Internet Engineering Task Force (IETF) Z. Zhang
Request for Comments: 9081 L. Giuliano
Updates: 6514 Juniper Networks
Category: Standards Track July 2021
ISSN: 2070-1721
Interoperation between Multicast Virtual Private Network (MVPN) and
Multicast Source Directory Protocol (MSDP) Source-Active Routes
Abstract
This document specifies the procedures for interoperation between
Multicast Virtual Private Network (MVPN) Source-Active (SA) routes
and customer Multicast Source Discovery Protocol (MSDP) SA routes,
which is useful for MVPN provider networks offering services to
customers with an existing MSDP infrastructure. Without the
procedures described in this document, VPN-specific MSDP sessions are
required among the Provider Edge (PE) routers that are customer MSDP
peers. This document updates RFC 6514.
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/rfc9081.
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Table of Contents
1. Introduction
1.1. MVPN RPT-SPT Mode
2. Terminology
2.1. Requirements Language
3. Specification
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
Section 14 ("Supporting PIM-SM without Inter-Site Shared C-Trees") of
[RFC6514] specifies the procedures for MVPN PEs to discover (C-S,C-G)
via MVPN Source-Active A-D routes and then send Source Tree Join
(C-S,C-G) C-multicast routes towards the ingress PEs to establish
shortest path trees (SPTs) for customer Any-Source Multicast (ASM)
flows for which they have downstream receivers. (C-*,C-G)
C-multicast routes are not sent among the PEs, so inter-site shared
C-Trees are not used, and the method is generally referred to as
"spt-only" mode.
With this mode, the MVPN Source-Active routes are functionally
similar to MSDP Source-Active messages. For a VPN, one or more of
the PEs, say PE1, either acts as a C-RP and learns of (C-S,C-G) via
PIM Register messages or has MSDP sessions with some MSDP peers and
learns of (C-S,C-G) via MSDP SA messages. In either case, PE1 will
then originate MVPN SA routes for other PEs to learn (C-S,C-G).
[RFC6514] only specifies that a PE receiving the MVPN SA routes, say
PE2, will advertise Source Tree Join (C-S,C-G) C-multicast routes if
it has corresponding (C-*,C-G) state learnt from its Customer Edge
(CE). PE2 may also have MSDP sessions for the VPN with other C-RPs
at its site, but [RFC6514] does not specify that PE2 advertises MSDP
SA messages to those MSDP peers for the (C-S,C-G) that it learns via
MVPN SA routes. PE2 would need to have an MSDP session with PE1
(that advertised the MVPN SA messages) to learn the sources via MSDP
SA messages for it to advertise the MSDP SA to its local peers. To
make things worse, unless blocked by policy control, PE2 would in
turn advertise MVPN SA routes because of those MSDP SA messages that
it receives from PE1, which are redundant and unnecessary. Also
notice that the PE1-PE2 MSDP session is VPN specific (i.e., only for
a single VPN), while the BGP sessions over which the MVPN routes are
advertised are not.
If a PE does advertise MSDP SA messages based on received MVPN SA
routes, the VPN-specific MSDP sessions with other PEs are no longer
needed. Additionally, this MVPN/MSDP SA interoperation has the
following inherent benefits for a BGP-based solution.
* MSDP SA refreshes are replaced with BGP hard state.
* Route reflectors can be used instead of having peer-to-peer
sessions.
* VPN extranet [RFC2764] mechanisms can be used to propagate
(C-S,C-G) information across VPNs with flexible policy control.
While MSDP Source-Active routes contain the source, group, and RP
addresses of a given multicast flow, MVPN Source-Active routes only
contain the source and group. MSDP requires the RP address
information in order to perform MSDP peer Reverse Path Forwarding
(RPF). Therefore, this document describes how to convey the RP
address information into the MVPN Source-Active route using an
Extended Community so this information can be shared with an existing
MSDP infrastructure.
The procedures apply to Global Table Multicast (GTM) [RFC7716] as
well.
1.1. MVPN RPT-SPT Mode
For comparison, another method of supporting customer ASM is
generally referred to as "rpt-spt" mode. Section 13 ("Switching from
a Shared C-Tree to a Source C-Tree") of [RFC6514] specifies the MVPN
SA procedures for that mode, but those SA routes are a replacement
for PIM-ASM assert and (s,g,rpt) prune mechanisms, not for source
discovery purposes. MVPN/MSDP SA interoperation for the "rpt-spt"
mode is outside the scope of this document. In the rest of the
document, the "spt-only" mode is assumed.
2. Terminology
Familiarity with MVPN [RFC6513] [RFC6514] and MSDP [RFC3618]
protocols and procedures is assumed. Some terminology is listed
below for convenience.
ASM: Any-Source Multicast
SPT: source-specific Shortest Path Tree
RPT: Rendezvous Point Tree
C-S: a multicast source address, identifying a multicast
source located at a VPN customer site
C-G: a multicast group address used by a VPN customer
C-RP: a multicast Rendezvous Point for a VPN customer
C-multicast: a multicast for a VPN customer
EC: Extended Community
GTM: Global Table Multicast, i.e., a multicast in the
default or global routing table vs. a VPN Routing and
Forwarding (VRF) table
2.1. Requirements Language
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.
