Internet Engineering Task Force (IETF) IJ. Wijnands
Request for Comments: 7441 Cisco Systems, Inc.
Updates: 6514 E. Rosen
Category: Standards Track Juniper Networks, Inc.
ISSN: 2070-1721 U. Joorde
January 2015 Encoding Multipoint LDP (mLDP) Forwarding Equivalence Classes (FECs)
in the NLRI of BGP MCAST-VPN Routes
Many service providers offer "BGP/MPLS IP VPN" service to their
customers. Existing IETF standards specify the procedures and
protocols that a service provider uses in order to offer this service
to customers who have IP unicast and IP multicast traffic in their
VPNs. It is also desirable to be able to support customers who have
MPLS multicast traffic in their VPNs. This document specifies the
procedures and protocol extensions that are needed to support
customers who use the Multipoint LDP (mLDP) as the control protocol
for their MPLS multicast traffic. Existing standards do provide some
support for customers who use mLDP, but only under a restrictive set
of circumstances. This document generalizes the existing support to
include all cases where the customer uses mLDP, without any
restrictions. 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 5741.
Information about the current status of this document, any errata,
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Table of Contents
1. Introduction ....................................................22. Why This Document is Needed .....................................33. Encoding an mLDP FEC in the MCAST-VPN NLRI ......................54. Wildcards .......................................................75. IANA Considerations .............................................76. Security Considerations .........................................87. References ......................................................97.1. Normative References .......................................97.2. Informative References .....................................9
Authors' Addresses ................................................101. Introduction
Many service providers (SPs) offer BGP/MPLS IP VPN service to their
customers. When a customer has IP multicast traffic in its VPN, the
service provider needs to signal the customer multicast states across
the backbone. A customer with IP multicast traffic is typically
using PIM (Protocol Independent Multicast) [PIM] and/or IGMP
(Internet Group Management Protocol) [IGMP] as the multicast control
protocol in its VPN. The IP multicast states of these protocols are
commonly denoted as "(S,G)" and/or "(*,G)" states, where "S" is a
multicast source address and "G" is a multicast group address.
[MVPN-BGP] specifies the way an SP may use BGP to signal a customer's
IP multicast states across the SP backbone. This is done by using
Multiprotocol BGP Updates whose Subsequent Address Family Identifier
(SAFI) values contain the codepoint for MCAST-VPN (as defined in
[MVPN-BGP]). The NLRI (Network Layer Reachability Information) field
of these BGP Updates includes a customer Multicast Source field and a
customer Multicast Group field, thus enabling the customer's (S,G) or
(*,G) states to be encoded in the NLRI.
It is also desirable for the BGP/MPLS IP VPN service to be able to
support customers who are using MPLS multicast, either instead of or
in addition to IP multicast. This document specifies the procedures
and protocol extensions needed to support customers who use mLDP
[mLDP] to create and maintain Point-to-Multipoint (P2MP) and/or
Multipoint-to-Multipoint (MP2MP) Label Switched Paths (LSPs). While
mLDP is not the only protocol that can be used to create and maintain
multipoint LSPs, consideration of other MPLS multicast control
protocols is outside the scope of this document.
When a customer is using mLDP in its VPN, the customer multicast
states associated with mLDP are denoted by an mLDP FEC Element
(Forwarding Equivalence Class Element; see [mLDP]) instead of by an
(S,G) or (*,G). Thus, it is necessary to have a way to encode a
customer's mLDP FEC Elements in the NLRI field of the BGP MCAST-VPN
While [MVPN-BGP] does specify a way of encoding an mLDP FEC Element
in the MCAST-VPN NLRI field, the encoding specified therein makes a
variety of restrictive assumptions about the customer's use of mLDP.
(These assumptions are described in Section 2 of this document.) The
purpose of this document is to update RFC 6514 [MVPN-BGP] so that
customers using mLDP in their VPNs can be supported even when those
assumptions do not hold.
Some SPs use the MVPN procedures to provide "global table multicast"
service (i.e., multicast service that is not in the context of a VPN)
to customers. Methods for doing this are specified in [GTM] and in
[SEAMLESS-MCAST]. The procedures described in this document can be
used along with the procedures of [GTM] or [SEAMLESS-MCAST] to
provide global table multicast service to customers that use MPLS
multicast in a global table context.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Why This Document Is Needed
An mLDP FEC Element consists of a FEC Type, a Root Node, and an
Opaque Value. mLDP uses several FEC Types and, in particular, uses
the FEC Type to distinguish between P2MP LSPs and MP2MP LSPs.
Section 11.1.2 of [MVPN-BGP] ("Originating Routes: mLDP as the
C-Multicast Protocol") states:
Whenever a PE [Provider Edge router] receives, from one of its CEs
[Customer Edge routers], a P2MP Label Map <X, Y, L> over interface
I, where X is the Root Node Address, Y is the Opaque Value, and L
is an MPLS label ... the PE constructs a Source Tree Join
C-multicast route whose MCAST-VPN NLRI contains X as the Multicast
Source field, and Y as the Multicast Group field.
