G) state.
* The (S,G) state binds the flow to a source-specific tree rooted at
the source IP address. When multiple sources share the same IP
address, the resulting source-specific trees ensure that each LHR
or RP resides on at most one tree.
This workaround, which often uses anycast addresses, is suitable for
Warm Standby (WS) redundancy solutions (Section 4). However, it is
not effective for Hot Standby (HS) redundancy scenarios (Section 5),
and it introduces challenges when sources need to be reachable via IP
unicast or when multiple sources with the same IP address are
attached to the same Broadcast Domain. In scenarios where multiple
multicast sources stream traffic to the same group using EVPN
Optimized Inter-Subnet Multicast (OISM), there is not necessarily any
(S,G) state created for the redundant sources. In such cases, the
Last Hop Routers may only have a (*,G) state, and there may not be an
RP router to create an (S,G) state.
This document extends [RFC9251] and [RFC9625] to address scenarios
where IP multicast source redundancy exists. Specifically, it
defines procedures for EVPN Provider Edge (PE) devices/routers to
ensure that receivers do not experience packet duplication when two
or more sources send identical IP multicast flows into the tenant
domain. These procedures are limited to the context of [RFC9251] and
[RFC9625]; handling redundant sources in other multicast solutions is
beyond the scope of this document.
1.1. Terminology
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.
BD: Broadcast Domain. An emulated Ethernet, such that two systems
on the same BD will receive each other's link-local broadcasts.
In this document, "BD" also refers to the instantiation of a
Broadcast Domain on an EVPN PE. An EVPN PE can be attached to one
or multiple BDs of the same tenant.
BUM: Broadcast, Unknown Unicast, and Multicast traffic.
DF: Designated Forwarder. As defined in [RFC7432], an Ethernet
Segment (ES) may be multi-homed (attached to more than one PE).
An
Ethernet Segment ES may also contain multiple BDs of one or more EVIs. For each
such EVI, one of the PEs attached to the segment becomes that
EVI's DF for that segment. Since a BD may belong to only one EVI,
we can speak unambiguously of the BD's DF for a given segment.
Downstream PE: In this document, a Downstream downstream PE is referred to as
the EVPN PE that is connected to the IP Multicast receivers and
gets the IP Multicast flows from remote EVPN PEs.
EVI: EVPN Instance.
G-traffic: Any frame with an IP payload whose destination IP Destination Address address
is a multicast group G.
G-source: Any system sourcing IP multicast traffic to group G.
Hot Standby Redundancy: The multicast source redundancy procedure
defined in this document, by which the upstream PEs forward the
redundant multicast flows to the downstream PEs, and the
downstream PEs make sure only one flow is forwarded to the
interested attached receivers.
IGMP: Internet Group Management Protocol [RFC3376].
I-PMSI: Inclusive Multicast Tree or Inclusive Provider Multicast
Service Interface. While defined in [RFC6513], in this document
it is only applicable to EVPN and refers to the default multicast
tree for a given BD. All the EVPN PEs that are attached to a
specific BD belong to the I-PMSI for the BD. The I-PMSI trees are
signaled by EVPN Inclusive Multicast Ethernet Tag (IMET) routes.
IMET route: EVPN Inclusive Multicast Ethernet Tag route, as in
[RFC7432].
MLD: Multicast Listener Discovery [RFC3810].
MVPN: Multicast Virtual Private Networks, as in [RFC6513].
OISM: Optimized Inter-Subnet Multicast, as in [RFC9625].
PE: Provider Edge.
PIM: Protocol Independent Multicast, as in [RFC7761].
P-tunnel: The term "Provider tunnel" refers to the type of tree
employed by an upstream EVPN PE to forward multicast traffic to
downstream PEs. The P-tunnels supported in this document include
Ingress Replication (IR), Assisted Replication (AR) [RFC9574], Bit
Indexed Explicit Replication (BIER) [RFC8296], multicast Label
Distribution Protocol (mLDP), and Point-to-Multipoint (P2MP) RSVP
- Traffic Engineering (RSVP-TE) extensions.
Redundant G-source: A host or router transmitting a Single Flow
Group (SFG) within a tenant network, where multiple hosts or
routers are also transmitting the same SFG. Redundant G-sources
transmitting the same SFG should have distinct IP addresses;
however, they may share the same IP address if located in
different Broadcast Domains (BDs) within the same tenant network.
For the purposes of this document, redundant G-sources are assumed
to not exhibit "bursty" traffic behavior.
S-ES and S-ESI: Multicast Source Ethernet Segment and multicast
Source Ethernet Segment Identifier. The Ethernet Segment ES and
Ethernet Segment Identifier ESI associated to
a G-source.
S-PMSI: Selective Multicast Tree or Selective Provider Multicast
Service Interface. As defined in [RFC6513], this term refers to a
multicast tree to which only the PEs interested in a specific
Broadcast Domain belong. In the context of this document, it is
specific to EVPN. Two types of EVPN S-PMSIs are supported:
S-PMSIs with Auto-Discovery routes: These S-PMSIs require the
upstream PE to advertise S-PMSI Auto-Discovery (S-PMSI A-D)
routes, as described in [RFC9572]. Downstream PEs interested
in the multicast traffic join the S-PMSI tree following the
procedures specified in [RFC9572].
S-PMSIs without Auto-Discovery Routes: These S-PMSIs do not
require the advertisement of S-PMSI A-D routes. Instead, they
rely on the forwarding information provided by Inclusive
Multicast Ethernet Tag (IMET) IMET routes.
Upstream PEs forward IP multicast flows only to downstream PEs
that advertise Selective Multicast Ethernet Tag (SMET) routes
for the specific flow. These S-PMSIs are supported exclusively
with the following P-tunnel types: Ingress Replication (IR), Assisted Replication
(AR), IR, AR, and Bit Indexed Explicit Replication (BIER). BIER.
SFG: Single Flow Group. A multicast group that represents traffic
containing a single flow. Multiple sources, which may have the
same or different IP addresses, can transmit traffic for an SFG.
An SFG can be represented in two forms:
(*,G): Indicates that any source transmitting multicast traffic
to group G is considered a redundant G-source for the SFG.
(S,G): Indicates that S is a prefix of any length. In this
representation, a source is deemed a redundant G-source for the
SFG if its address matches the specified prefix S.
SMET route: Selective Multicast Ethernet Tag route, as in [RFC9251].
(S,G) and (*,G): Used to describe multicast packets or multicast
state. "S" stands for Source (IP address of the multicast
traffic), and "G" stands for the Group or multicast destination IP
address of the group. An (S,G) multicast packet refers to an IP
packet with source IP address "S" and destination IP address "G",
and it is forwarded on a multicast router if there is a
corresponding state for (S,G). A (*,G) multicast packet refers to
an IP packet with "any" source IP address and a destination IP
address "G", and it is forwarded on a multicast router based on
the existence of the corresponding (*,G) state. The document uses
variations of these terms. For example, (S1,G1) represents the
multicast packets or multicast state for source IP address "S1"
and group IP address "G1".
Upstream PE: In this document, an Upstream upstream PE refers to either the
EVPN PE that is directly connected to the IP multicast source or
the PE closest to the source. The Upstream upstream PE receives IP
multicast flows through local Attachment Circuits (ACs).
Warm Standby Redundancy: A multicast source redundancy mechanism
defined in this document, wherein the upstream PEs connected to
redundant sources within the same tenant ensure that only one
source of a given flow transmits multicast traffic to the
interested downstream PEs at any given time.
This document also assumes familiarity with the terminology of
[RFC7432], [RFC4364], [RFC6513], [RFC6514], [RFC9251], [RFC9625],
[RFC9136], and [RFC9572].
1.2. Background on IP Multicast Delivery in EVPN Networks
IP multicast facilitates the delivery of a single copy of a packet
from a source (S) to a group of receivers (G) along a multicast tree.