3. Specification
The MVPN PEs that act as customer RPs or have one or more MSDP
sessions in a VPN (or the global table in case of GTM) are treated as
an MSDP mesh group for that VPN (or the global table). In the rest
of the document, it is referred to as the PE mesh group. This PE
mesh group MUST NOT include other MSDP speakers and is integrated
into the rest of the MSDP infrastructure for the VPN (or the global
table) following normal MSDP rules and practices.
When an MVPN PE advertises an MVPN SA route following procedures in
[RFC6514] for the "spt-only" mode, it MUST attach an "MVPN SA RP-
address Extended Community". This is a Transitive IPv4-Address-
Specific Extended Community. The Local Administrator field is set to
zero, and the Global Administrator field is set to an RP address
determined as the following:
* If the (C-S,C-G) is learnt as a result of the PIM Register
mechanism, the local RP address for the C-G is used.
* If the (C-S,C-G) is learnt as a result of incoming MSDP SA
messages, the RP address in the selected MSDP SA message is used.
In addition to the procedures in [RFC6514], an MVPN PE may be
provisioned to generate MSDP SA messages from received MVPN SA
routes, with or without local policy control. If a received MVPN SA
route triggers an MSDP SA message, the MVPN SA route is treated as if
a corresponding MSDP SA message was received from within the PE mesh
group and normal MSDP procedure is followed (e.g., an MSDP SA message
is advertised to other MSDP peers outside the PE mesh group). The
(S,G) information comes from the (C-S,C-G) encoding in the MVPN SA
Network Layer Reachability Information (NLRI), and the RP address
comes from the "MVPN SA RP-address EC" mentioned above. If the
received MVPN SA route does not have the EC (this could be from a
legacy PE that does not have the capability to attach the EC), the
local RP address for the C-G is used. In that case, it is possible
that the RP inserted into the MSDP SA message for the C-G is actually
the MSDP peer to which the generated MSDP message is advertised,
causing the peer to discard it due to RPF failure. To get around
that problem, the peer SHOULD use local policy to accept the MSDP SA
message.
An MVPN PE MAY treat only the best MVPN SA route selected by the BGP
route selection process (instead of all MVPN SA routes) for a given
(C-S,C-G) as a received MSDP SA message (and advertise the
corresponding MSDP message). In that case, if the selected best MVPN
SA route does not have the "MVPN SA RP-address EC" but another route
for the same (C-S, C-G) does, then the next best route with the EC
SHOULD be chosen. As a result, if/when the best MVPN SA route with
the EC changes, a new MSDP SA message is advertised if the RP address
determined according to the newly selected MVPN SA route is different
from before. The MSDP SA state associated with the previously
advertised MSDP SA message with the older RP address will be timed
out.
4. Security Considerations
[RFC6514] specifies the procedure for a PE to generate an MVPN SA
upon discovering a (C-S,C-G) flow (e.g., via a received MSDP SA
message) in a VPN. This document extends this capability in the
reverse direction -- upon receiving an MVPN SA route in a VPN,
generate a corresponding MSDP SA and advertise it to MSDP peers in
the same VPN. As such, the capabilities specified in this document
introduce no additional security considerations beyond those already
specified in [RFC6514] and [RFC3618]. Moreover, the capabilities
specified in this document actually eliminate the control message
amplification that exists today where VPN-specific MSDP sessions are
required among the PEs that are customer MSDP peers, which lead to
redundant messages (MSDP SAs and MVPN SAs) being carried in parallel
between PEs.
5. IANA Considerations
IANA registered the following in the "Transitive IPv4-Address-
Specific Extended Community Sub-Types" registry:
+=======+=======================================+
| Value | Description |
+=======+=======================================+
| 0x20 | MVPN SA RP-address Extended Community |
+-------+---------------------------------------+
Table 1
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words 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>.
[RFC3618] Fenner, B., Ed. and D. Meyer, Ed., "Multicast Source
Discovery Protocol (MSDP)", RFC 3618,
DOI 10.17487/RFC3618, October 2003,
<https://www.rfc-editor.org/info/rfc3618>.
[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, DOI 10.17487/RFC2764, February 2000,
<https://www.rfc-editor.org/info/rfc2764>.
[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>.
[RFC7716] Zhang, J., Giuliano, L., Rosen, E., Ed., Subramanian, K.,
and D. Pacella, "Global Table Multicast with BGP Multicast
VPN (BGP-MVPN) Procedures", RFC 7716,
DOI 10.17487/RFC7716, December 2015,
<https://www.rfc-editor.org/info/rfc7716>.
Acknowledgements
The authors thank Eric Rosen, Vinod Kumar, Yajun Liu, Stig Venaas,
Mankamana Mishra, Gyan Mishra, Qin Wu, and Jia He for their reviews,
comments, questions, and suggestions for this document.
Authors' Addresses
Zhaohui Zhang
Juniper Networks
Email: zzhang@juniper.net
Lenny Giuliano
Juniper Networks
Email: lenny@juniper.net
