Interoperation between Multicast Virtual Private Network (MVPN) and Multicast Source Directory Protocol (MSDP) Source-Active Routes
RFC 9081
Zhaohui Zhang
Juniper Networks
zzhang@juniper.net
Lenny Giuliano
Juniper Networks
lenny@juniper.net
July 2021
BESS
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.
Introduction
Section "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.
MVPN RPT-SPT Mode
For comparison, another method of supporting customer ASM is generally referred to as "rpt-spt" mode. Section "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.
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
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.
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.
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.
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
References
Normative References
Informative References
[RFC6514]
[RFC2764]
[RFC7716]
[RFC6513]
[RFC3618]
[RFC2119]
[RFC8174]
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.