In an EVPN tenant domain, the multicast tree can be constructed where
the source (S) and the receivers for the multicast group (G) are
either connected to the same Broadcast Domain (BD) or to different
Broadcast Domains. The former scenario is referred to as "Intra-
subnet IP Multicast forwarding", while the latter is referred to as
"Inter-subnet IP Multicast forwarding".
1.2.1. Intra-Subnet IP Multicast Forwarding
When the source S1 and the receivers interested in G1 are connected
to the same Broadcast Domain, the EVPN network can deliver IP
multicast traffic to the receivers using two different approaches, as
illustrated in Figure 1:
S1 + S1 +
(a) + | (b) + |
| | (S1,G1) | | (S1,G1)
PE1 | | PE1 | |
+-----+ v +-----+ v
|+---+| |+---+|
||BD1|| ||BD1||
|+---+| |+---+|
+-----+ +-----+
+-------|-------+ +-------|
| | | | |
v v v v v
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|+---+| |-----| |-----| |+---+| |+---+| |+---+|
||BD1|| ||BD1|| ||BD1|| ||BD1|| ||BD1|| ||BD1||
|+---+| |-----| |-----| |+---+| |+---+| |+---+|
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
PE2| PE3| PE4| PE2| PE3| PE4
- | - - - | - | - | - - - | -
| | | | | | | | |
v v v v v
| R1 R2 | R3 | R1 R2 | R3
- - - G1- - - - - - G1- - -
Figure 1: Intra-Subnet IP Multicast
Model (a): IP Multicast Delivery as BUM Traffic
The upstream PE sends the IP Multicast flows to all downstream
PEs, even to PEs with non-interested receivers, such as, e.g., PE4
in Figure 1. To optimize this behavior, downstream PEs can snoop
IGMP/MLD messages from receivers to build Layer 2 multicast state.
For instance, PE4 could avoid forwarding (S1,G1) to R3, since R3
has not expressed interest in (S1,G1).
Model (b): Optimized Delivery with S-PMSI
Model (b) in Figure 1 uses a "Selective Provider Multicast Service
Interface (S-PMSI)" to optimize the delivery of the (S1,G1) flow.
* For example, if PE1 uses "Ingress Replication (IR)", "IR", it will forward (S1,G1) only to
downstream PEs that have issued a
"Selective Multicast Ethernet Tag (SMET)" "SMET" route for (S1,G1),
such as PE2 and PE3.
* If PE1 uses a P-tunnel type other than IR (e.g., Assisted
Replication (AR) AR or Bit Indexed Explicit Replication (BIER)), BIER),
PE1 will advertise an "S-PMSI Auto-Discovery (A-D)" route for
(S1,G1). Downstream PEs, such as PE2 and PE3, will then join
the corresponding multicast tree to receive the flow.
Procedures for Model (b) are specified in [RFC9251].
1.2.2. Inter-Subnet IP Multicast Forwarding
When the sources and receivers are connected to different BDs within
the same tenant domain, the EVPN network can deliver IP multicast
traffic using either Inclusive or Selective Multicast Trees, as
illustrated in Figure 2 with models (a) and (b), respectively.
S1 + S1 +
(a) + | (b) + |
| | (S1,G1) | | (S1,G1)
PE1 | | PE1 | |
+-----+ v +-----+ v
|+---+| |+---+|
||BD1|| ||BD1||
|+---+| |+---+|
+-----+ +-----+
+-------|-------+ +-------|
| | | | |
v v v v v
+-----+ +-----+ +-----+ +-----+ +-----+ +-----+
|+---+| |+---+| |+---+| |+---+| |+---+| |+---+|
||SBD|| ||SBD|| ||SBD|| ||SBD|| ||SBD|| ||SBD||
|+-|-+| |+-|-+| |+---+| |+-|-+| |+-|-+| |+---+|
| VRF | | VRF | | VRF | | VRF | | VRF | | VRF |
|+-v-+| |+-v-+| |+---+| |+-v-+| |+-v-+| |+---+|
||BD2|| ||BD3|| ||BD4|| ||BD2|| ||BD3|| ||BD4||
|+-|-+| |+-|-+| |+---+| |+-|-+| |+-|-+| |+---+|
+--|--+ +--|--+ +-----+ +--|--+ +--|--+ +-----+
PE2| PE3| PE4 PE2| PE3| PE4
- | - - - | - - | - - - | -
| | | | | | | |
v v v v
| R1 R2 | R3 | R1 R2 | R3
- - - G1- - - - - - G1- - -
Figure 2: Inter-Subnet IP Multicast
As defined in [RFC9625], inter-subnet multicast forwarding in EVPN is
optimized by ensuring IP multicast flows are sent within the context
of the source BD. If a downstream PE is not connected to the source
BD, the IP multicast flow is delivered to the Supplementary Broadcast
Domain (SBD), as shown in Figure 2.
* Inclusive and Selective Multicast Trees
Model (a): Inclusive Multicast Tree
In this model, the Inclusive Multicast Tree for BD1 on PE1
delivers (S1,G1) to all downstream PEs, such as PE2, PE3, and
PE4, in the context of the SBD. Each downstream PE then
locally routes the flow to its Attachment Circuits, ACs, ensuring delivery to
interested receivers.
Model (b): Selective Multicast Tree
In this model, PE1 optimizes forwarding by delivering (S1,G1)
only to downstream PEs that explicitly indicate interest in the
flow via Selective Multicast Ethernet Tag (SMET) SMET routes. If the P-tunnel type is "Ingress Replication (IR)", "Assisted
Replication (AR)", "IR", "AR", or "Bit Indexed Explicit Replication
(BIER)",
"BIER", PE1 does not need to advertise an S-PMSI A-D route.
Downstream PEs join the multicast tree based on the SMET routes
advertised for (S1,G1).
* Advantages of [RFC9625]
[RFC9625] extends the procedures defined in [RFC9251] to support
both intra- and inter-subnet multicast forwarding for EVPN. It
ensures that every upstream PE attached to a source is aware of
all downstream PEs within the same tenant domain that have
interest in specific flows. This is achieved through the
advertisement of SMET routes with the SBD Route Target, which are
imported by all upstream PEs.
* Elimination of Complexity
The inter-subnet multicast mechanism defined in [RFC9625]
eliminates the need for: Rendezvous Points (RPs), Shared shared trees,
Upstream Multicast Hop
upstream multicast hop selection, or other complex conventional
multicast routing techniques.
By leveraging the EVPN framework, inter-subnet multicast forwarding
achieves efficient delivery without introducing unnecessary overhead
or dependencies on classic IP multicast protocols.
1.3. Multi-Homed IP Multicast Sources in EVPN
Unlike conventional multicast routing technologies, multi-homed PEs
connected to the same source do not create IP multicast packet
duplication when utilizing a multi-homed Ethernet Segment. ES. Figure 3 illustrates
this scenario, where two multi-homed PEs (PE1 and PE2) are attached
to the same source S1. The source S1 is connected via a Layer 2
switch (SW1) to an all-active Ethernet Segment ES (ES-1), with a Link Aggregation
Group (LAG) extending to PE1 and PE2.