In other words, the Root Node of the mLDP FEC Element appears in the
Multicast Source field, and the Opaque Value of the mLDP FEC Element
appears in the Multicast Group field.
This method of encoding an mLDP FEC in an MCAST-VPN NLRI can only be
used if all of the following conditions hold:
1. A customer using mLDP is not also using PIM/IGMP.
The encoding in [MVPN-BGP] does not specify any way in which one
can determine, upon receiving a BGP Update, whether the Multicast
Group field contains an IP address or whether it contains an mLDP
FEC Element Opaque Value. Therefore, it might not uniquely
identify a customer multicast state if the customer is using both
PIM/IGMP and mLDP in its VPN.
2. A customer using mLDP is using only the mLDP P2MP FEC Element and
is not using the mLDP MP2MP FEC Element.
The encoding in [MVPN-BGP] does not specify any way to encode the
type of the mLDP FEC Element; it just assumes it to be a P2MP FEC
3. A customer using mLDP is using only an mLDP Opaque Value type for
which the Opaque Value is exactly 32 bits or 128 bits long.
The use of Multicast Group fields that have other lengths is
declared by [MVPN-BGP] to be "out of scope" of that document
(see, e.g., Section 4.3 of that document).
This condition holds if the customer uses only the mLDP "Generic
LSP Identifier" Opaque Value type (defined in [mLDP]). However,
mLDP supports many other Opaque Value types whose length is not
restricted to be 32 or 128 bits.
The purpose of this document is to update [MVPN-BGP] so that
customers using mLDP can be supported, even when these conditions do
3. Encoding an mLDP FEC in the MCAST-VPN NLRI
When mLDP is used as the customer multicast control protocol,
[MVPN-BGP] presupposes that an mLDP FEC Element will be encoded in
the NLRI of the following three MCAST-VPN route types:
- C-multicast Source Tree Join route,
- S-PMSI A-D route, and
- Leaf A-D route.
The other four MCAST-VPN route types defined in [MVPN-BGP] do not
ever need to carry mLDP FEC Elements. The C-multicast Shared Tree
Join route and the Source Active A-D route are used to communicate
state about unidirectional shared trees; since mLDP does not have
unidirectional shared trees, these routes are not used to signal mLDP
states. The Intra-AS I-PMSI A-D route and the Inter-AS I-PMSI A-D
route do not identify specific customer multicast states and hence do
not carry any information that is specific to the customer's
multicast control protocol.
This document defines three new route types:
- C-Multicast Source Tree Join route for C-multicast mLDP,
- S-PMSI A-D route for C-multicast mLDP, and
- Leaf A-D route for C-multicast mLDP.
The term "C-multicast mLDP" in the names of these route types is
intended to indicate that the NLRI of these routes contains mLDP FEC
Each of these route types corresponds to a route type defined in
[MVPN-BGP]. IANA has been requested to allocate codepoints for these
three route types such that (a) the high-order two bits have the
value 0x01, and (b) the low-order six bits have the same value as the
codepoints for the corresponding route types from [MVPN-BGP].
In general, the procedures defined in other MVPN specifications for
the C-Multicast Source Tree Join route, the S-PMSI A-D route, and the
Leaf A-D route also apply to the C-Multicast Source Tree Join route
for C-multicast mLDP, the S-PMSI A-D route for C-multicast mLDP, and
the Leaf A-D route for C-multicast mLDP, respectively. However, the
NLRI of these three new route types is constructed differently than
the NLRI of the corresponding routes from [MVPN-BGP]: the Multicast
Source Length, Multicast Source, Multicast Group Length, and
Multicast Group fields are omitted, and in their place is a single
mLDP FEC Element, as defined in [mLDP]. (See Section 2.2 of [mLDP]
for a diagram of the mLDP FEC Element.)
As a result, the NLRI of an S-PMSI A-D route for C-multicast mLDP
will consist of a Route Distinguisher, followed by the mLDP FEC,
followed by the Originating Router's IP Address field.
The NLRI of a C-multicast Source Tree Join route for C-multicast mLDP
will consist of a Route Distinguisher, followed by the Source AS,
followed by the mLDP FEC.
In a Leaf A-D route for C-multicast mLDP that has been derived from
an S-PMSI A-D route for C-multicast mLDP, the Route Key field remains
the NLRI of the S-PMSI A-D route from which it was derived.
In a Leaf A-D route for C-multicast mLDP that has not been derived
from an S-PMSI A-D, the Route Key field is as specified in
[SEAMLESS-MCAST], except that the Multicast Source Length, Multicast
Source, Multicast Group Length, and Multicast Group fields are
omitted, and in their place is a single mLDP FEC Element. Thus, the
Route Key field consists of a Route Distinguisher, an mLDP FEC
Element, and the IP address of the Ingress PE router.