S1
|
v
+-----+
| SW1 |
+-----+
+---- | |
(S1,G1)| +----+ +----+
IGMP/MLD | | all-active |
J(S1,G1) PE1 v | ES-1 | PE2
+----> +-----------|---+ +---|-----------+
| +---+ +---+ | | +---+ |
R1 <-----|BD2| |BD1| | | |BD1| |
| +---+---+---+ | | +---+---+ |
+----| |VRF| | | | |VRF| |----+
| | +---+---+ | | | +---+---+ | |
| | |SBD| | | | |SBD| | |
| | +---+ | | | +---+ | |
| +------------|--+ +---------------+ |
| | |
| | |
| | |
| EVPN | ^ |
| OISM v PE3 | SMET |
| +---------------+ | (*,G1) |
| | +---+ | | |
| | |SBD| | |
| | +---+---+ | |
+--------------| |VRF| |----------------+
| +---+---+---+ |
| |BD2| |BD3| |
| +-|-+ +-|-+ |
+---|-------|---+
^ | | ^
IGMP/MLD | v v | IGMP/MLD
J(*,G1) | R2 R3 | J(S1,G1)
Figure 3: All-Active Multi-Homing and OISM
When S1 transmits the (S1,G1) flow, SW1 selects a single link within
the all-active Ethernet Segment ES to forward the flow, as per [RFC7432]. In this
example, assuming PE1 is the receiving PE for BD1, the multicast flow
is forwarded once BD1 establishes multicast state for (S1,G1) or
(*,G1). In Figure 3:
* Receiver R1 receives (S1,G1) directly via the IRB (Integrated
Routing and Bridging) interface, defined in [RFC9135], following
the procedures in [RFC9625].
* Receivers R2 and R3, upon issuing IGMP/MLD reports, trigger PE3 to
advertise an SMET (*,G1) route. This creates multicast state in
PE1's BD1, enabling PE1 to forward the multicast flow to PE3's
SBD. PE3 subsequently delivers the flow to R2 and R3 as defined
in [RFC9625].
Requirements for Multi-Homed IP Multicast Sources:
* When IP multicast source multi-homing is needed, EVPN multi-homed
Ethernet Segments
ESes MUST be used.
* EVPN multi-homing ensures that only one upstream PE forwards a
given multicast flow at a time, preventing packet duplication at
downstream PEs.
* The SMET route for a multicast flow ensures that all upstream PEs
in the multi-homed Ethernet Segment ES maintain state for the flow. This allows
for immediate failover, as the backup PE can seamlessly take over
forwarding in case of an upstream PE failure.
This document assumes that multi-homed PEs connected to the same
source always utilize multi-homed Ethernet Segments. ESes.
1.4. The Need for Redundant IP Multicast Sources in EVPN
While multi-homing PEs to the same IP multicast G-source provides a
certain level of resiliency, multicast applications are often
critical in operator networks, necessitating a higher level of
redundancy. This document assumes the following:
a. Redundant G-sources: Redundant G-sources for an SFG may exist
within the EVPN tenant network. A redundant G-source is defined
as a host or router transmitting an SFG stream in a tenant
network where another host or router is also sending traffic to
the same SFG.
b. G-source placement: Redundant G-sources may reside in the same BD
or in different BDs of the tenant network. There must be no
restrictions on the locations of receiver systems within the
tenant.
c. G-source attachment to EVPN PEs: Redundant G-sources may be
either single-homed to a single EVPN PE or multi-homed to
multiple EVPN PEs.
d. Packet duplication avoidance: The EVPN PEs must ensure that
receiver systems do not experience duplicate packets for the same
SFG.
This framework ensures that EVPN networks can effectively support
redundant multicast sources while maintaining high reliability and
operational efficiency.
2. Solution Overview
An SFG can be represented as (*,G) if any source transmitting
multicast traffic to group G is considered a redundant G-source.
Alternatively, this document allows an SFG to be represented as
(S,G), where the source IP address S is a prefix of variable length.
In this case, a source is deemed a redundant G-source for the SFG if
its address falls within the specified prefix. The use of variable-
length prefixes in source advertisements via S-PMSI A-D routes is
permitted in this document only for the specific application of
redundant G-sources.
This document describes two solutions for handling redundant
G-sources:
* Warm Standby Solution
* Hot Standby Solution
2.1. Warm Standby Solution
The Warm Standby solution is an upstream PE-based solution, where
downstream PEs do not participate in the procedures. In this
solution, all upstream PEs connected to redundant G-sources for an
SFG (*,G) or (S,G) elect a "Single Forwarder (SF)" among themselves.
After the Single Forwarder is elected, the upstream PEs apply Reverse
Path Forwarding checks to the multicast state for the SFG:
* Non-Single Forwarder Behavior: A non-Single Forwarder upstream PE
discards all (*,G) or (S,G) packets received over its local
Attachment Circuit. AC.
* Single Forwarder Behavior: The Single Forwarder accepts and
forwards (*,G) or (S,G) packets received on a single local
Attachment Circuit AC for
the SFG. If packets are received on multiple local Attachment Circuits, ACs, the
Single Forwarder discards packets on all but one. The selection
of the Attachment Circuit AC for forwarding is a local implementation detail.
In the event of a failure of the Single Forwarder, a new Single
Forwarder is elected among the upstream PEs. This election process
requires BGP extensions on existing EVPN routes, which are detailed
in Sections 3 and 4.
2.2. Hot Standby Solution
The Hot Standby solution relies on downstream PEs to prevent
duplication of SFG packets. Upstream PEs, aware of locally connected
G-sources, append a unique Ethernet Segment Identifier (ESI) ESI label to multicast packets for each
SFG. Downstream PEs receive SFG packets from all upstream PEs
attached to redundant G-sources and avoid duplication by performing a
Reverse Path Forwarding check on the (*,G) state for the SFG:
* Packet Filtering: A downstream PE discards (*,G) packets received
from the "wrong G-source."
* Wrong G-source Identification: The "wrong G-source" is identified
using an ESI label that differs from the ESI label associated with
the selected G-source.
* ESI Label Usage: In this solution, the ESI label is used for
"ingress filtering" at the downstream PE, rather than for "egress
filtering" as described in [RFC7432]. In [RFC7432], the ESI label
indicates which egress Attachment Circuits ACs must be excluded when forwarding BUM
traffic. Here, the ESI label identifies ingress traffic that
should be discarded by the downstream PE.
Control plane and data plane extensions to [RFC7432] are required to
support ESI labels for SFGs forwarded by upstream PEs. Upon failure
of the selected G-source, the downstream PE switches to a different
G-source and updates its Reverse Path Forwarding check for the (*,G)
state. These extensions and procedures are described in Sections 3
and 5.
2.3. Solution Selection
Operators should select a solution based on their specific
requirements:
* The Warm Standby solution is more bandwidth-efficient but incurs
longer failover times in the event of a G-source or upstream PE
failure. Additionally, only the upstream PEs connected to
redundant G-sources for the same SFG need to support the new
procedures in the Warm Standby solution.
* The Hot Standby solution is recommended for scenarios requiring
fast failover times, provided that the additional bandwidth
consumption (due to multiple transmissions of SFG packets to
downstream PEs) is acceptable.
2.4. System Support
This document does not mandate support for both solutions on a single
system. If one solution is implemented, support for the other is
OPTIONAL.
3. BGP EVPN Extensions
This document introduces the following BGP EVPN extensions:
3.1. Single Flow Group Flag in the Multicast Flags Extended Community
A new Single Flow Group (SFG) flag is defined within the Multicast
Flags Extended Community. This flag has been registered as bit 4 in
the "Multicast Flags Extended Community" registry (see Table 1). The
SFG flag is set in S-PMSI A-D routes that carry (*,G) or (S,G) Single
Flow Group information in the BGP EVPN Network Layer Reachability
Information (NLRI).
3.2. ESI Label Extended Community in S-PMSI A-D Routes
The Hot Standby solution requires the advertisement of one or more
ESI Label Extended Communities [RFC7432] alongside the S-PMSI A-D
routes. These extended communities encode the ESI values associated
with an S-PMSI A-D (*,G) or (S,G) route that advertises the presence
of a Single Flow Group.
Key considerations include:
1. When advertised with the S-PMSI A-D routes, only the ESI Label label
value in the extended community is relevant to the procedures
defined in this document.
2. The Flags field within the extended community MUST be set to
"0x00" on transmission and MUST be ignored on reception.
[RFC7432] specifies the use of the ESI Label Extended Community in
conjunction with the A-D per ES route. This document extends the
applicability of the ESI Label Extended Community by allowing its
inclusion multiple times (with different ESI Label label values) alongside
the EVPN S-PMSI A-D route. These extensions enable the precise
encoding and advertisement of SFG-related information, facilitating
efficient multicast traffic handling in EVPN networks.
4. Warm Standby (WS) Solution for Redundant G-Sources
This section specifies the Warm Standby solution for handling
redundant multicast sources (G-sources). Note that while the
examples use IPv4 addresses, the solution supports both IPv4 and IPv6
sources.
4.1. Specification
The Warm Standby solution follows these general procedures:
1. Configuration of the upstream PEs
Upstream PEs, potentially connected to redundant G-sources, are
configured to recognize:
* The multicast groups that carry an SFG in the tenant domain.
* The local Broadcast Domains that may host redundant G-sources
The SFG configuration applies to either "any" source, i.e., (*)
or to a specific "source prefix" (e.g., "192.0.2.0/30" or
"2001:db8::/120"). For instance, if the IPv4 prefix is
192.0.2.0/30, the sources 192.0.2.1 and 192.0.2.2 are considered
redundant G-sources for the SFG, while 192.0.2.10 is not. In a
different example for IPv6, if the prefix is 2001:db8::/120,
sources 2001:db8::1 or 2001:db8::2 are considered redundant
G-sources for the SFG, but 2001:db8:1::1 is not.
Example Configuration (Figure 4):
* PE1 is configured to recognize G1 as an SFG for any source,
with potential redundant G-sources attached to BD1 and BD2.
* Alternatively, PE1 may recognize G1 as an SFG for sources
within 192.0.2.0/30 (or 2001:db8::/120), with redundant
G-sources in BD1 and BD2.
2. Signaling the location of a G-source for an SFG
Upon receiving the first IP multicast packet for a configured SFG
on a Broadcast Domain, an upstream PE (e.g., PE1):
* MUST advertise an S-PMSI A-D route for the SFG:
- An (*,G) route if the SFG is configured for any source.
- An (S,G) route (where S can have any prefix length) if the
SFG is configured for a source prefix.
* MUST include the following attributes in the S-PMSI A-D route:
- Route Targets (RTs): The Supplementary Broadcast Domain
Route Target (SBD-RT), if applicable, and the Broadcast
Domain Route Target (BD-RT) of the Broadcast Domain
receiving the traffic. The SBD-RT is needed so that the
route is imported by all PEs attached to the tenant domain
in an OISM solution.
- Multicast Flags Extended Community: MUST include the SFG
flag to indicate that the route conveys an SFG.
- Designated Forwarder Election Extended Community: Specifies
the algorithm and preferences for the Single Forwarder
election, using the Designated Forwarder DF election defined in [RFC8584].
* Advertises the route:
- Without a PMSI Tunnel Attribute if using Ingress
Replication (IR), Assisted Replication (AR), IR, AR, or Bit Indexed
Explicit Replication (BIER). BIER.
- With a PMSI Tunnel Attribute for other tunnel types.
* MUST withdraw the S-PMSI A-D route when the SFG traffic
ceases. A timer is RECOMMENDED to detect inactivity and
trigger route withdrawal.
3. Single Forwarder Election on the upstream PEs
If an upstream PE receives one or more S-PMSI A-D routes for the
same SFG from remote PEs, it performs Single Forwarder Election
based on the Designated Forwarder Election Extended Community.
* Two routes are considered part of the same SFG if they are
advertised for the same tenant and match in the following
fields:
- Multicast Source Length
- Multicast Source
- Multicast Group Length
- Multicast Group
* Election Rules:
1. A consistent DF Election Algorithm MUST be used across all
upstream PEs for the Single Forwarder election. In OISM
networks, the Default Designated Forwarder Election
Algorithm MUST NOT be used if redundant G-sources are
attached to Broadcast Domains with different Ethernet
Tags.
2. In case of a mismatch in the DF Election Algorithm or
capabilities, the tie-breaker is the lowest PE IP address
(as advertised in the Originator Address field of the
S-PMSI A-D route).
4. Reverse Path Forwarding Checks on the Upstream PEs
All PEs with a local G-source for an SFG apply a Reverse Path
Forwarding check to the (*,G) or (S,G) state based on the Single
Forwarder election result:
1. Non-Single Forwarder PEs: MUST discard all (*,G) or (S,G)
packets received on local Attachment Circuits. ACs.
2. Single Forwarder PEs: MUST forward (*,G) or (S,G) packets
received on one (and only one) local Attachment Circuit. AC.
Key Features of the Warm Standby Solution:
* The solution ensures redundancy for SFGs without requiring
upgrades to downstream PEs (where no redundant G-sources are
connected).
* Existing procedures for non-SFG G-sources remain unchanged.
* Redundant G-sources can be either single-homed or multi-homed.
Multi-homing does not alter the above procedures.
Examples of the Warm Standby solution are provided in Sections 4.2
and 4.3.
4.2. Warm Standby Example in an OISM Network
Figure 4 illustrates an example where S1 and S2 are redundant
G-sources for the Single Flow Group (*,G1).
S1 (Single S2
| Forwarder) |
(S1,G1)| (S2,G1)|
| |
PE1 | PE2 |
+--------v---+ +--------v---+
S-PMSI | +---+ | | +---+ | S-PMSI
(*,G1) | +---|BD1| | | +---|BD2| | (*,G1)
Pref200 | |VRF+---+ | | |VRF+---+ | Pref100
|SFG |+---+ | | | |+---+ | | SFG|
| +----|SBD|--+ | |-----------||SBD|--+ |---+ |
v | |+---+ | | |+---+ | | v
| +---------|--+ +------------+ |
SMET | | | SMET
(*,G1) | | (S1,G1) | (*,G1)
| +--------+------------------+ |
^ | | | | ^
| | | EVPN | | |
| | | OISM | | |
| | | | | |
PE3 | | PE4 | | PE5
+--------v---+ +------------+ | +------------+
| +---+ | | +---+ | | | +---+ |
| +---|SBD| |-------| +---|SBD| |--|---| +---|SBD| |
| |VRF+---+ | | |VRF+---+ | | | |VRF+---+ |
|+---+ | | |+---+ | | | |+---+ | |
||BD3|--+ | ||BD4|--+ | +--->|BD1|--+ |
|+---+ | |+---+ | |+---+ |
+------------+ +------------+ +------------+
| ^ | ^
| | IGMP/MLD | | IGMP/MLD
R1 | J(*,G1) R3| J(*,G1)
Figure 4: Warm Standby Solution for Redundant G-Sources
The Warm Standby procedure is as follows:
1. Configuration of the upstream PEs (PE1 and PE2)
* PE1 and PE2 are configured to recognize G1 as a Single Flow
Group for any source.
* Redundant G-sources for G1 may be attached to BD1 (for PE1)
and BD2 (for PE2).
2. Signaling the location of S1 and S2 for (*,G1)
Upon receiving traffic for G1 on a local Attachment Circuit: AC:
* PE1 and PE2 originate S-PMSI A-D routes for (*,G1), including:
- the Supplementary Broadcast Domain Route Target (SBD-RT), SBD-RT,
- the Designated Forwarder Election Extended Community, and
- the SFG flag in the Multicast Flags Extended Community.
* These routes indicate the presence of a Single Flow Group.
3. Single Forwarder Election
* Based on the Designated Forwarder Election Extended Community,
PE1 and PE2 perform Single Forwarder election.
* Assuming they use Preference-based Election [RFC9785], PE1
(with a higher preference) is elected as the Single Forwarder
for (*,G1).
4. Reverse Path Forwarding check on the PEs attached to a redundant
G-source
a. Non-Single Forwarder Behavior: PE2 discards all (*,G1)
packets received on its local Attachment Circuit. AC.
b. Single Forwarder Behavior: PE1 forwards (*,G1) packets
received on one (and only one) local Attachment Circuit. AC.