Please note that [BGP-ERR], Section 5.4 ("Typed NLRI") is applicable
if the Route Type field of the NLRI of a received MCAST-VPN route
contains an unrecognized value. Any such routes will be discarded.
An mLDP FEC Element contains an address family field whose value is
from IANA's "Address Family Numbers" registry. The value of the
address family field identifies the address family of the Root Node
Address field of the FEC Element. When an mLDP FEC Element is
encoded into the NLRI of a BGP Update whose SAFI is MCAST-VPN, the
address family of the Root Node Address (as indicated in the FEC
Element) MUST correspond to the address family that is identified in
the Address Family Identifier (AFI) field of that BGP Update. These
two address family fields are considered to correspond to each other
under the following conditions:
- they contain identical values,
- the BGP Update's AFI field identifies IPv4 as the address family,
and the mLDP FEC Element identifies "Multi-Topology IPv4" as the
address family of the Root Node, or
- the BGP Update's AFI field identifies IPv6 as the address family,
and the mLDP FEC Element identifies "Multi-Topology IPv6" as the
address family of the Root Node.
For more information about the "multi-topology" address families, see
[LDP-MT] and [mLDP-MT].
[MVPN-WILDCARDS] specifies encodings and procedures that allow
"wildcards" to be specified in the NLRI of S-PMSI A-D routes. A set
of rules are given that specify when a customer multicast flow
"matches" a given S-PMSI A-D route whose NLRI contains wildcards.
However, the use of these wildcards is defined only for the case
where the customer is using PIM as its multicast control protocol.
The use of wildcards when the customer is using mLDP as its multicast
control protocol is outside the scope of this document.
5. IANA Considerations
[MVPN-BGP] does not create a registry for the allocation of new
MCAST-VPN Route Type values. In retrospect, it seems that it should
have done so. IANA has created a new registry called "BGP MCAST-VPN
Route Types", which references this document and [MVPN-BGP]. The
registry has been created under the top-level registry: "Border
Gateway Protocol (BGP) Parameters"
<http://www.iana.org/assignments/bgp-parameters>. The allocation
policy is "Standards Action". Values may be assigned from one of
- Range 0x01-0x3f: Generic/PIM Range. Values are assigned from this
range when the NLRI format associated with the route type
presupposes that PIM or IGMP is the C-multicast control protocol
or when the NLRI format associated with the route type is
independent of the C-multicast control protocol.
- Range 0x43-0x7f: mLDP Range. Values are assigned from this range
when the NLRI format associated with the route type presupposes
that mLDP is the C-multicast control protocol.
- Range 0x80-0xff: This range is reserved; values should not be
assigned from this range.
In general, whenever an assignment is requested from this registry,
two codepoints should be requested at the same time: one from the
Generic/PIM range and one from the mLDP range. The two codepoints
should have the same low-order 6 bits. If one of the two codepoints
is not actually needed, it should be registered anyway and marked as
The "BGP MCAST-VPN Route Types" contains the following initial
Value Meaning Reference
--------- ---------------------------------- -------------------
0x00 Reserved This RFC
0x01 Intra-AS I-PMSI A-D route This RFC, [RFC6514]
0x02 Inter-AS I-PMSI A-D route This RFC, [RFC6514]
0x03 S-PMSI A-D route This RFC, [RFC6514]
0x04 Leaf A-D route This RFC, [RFC6514]
0x05 Source Active A-D route This RFC, [RFC6514]
0x06 Shared Tree Join route This RFC, [RFC6514]
0x07 Source Tree Join route This RFC, [RFC6514]
0x08-0x3f Unassigned (Generic/PIM range) This RFC
0x40-0x42 Reserved This RFC
0x43 S-PMSI A-D route for
C-multicast mLDP This RFC
0x44 Leaf A-D route for C-multicast mLDP This RFC
0x45-0x46 Reserved This RFC
0x47 Source Tree Join route for
C-multicast mLDP This RFC
0x48-0x7f Unassigned (mLDP range) This RFC
0x80-0xff Reserved This RFC
6. Security Considerations
This document specifies a method of encoding an mLDP FEC Element in
the NLRI of some of the BGP Update messages that are specified in
[MVPN-BGP]. The security considerations of [mLDP] and of [MVPN-BGP]
are applicable, but no new security considerations are raised.
[PIM] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006,
Rekhter, Y., Aggarwal, R., Morin, T., Grosclaude, I.,
Leymann, N., and S. Saad, "Inter-Area P2MP Segmented
LSPs", Work in Progress, draft-ietf-mpls-seamless-
mcast-14, June 2014.
The authors wish to thank Pradosh Mohapatra and Saquib Najam for
their ideas and comments. We also thank Yakov Rekhter and Martin
Vigoureux for their comments.
Cisco Systems, Inc.
De kleetlaan 6a Diegem 1831
Eric C. Rosen
Juniper Networks, Inc.
10 Technology Park Drive
Westford, MA 01886