The outcome:
* Upon receiving IGMP/MLD reports for (*,G1) or (S,G1), downstream
PEs (PE3 and PE5) issue SMET routes to pull the multicast Single
Flow Group traffic from PE1 only.
* In the event of a failure of S1, the Attachment Circuit AC connected to S1, or PE1
itself, the S-PMSI A-D route for (*,G1) is withdrawn by PE1.
* As a result, PE2 is promoted to Single Forwarder, ensuring
continued delivery of (*,G1) traffic.
4.3. Warm Standby Example in a Single-BD Tenant Network
Figure 5 illustrates an example where S1 and S2 are redundant
G-sources for the Single Flow Group (*,G1). In this case, all
G-sources and receivers are connected to the same BD1, and there is
no Supplementary Broadcast Domain (SBD). SBD.
S1 (Single S2
| Forwarder) |
(S1,G1)| (S2,G1)|
| |
PE1 | PE2 |
+--------v---+ +--------v---+
S-PMSI | +---+ | | +---+ | S-PMSI
(*,G1) | |BD1| | | |BD1| | (*,G1)
Pref200 | +---+ | | +---+ | Pref100
|SFG | | | | | SFG|
| +---| | |-----------| |---+ |
v | | | | | | | v
| +---------|--+ +------------+ |
SMET | | | SMET
(*,G1) | | (S1,G1) | (*,G1)
| +--------+------------------------+ |
^ | | | | ^
| | | EVPN | | |
| | | | | |
| | | | | |
PE3 | | PE4 | | PE5
+--------v---+ +------------+ +-|----------+
| +---+ | | +---+ | | | +---+ |
| |BD1| |-------| |BD1| |------| +--->|BD1| |
| +---+ | | +---+ | | +---+ |
| | | | | |
| | | | | |
| | | | | |
+------------+ +------------+ +------------+
| ^ | ^
| | IGMP/MLD | | IGMP/MLD
R1 | J(*,G1) R3 | J(*,G1)
Figure 5: WS Solution for Redundant G-Sources in the Same BD
The procedures for the Warm Standby solution in this example are
identical to those described in Section 4.2, with the following
distinction:
* Signaling the S-PMSI A-D Routes
- Upon receiving traffic for the Single Flow Group (*,G1), PE1
and PE2 advertise S-PMSI A-D routes.
- These routes include only the BD1-RT (Broadcast Domain 1 Route
Target) as there is no Supplementary Broadcast Domain (SBD) SBD in this scenario.
This example represents a specific sub-case of the broader procedure
detailed in Section 4.2, adapted to a single Broadcast Domain
environment. The absence of an SBD simplifies the configuration, as
all signaling remains within the context of BD1.
5. Hot Standby Solution for Redundant G-Sources
This section specifies the Hot Standby solution for handling
redundant multicast sources (G-sources). The solution supports both
IPv4 and IPv6 sources.
5.1. Specification
The Hot Standby solution is designed for scenarios requiring fast
failover in the event of a G-source or upstream PE failure. It
assumes that additional bandwidth consumption in the tenant network
is acceptable. The procedure is as follows:
1. Configuration of PEs
* Upstream PEs are configured to identify Single Flow Groups in
the tenant domain. This includes groups for any source or a
source prefix containing redundant G-sources.
* Each redundant G-source MUST be associated with an Ethernet
Segment ES on the
upstream PEs. This applies to both single-
homed single-homed and multi-homed multi-
homed G-sources. For both, single-homed and multi-homed
G-sources, ESI labels are essential for Reverse Path
Forwarding checks on downstream PEs. The term S-ESI is used
to denote the ESI associated with a redundant G-source.
* Unlike the Warm Standby solution, the Hot Standby solution
requires downstream PEs to support the procedure.
* Downstream PEs:
- Do not need explicit configuration for Single Flow Groups
or their ESIs (since they get that information from the
upstream PEs).
- Dynamically select an ESI for each Single Flow Group based
on local policy (hence different downstream PEs may select
different Ethernet Segment Identifiers) ESIs) and program a Reverse Path Forwarding check
to discard (*,G) or (S,G) packets from other ESIs.
2. Signaling the location of a G-source for a given SFG and its
association to the local Ethernet Segments ESes
An upstream PE configured for Hot Standby procedures:
* MUST advertise an S-PMSI A-D route for each Single Flow Group.
These routes:
- Use the Broadcast Domain Route Target (BD-RT) and, if
applicable, the Supplementary Broadcast Domain Route Target
(SBD-RT) SBD-RT so that the routes are imported in
all the PEs of the tenant domain.
- MUST include ESI Label Extended Communities to convey the
S-ESI labels associated with the Single Flow Group. These
ESI labels match the labels advertised by the EVPN A-D per
ES routes for each S-ES.
* MAY include a PMSI Tunnel Attribute, depending on the tunnel
type, as specified in the Warm Standby procedure.
* MUST trigger the S-PMSI A-D route advertisement based on the
SFG configuration (and not based on the reception of traffic).
3. Distribution of DCB ESI Labels labels and G-source ES routes
* Upstream PEs advertise corresponding EVPN routes:
- EVPN Ethernet Segment ES routes for the local S-ESIs. ES routes are used
for regular Designated Forwarder DF Election for the S-ES. This document does
not introduce any change in the procedures related to the
EVPN ES routes.
- A-D per EVI and A-D per ES routes for tenant-specific
traffic. If the SBD exists, the EVPN A-D per EVI and A-D
per ES routes MUST include the route target SBD-RT, since
they have to be imported by all the PEs in the tenant
domain.
* ESI Label Procedures: label procedures:
- The EVPN A-D per ES routes convey the S-ESI labels that the
downstream PEs use to implement Reverse Path Forwarding
checks for SFGs.
- All packets for a given G-source MUST carry the same S-ESI
label. For example, if two redundant G-sources are multi-
homed to PE1 and PE2 via S-ES-1 and S-ES-2, PE1 and PE2
MUST allocate the same ESI label "Lx" for S-ES-1, and they
MUST allocate the same ESI label "Ly" for S-ES-2. In
addition, Lx and Ly MUST be different.
- S-ESI labels are allocated as Domain-wide Common Block
(DCB) labels and follow the procedures in [RFC9573]. In
addition, the PE indicates that these ESI labels are DCB
labels by using the extensions described in Section 5.2.
4. Processing of EVPN A-D per ES/EVI routes and Reverse Path
Forwarding check on the downstream PEs
The EVPN A-D per ES/EVI routes are received and imported in all
the PEs in the tenant domain. Downstream PEs process received
EVPN A-D per ES/EVI routes based on their configuration:
* The PEs attached to the same Broadcast Domain of the route
target BD-RT that is included in the EVPN A-D per ES/EVI
routes process the routes as in [RFC7432] and [RFC8584]. If
the receiving PE is attached to the same Ethernet Segment ES as indicated in
the route, split-horizon procedures [RFC7432] are followed and
the Designated Forwarder DF Election candidate list is modified as in [RFC8584] if
the Ethernet Segment ES supports the AC-DF (Attachment Circuit (AC influenced Designated Forwarder) DF) capability.
* The PEs that are not attached to the Broadcast Domain
identified by the BD-RT but are attached to the Supplementary
Broadcast Domain SBD of the
received SBD-RT MUST import the EVPN A-D per ES/EVI routes and
use them for redundant G-source mass withdrawal, as explained
later.
* Upon importing EVPN A-D per ES routes corresponding to
different S-ESes, a PE MUST select a primary S-ES based on
local policy, and add a Reverse Path Forwarding check to the
(*,G) or (S,G) state in the Broadcast Domain or Supplementary
Broadcast Domain. SBD. This
Reverse Path Forwarding check discards all ingress packets to
(*,G)/(S,G) that are not received with the ESI label of the
primary S-ES.
5. G-traffic forwarding for redundant G-sources and fault detection
* Traffic Forwarding forwarding with S-ESI Labels: labels:
- When there is an existing (*,G) or (S,G) state for the SFG
with output interface list entries associated with remote
EVPN PEs, the upstream PE will add an S-ESI label to the
bottom of the stack when forwarding G-traffic received on
an S-ES. This label is allocated from a domain-wide common
block DCB as described
in Step 3.
- If point-to-multipoint or BIER PMSIs are used, this
procedure does not introduce new data path requirements on
the upstream PEs, apart from allocating the S-ESI label
from the domain-wide common block DCB as per [RFC9573]). However, when Ingress Replication IR or Assisted Replication AR are
employed, this document extends the procedures defined in
[RFC7432]. In these scenarios, the upstream PE pushes the
S-ESI labels on packets not only destinated destined for PEs sharing
the ES but also for all PEs within the tenant domain. This
ensures that downstream PEs receive all the multicast
packets from the redundant G-sources with an S-ESI label,
regardless of the PMSI type or local ESes. Downstream PEs
will discard any packet carrying an S-ESI label different
from the primary S-ESI label (associated with the selected
primary S-ES), as outlined in Step 4.
* Handling Route Withdrawals route withdrawals and Fault Detection fault detection
- If the last EVPN A-D per EVI or the last EVPN A-D per ES
route for the primary S-ES is withdrawn, the downstream PE
will immediately select a new primary S-ES and update the
Reverse Path Forwarding check accordingly.
- For scenarios where the same S-ES is used across multiple
tenant domains by the upstream PEs, the withdrawal of all
the EVPN A-D per-ES routes associated with an S-ES enables
a mass withdrawal mechanism. This allows the downstream PE
to simultaneously update the Reverse Path Forwarding check
for all tenant domains that rely on the same S-ES.
* Removal of Reverse Path Forwarding Checks checks on S-PMSI Withdrawal withdrawal
- The withdrawal of the last EVPN S-PMSI A-D route for a
given (*,G) or (S,G) that represents an SFG SHOULD result
in the downstream PE removing the S-ESI label-based Reverse
Path Forwarding check for that (*,G) or (S,G).
This document supports the use of Context Context-Specific Label Space ID
Extended Communities, as described in [RFC9573], for scenarios where
S-ESI labels are allocated within context context-specific label spaces.
When the context context-specific label space ID extended community is
advertised along with the ESI label in an EVPN A-D per ES route, the
ESI label is from a context context-specific label space identified by the Domain-wide Common Block (DCB)
DCB label in the Extended Community.
5.2. Extensions for the Advertisement of DCB Labels
Domain-wide Common Block
DCB labels are specified in [RFC9573], and this document makes use of
them as outlined in Section 5.1. [RFC9573] assumes that Domain-wide Common Block DCB labels
are applicable only to Multipoint-to-Multipoint, Point-to-Multipoint,
or BIER tunnels. Additionally, it specifies that when a PMSI label
is a Domain-wide
Common Block DCB label, the ESI label used for multi-homing is also a
Domain-wide Common Block DCB
label.
This document extends the use of DCB-allocated ESI labels with the
following provisions:
a. DCB-allocated ESI labels MAY be used with Ingress Replication IR tunnels and
b. DCB-allocated ESI labels MAY be used by PEs that do not use DCB-
allocated PMSI labels.
These control plane extensions are indicated in the EVPN A-D per ES
routes for the relevant S-ESs S-ESes by:
1. Adding the ESI-DCB-flag (Domain-wide Common Block (DCB flag) to the ESI Label label Extended
Community, or
2. Adding the Context Context-Specific Label Space ID extended community
The encoding of the DCB-flag within the ESI Label Extended Community
is shown below:
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=0x01 | Flags(1 octet)| Reserved=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved=0 | ESI Label |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: ESI Label Extended Community
This document defines the DCB-flag as follows:
* Bit 5 of the Flags octet in the ESI Label Extended Community is
defined as the ESI-DCB-flag by this document.
* When the ESI-DCB-flag is set, it indicates that the ESI label is a
DCB label.
Criteria for identifying a DCB label:
An ESI label is considered a DCB label if either of the following
conditions is met:
a. The ESI label is encoded in an ESI Label Extended Community with
the ESI-DCB-flag set.
b. The ESI label is signaled by a PE that has advertised a PMSI
label that is a DCB label.
As in [RFC9573], this document also permits the use of context context-
specific label space ID extended community. When this extended
community is advertised along with the ESI label in an EVPN A-D per
ES route, it indicates that the ESI label is from a context context-specific
label space identified by the DCB label in the Extended Community.
5.3. Use of BFD in the HS Solution
In addition to utilizing the state of the EVPN A-D per EVI, EVPN A-D
per ES, or S-PMSI A-D routes to adjust the Reverse Path Forwarding
checks for (*,G) or (S,G) as discussed in Section 5.1, the
Bidirectional Forwarding Detection (BFD) protocol MAY be employed to
monitor the status of the multipoint tunnels used to forward the SFG
packets from redundant G-sources.
BFD integration:
* The BGP-BFD Attribute is advertised alongside the S-PMSI A-D or
Inclusive Multicast Ethernet Tag
IMET routes, depending on whether Inclusive PMSI or Selective PMSI
trees are being utilized.
* The procedures outlined in [RFC9780] are followed to bootstrap
multipoint BFD sessions on the downstream PEs.
5.4. Hot Standby Example in an OISM Network
This section describes the Hot Standby model applied in an Optimized
Inter-Subnet Multicast (OISM) network. Figures 7 and 8 illustrate
scenarios with multi-homed and single-homed redundant G-sources,
respectively.
5.4.1. Multi-Homed Redundant G-Sources
Scenario (Figure 7):
* S1 and S2 are redundant G-sources for the Single Flow Group
(*,G1), connected to BD1.
* S1 and S2 are all-active multi-homed to upstream PEs (PE1 and
PE2).
* Receivers are connected to downstream PEs (PE3 and PE5) in BD3 and
BD1, respectively.
* S1 and S2 are connected to the multi-homing PEs using a LAG.
Multicast traffic can traverse either link.
* In this model, downstream PEs receive duplicate G-traffic for
(*,G1) and must use Reverse Path Forwarding checks to avoid
delivering duplicate packets to receivers.
S1(ESI-1) S2(ESI-2)
| |
| +----------------------+
(S1,G1)| | (S2,G1)|
+----------------------+ |
PE1 | | PE2 | |
+--------v---+ +--------v---+
| +---+ | | +---+ | S-PMSI
S-PMSI | +---|BD1| | | +---|BD1| | (*,G1)
(*,G1) | |VRF+---+ | | |VRF+---+ | SFG
SFG |+---+ | | | |+---+ | | | ESI1,2
ESI1,2 +---||SBD|--+ | |-----------||SBD|--+ | |---+ |
| | |+---+ | | EVPN |+---+ | | | v
v | +---------|--+ OISM +---------|--+ |
| | | |
| | (S1,G1) | |
SMET | +---------+------------------+ | | SMET
(*,G1) | | | | | (*,G1)
^ | | +----------------------------+---+ | ^
| | | | (S2,G1) | | | |
| | | | | | | |
PE3 | | | PE4 | | | PE5
+-------v-v--+ +------------+ | | +------------+
| +---+ | | +---+ | | | | +---+ |
| +---|SBD| +-------| +---|SBD| |--|-|-| +---|SBD| |
| |VRF+---+ | | |VRF+---+ | | | | |VRF+---+ |
|+---+ | | |+---+ | | | | |+---+ | |
||BD3|--+ | ||BD4|--+ | | +->|BD1|--+ |
|+---+ | |+---+ | +--->+---+ |
+------------+ +------------+ +------------+
| ^ | ^
| | IGMP/MLD | | IGMP/MLD
R1 | J(*,G1) R3 | J(*,G1)
Figure 7: Hot Standby Solution for Multi-Homed Redundant
G-Sources in OISM
The procedure is as follows:
1. Configuration of the PEs:
* PE1 and PE2 are configured to recognize (*,G1) as a Single
Flow Group.
* Redundant G-sources use S-ESIs: ESI-1 for S1 and ESI-2 for S2.
* The Ethernet Segments ESes (ES-1 and ES-2) are configured on both PEs. ESI
labels are allocated from a Domain-wide Common Block
(DCB) DCB [RFC9573] - ESI-label-1 for
ESI-1 and ESI-label-2 for ESI-2.
* The downstream PEs (PE3, PE4, and PE5) are configured to
support Hot Standby mode and select the G-source with, e.g.,
lowest ESI value.
2. Advertisement of the EVPN routes:
* PE1 and PE2 advertise S-PMSI A-D routes for (*,G1), including:
- Route Targets: BD1-RT and SBD-RT.
- ESI Label Extended Communities for ESI-label-1 and ESI-
label-2.
- The SFG flag indicating that (*,G1) represents a Single
Flow Group.
* EVPN ES and A-D per ES/EVI routes are also advertised for
ESI-1 and ESI-2. These include SBD-RT for downstream PE
import. The EVPN A-D per ES routes contain ESI-label-1 for
ESI-1 (on both PEs) and ESI-label-2 for ESI-2 (also on both
PEs).
3. Processing of EVPN A-D per ES/EVI routes and Reverse Path
Forwarding check on Downstream downstream PEs:
* PE1 and PE2 receive each other's ES and A-D per ES/EVI routes.
Designated Forwarder
DF Election and programming of the ESI labels for egress
split-horizon filtering follow, as specified in [RFC7432] and
[RFC8584].
* PE3/PE4 import the EVPN A-D per ES/EVI routes in the SBD; PE5
imports them in BD1.
* As downstream PEs, PE3 and PE5 use the EVPN A-D per ES/EVI
routes to program Reverse Path Forwarding checks.
* The primary S-ESI for (*,G1) is selected based on local policy
(e.g., lowest ESI value), and therefore packets with ESI-
label-2 are discarded if ESI-label-1 is selected as the
primary label.
4. Traffic forwarding and fault detection:
* PE1 receives (S1,G1) traffic and forwards it with ESI-label-1
in the context of BD1. This traffic passes Reverse Path
Forwarding checks on downstream PEs (PE3 and PE5, since PE4
has no local interested receivers) and is delivered to
receivers.
* PE2 receives (S2,G1) traffic and forwards it with ESI-label-2.
This traffic fails the Reverse Path Forwarding check on PE3
and PE5 and is discarded.
* If the link between S1 and PE1 fails, PE1 withdraws the EVPN
ES and A-D routes for ESI-1. S1 forwards the (S1,G1) traffic
to PE2 instead. PE2 continues forwarding (S2,G1) traffic
using ESI-label-2 and now also forwards (S1,G1) with ESI-
label-1. The Reverse Path Forwarding checks do not change in
PE3/PE5.
* If all links to S1 fail, PE2 also withdraws the EVPN ES and
A-D routes for ESI-1 and downstream PEs update the Reverse
Path Forwarding checks to accept ESI-label-2 traffic.
5.4.2. Single-Homed Redundant G-Sources
Scenario (Figure 8):
* S1 is single-homed to PE1 using ESI-1, and S2 is single-homed to
PE2 using ESI-2.
* The scenario is a subset of the multi-homed case. Only one PE
advertises EVPN A-D per ES/EVI routes for each S-ESI.
S1(ESI-1) S2(ESI-2)
| |
(S1,G1)| (S2,G1)|
| |
PE1 | PE2 |
+--------v---+ +--------v---+
| +---+ | | +---+ | S-PMSI
S-PMSI | +---|BD1| | | +---|BD2| | (*,G1)
(*,G1) | |VRF+---+ | | |VRF+---+ | SFG
SFG |+---+ | | | |+---+ | | | ESI2
ESI1 +---||SBD|--+ | |-----------||SBD|--+ | |---+ |
| | |+---+ | | EVPN |+---+ | | | v
v | +---------|--+ OISM +---------|--+ |
| | | |
| | (S1,G1) | |
SMET | +---------+------------------+ | | SMET
(*,G1) | | | | | (*,G1)
^ | | +--------------------------------+----+ | ^
| | | | (S2,G1) | | | |
| | | | | | | |
PE3 | | | PE4 | | | PE5
+-------v-v--+ +------------+ | +------v-----+
| +---+ | | +---+ | | | +---+ |
| +---|SBD| |-------| +---|SBD| |--|---| +---|SBD| |
| |VRF+---+ | | |VRF+---+ | | | |VRF+---+ |
|+---+ | | |+---+ | | | |+---+ | |
||BD3|--+ | ||BD4|--+ | +--->|BD1|--+ |
|+---+ | |+---+ | |+---+ |
+------------+ +------------+ +------------+
| ^ | ^
| | IGMP/MLD | | IGMP/MLD
R1 | J(*,G1) R3 | J(*,G1)
Figure 8: HS Solution for Single-Homed Redundant G-Sources in OISM
The procedures follow the same logic as described in the multi-homed
scenario, with the distinction that each ESI is specific to a single
PE.
Figures 7 and 8 demonstrate the application of the Hot Standby
solution, ensuring seamless traffic forwarding while avoiding
duplication in the presence of redundant G-sources.
5.5. Hot Standby Example in a Single-BD Tenant Network
The Hot Standby procedures described in Section 5.4 apply equally to
scenarios where the tenant network comprises a single Broadcast
Domain (e.g., BD1), irrespective of whether the redundant G-sources
are multi-homed or single-homed. In such cases:
* The advertised routes do not include the Supplementary Broadcast
Domain Route Target (SBD-RT). SBD-RT.
* All procedures are confined to the single BD1.
The absence of the SBD simplifies the configuration and limits the
scope of the Hot Standby solution to BD1, while maintaining the
integrity of the procedures for managing redundant G-sources.
6. Security Considerations
The same Security Considerations described in [RFC9625] are valid for
this document.
From a security perspective, out of the two methods described in this
document, the Warm Standby method is considered lighter in terms of
control plane, and therefore its impact is low on the processing
capabilities of the PEs. The Hot Standby method adds more burden on
the control plane of all the PEs of the tenant with sources and
receivers.
7. IANA Considerations
IANA has allocated bit 4 in the "Multicast Flags Extended Community"
registry that was introduced by [RFC9251]. This bit indicates that a
given (*,G) or (S,G) in an S-PMSI A-D route is associated with an
SFG. This bit is called "Single Flow Group" bit, and it is defined
as follows:
+=====+===================+===============+
| Bit | Name | Reference |
+=====+===================+===============+
| 4 | Single Flow Group | This Document |
+-----+-------------------+---------------+
Table 1
IANA has allocated bit 5 in the "EVPN ESI Label Extended Community
Flags" registry that was introduced by [RFC9746]. This bit is the
ESI-DCB flag and indicates that the ESI label contained in the ESI
Label Extended Community is a Domain-wide Common Block DCB label. This bit is defined as
follows:
+=====+==============+===============+
+=====+=========+===============+
| Bit | Name | Reference |
+=====+==============+===============+
+=====+=========+===============+
| 5 | ESI-DCB Flag | This Document |
+-----+--------------+---------------+
+-----+---------+---------------+
Table 2
8. References
8.1. Normative References
[RFC7432] Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
Ethernet VPN",
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 7432, 2119,
DOI 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>. 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6513] Rosen, E., Ed. and R. Aggarwal, Ed., "Multicast in MPLS/
BGP IP VPNs", RFC 6513, DOI 10.17487/RFC6513, February
2012, <https://www.rfc-editor.org/info/rfc6513>.
[RFC6514] Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
Encodings and Procedures for Multicast in MPLS/BGP IP
VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,
<https://www.rfc-editor.org/info/rfc6514>.
[RFC9251]
[RFC7432] Sajassi, A., Thoria, S., Mishra, M., Patel, K., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
Uttaro, J., Drake, J., and W. Lin, "Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) Proxies for Henderickx, "BGP MPLS-Based
Ethernet
VPN (EVPN)", VPN", RFC 9251, 7432, DOI 10.17487/RFC9251, June 2022,
<https://www.rfc-editor.org/info/rfc9251>.
[RFC9625] Lin, W., Zhang, Z., Drake, J., Rosen, E., Ed., Rabadan,
J., and A. Sajassi, "EVPN Optimized Inter-Subnet Multicast
(OISM) Forwarding", 10.17487/RFC7432, February
2015, <https://www.rfc-editor.org/info/rfc7432>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 9625,
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC9625, August
2024, <https://www.rfc-editor.org/info/rfc9625>. 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8584] Rabadan, J., Ed., Mohanty, S., Ed., Sajassi, A., Drake,
J., Nagaraj, K., and S. Sathappan, "Framework for Ethernet
VPN Designated Forwarder Election Extensibility",
RFC 8584, DOI 10.17487/RFC8584, April 2019,
<https://www.rfc-editor.org/info/rfc8584>.
[RFC2119] Bradner,
[RFC9251] Sajassi, A., Thoria, S., "Key words Mishra, M., Patel, K., Drake, J.,
and W. Lin, "Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) Proxies for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, Ethernet
VPN (EVPN)", RFC 8174, 9251, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. 10.17487/RFC9251, June 2022,
<https://www.rfc-editor.org/info/rfc9251>.
[RFC9573] Zhang, Z., Rosen, E., Lin, W., Li, Z., and IJ. Wijnands,
"MVPN/EVPN Tunnel Aggregation with Common Labels",
RFC 9573, DOI 10.17487/RFC9573, May 2024,
<https://www.rfc-editor.org/info/rfc9573>.
[RFC9625] Lin, W., Zhang, Z., Drake, J., Rosen, E., Ed., Rabadan,
J., and A. Sajassi, "EVPN Optimized Inter-Subnet Multicast
(OISM) Forwarding", RFC 9625, DOI 10.17487/RFC9625, August
2024, <https://www.rfc-editor.org/info/rfc9625>.
[RFC9746] Rabadan, J., Ed., Nagaraj, K., Lin, W., and A. Sajassi, "BGP
EVPN Multihoming Extensions for Split-Horizon Filtering",
RFC 9746, DOI 10.17487/RFC9746, March 2025,
<https://www.rfc-editor.org/info/rfc9746>.
8.2. Informative References
[RFC9136] Rabadan, J., Ed., Henderickx, W., Drake, J., Lin, W.,
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. Sajassi, "IP Prefix Advertisement in Ethernet VPN
(EVPN)",
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 9136, 3376, DOI 10.17487/RFC9136, 10.17487/RFC3376, October 2021,
<https://www.rfc-editor.org/info/rfc9136>.
[RFC9572] Zhang, Z., Lin, W., Rabadan, J., Patel, K., 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and A.
Sajassi, "Updates to EVPN Broadcast, Unknown Unicast, or
Multicast (BUM) Procedures", RFC 9572,
DOI 10.17487/RFC9572, May 2024,
<https://www.rfc-editor.org/info/rfc9572>.
[RFC9785] Rabadan, J., L. Costa, Ed., Sathappan, S., Lin, W., Drake, J., and
A. Sajassi, "Preference-Based EVPN Designated Forwarder
(DF) Election", "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 9785, 3810,
DOI 10.17487/RFC9785, 10.17487/RFC3810, June 2025,
<https://www.rfc-editor.org/info/rfc9785>. 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
2006, <https://www.rfc-editor.org/info/rfc4364>.
[RFC9780] Mirsky, G., Mishra, G., and D. Eastlake 3rd,
"Bidirectional Forwarding Detection (BFD) for Multipoint
Networks over Point-to-Multipoint MPLS Label Switched
Paths (LSPs)", RFC 9780, DOI 10.17487/RFC9780, May 2025,
<https://www.rfc-editor.org/info/rfc9780>.
[RFC7761] Fenner, B., Handley, M., Holbrook, H., Kouvelas, I.,
Parekh, R., Zhang, Z., and L. Zheng, "Protocol Independent
Multicast - Sparse Mode (PIM-SM): Protocol Specification
(Revised)", STD 83, RFC 7761, DOI 10.17487/RFC7761, March
2016, <https://www.rfc-editor.org/info/rfc7761>.
[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in Ethernet VPN
(EVPN)", RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/info/rfc9135>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC8296] Wijnands, IJ., Ed., Rosen, E., Ed., Dolganow, A.,
Tantsura, J., Aldrin, S., and I. Meilik, "Encapsulation
for Bit Index Explicit Replication (BIER) in MPLS and Non-
MPLS Networks", RFC 8296, DOI 10.17487/RFC8296, January
2018, <https://www.rfc-editor.org/info/rfc8296>.
[RFC9135] Sajassi, A., Salam, S., Thoria, S., Drake, J., and J.
Rabadan, "Integrated Routing and Bridging in Ethernet VPN
(EVPN)", RFC 9135, DOI 10.17487/RFC9135, October 2021,
<https://www.rfc-editor.org/info/rfc9135>.
[RFC9136] Rabadan, J., Ed., Henderickx, W., Drake, J., Lin, W., and
A. Sajassi, "IP Prefix Advertisement in Ethernet VPN
(EVPN)", RFC 9136, DOI 10.17487/RFC9136, October 2021,
<https://www.rfc-editor.org/info/rfc9136>.
[RFC9572] Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
Sajassi, "Updates to EVPN Broadcast, Unknown Unicast, or
Multicast (BUM) Procedures", RFC 9572,
DOI 10.17487/RFC9572, May 2024,
<https://www.rfc-editor.org/info/rfc9572>.
[RFC9574] Rabadan, J., Ed., Sathappan, S., Lin, W., Katiyar, M., and
A. Sajassi, "Optimized Ingress Replication Solution for
Ethernet VPNs (EVPNs)", RFC 9574, DOI 10.17487/RFC9574,
May 2024, <https://www.rfc-editor.org/info/rfc9574>.
[RFC9780] Mirsky, G., Mishra, G., and D. Eastlake 3rd,
"Bidirectional Forwarding Detection (BFD) for Multipoint
Networks over Point-to-Multipoint MPLS Label Switched
Paths (LSPs)", RFC 9780, DOI 10.17487/RFC9780, May 2025,
<https://www.rfc-editor.org/info/rfc9780>.
[RFC9785] Rabadan, J., Ed., Sathappan, S., Lin, W., Drake, J., and
A. Sajassi, "Preference-Based EVPN Designated Forwarder
(DF) Election", RFC 9785, DOI 10.17487/RFC9785, June 2025,
<https://www.rfc-editor.org/info/rfc9785>.
Acknowledgments
The authors would like to thank Mankamana Mishra, Ali Sajassi, Greg
Mirsky, and Sasha Vainshtein for their review and valuable comments.
Special thanks to Gunter Van de Velde for his excellent review, which
significantly enhanced the document's readability.
Contributors
In addition to the authors listed on the front page, the following
person has significantly contributed to this document:
Eric C. Rosen
Email: erosen52@gmail.com
Authors' Addresses
Jorge Rabadan (editor)
Nokia
520 Almanor Avenue
Sunnyvale, CA 94085
United States of America
Email: jorge.rabadan@nokia.com
Jayant Kotalwar
Nokia
520 Almanor Avenue
Sunnyvale, CA 94085
United States of America
Email: jayant.kotalwar@nokia.com
Senthil Sathappan
Nokia
520 Almanor Avenue
Sunnyvale, CA 94085
United States of America
Email: senthil.sathappan@nokia.com
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
Wen Lin
Juniper Networks
Email: wlin@juniper.net