IDR Working Group C. Lin
Internet Draft New H3C Technologies
Updates: 9085, 9086 (if approved) Z. Li
Intended status: Standards Track China Mobile
Expires: March 02, 2025 R. Pang
China Unicom
K. Talaulikar
Cisco Systems
M. Chen
New H3C Technologies
September 02, 2024
Segment Routing BGP Egress Peer Engineering over Layer 2 Bundle
Members
draft-ietf-idr-sr-epe-over-l2bundle-00
Abstract
There are deployments where the Layer 3 interface on which a BGP
peer session is established is a Layer 2 interface bundle. In order
to allow BGP-EPE to control traffic flows on individual member links
of the underlying Layer 2 bundle, BGP Peering SIDs need to be
allocated to individual bundle member links, and advertisement of
such BGP Peering SIDs in BGP-LS is required. This document describes
how to support Segment Routing BGP Egress Peer Engineering over
Layer 2 bundle members. This document updates [RFC9085] to allow the
L2 Bundle Member Attributes TLV to be added to the BGP-LS Attribute
associated with the Link NLRI of BGP peering link. This document
updates [RFC9085] and [RFC9086] to allow the PeerAdj SID TLV to be
included as a sub-TLV of the L2 Bundle Member Attributes TLV.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 02, 2025.
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Copyright Notice
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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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warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction...................................................3
1.1. Requirements Language.....................................4
2. Problem Statement..............................................4
3. Advertising Peer Adjacency Segment for L2 Bundle Member in BGP-LS
..................................................................5
3.1. SR-MPLS...................................................5
3.2. SRv6......................................................6
4. Manageability Considerations...................................7
5. MC-LAG Bundles Considerations..................................7
6. Security Considerations........................................8
7. IANA Considerations............................................8
8. References.....................................................8
8.1. Normative References......................................8
8.2. Informative References....................................9
Appendix A. Example...............................................9
Acknowledgements.................................................11
Authors' Addresses...............................................11
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1. Introduction
Segment Routing (SR) leverages the source routing paradigm. A node
steers a packet through an ordered list of instructions called
"segments". Segment Routing can be instantiated on both MPLS and
IPv6 data planes, which are referred to as SR-MPLS and SRv6.
BGP Egress Peer Engineering (BGP-EPE) allows an ingress Provider
Edge (PE) router within the domain to use a specific egress PE and a
specific external interface/neighbor to reach a particular
destination.
The SR architecture [RFC8402] defines three types of BGP Peering
Segments that may be instantiated at a BGP node:
o Peer Node Segment (PeerNode SID): instruction to steer to a
specific peer node
o Peer Adjacency Segment (PeerAdj SID): instruction to steer over a
specific local interface towards a specific peer node
o Peer Set Segment (PeerSet SID): instruction to load-balance to a
set of specific peer nodes
[RFC9087] illustrates a centralized controller-based BGP-EPE
solution involving SR path computation using the BGP Peering
Segments. A centralized controller learns the BGP Peering SIDs via
Border Gateway Protocol - Link State (BGP-LS) and then uses this
information to program a BGP-EPE policy. [RFC9086] defines the
extension to BGP-LS for advertisement of BGP Peering Segments along
with their BGP peering node information.
There are deployments where the Layer 3 interface on which a BGP
peer session is established is a Layer 2 interface bundle (L2
Bundle), for instance, a Link Aggregation Group (LAG) [IEEE802.1AX].
BGP-EPE may wish to control traffic flows on individual member links
of the underlying Layer 2 bundle. In order to do so, BGP Peering
SIDs need to be allocated to individual bundle member links, and
advertisement of such BGP Peering SIDs in BGP-LS is required.
This document describes how to support Segment Routing BGP Egress
Peer Engineering over Layer 2 bundle members.
This document updates [RFC9085] to allow the L2 Bundle Member
Attributes TLV to be added to the BGP-LS Attribute associated with
the Link NLRI of BGP peering link. This document updates [RFC9085]
and [RFC9086] to allow the PeerAdj SID TLV to be included as a sub-
TLV of the L2 Bundle Member Attributes TLV.
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1.1. 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.
2. Problem Statement
In the network depicted in Figure 1, B and C establish BGP peer
session on a Layer 2 bundle. Assume that, the member link 1 has the
largest available bandwidth. The operator of AS1 wishes to apply a
BGP-EPE policy to steer certain flows from AS1 to AS2 via member
link 1 of the Layer 2 bundle to ensure there is no over-
subscription.
L2 Bundle +--------+
/---member 1---\ | |
--+---member 2---+--C AS2 |
+--------+ / \---member 3---/ | |
| |/ +--------+
A AS1 B
| |\ +--------+
+--------+ \ | |
--------------------D AS3 |
| |
+--------+
Figure 1: BGP-EPE over L2 Bundle
The existing Peer Adjacency SID can be allocated to the Layer 3
interface between B and C, which is a Layer 2 interface bundle. If
steered by that Peer Adjacency SID, the traffic will be forwarded by
load balancing among all the bundle member links. So, the existing
mechanism cannot meet the requirement of steering traffic flows via
individual member link.
In order to support BGP Egress Peer Engineering over Layer 2 bundle
members, a BGP router needs to have the ability to assign Peer
Adjacency Segments for member links. And, the Peer Adjacency
Segments of bundle members need to be advertised in BGP-LS, which
will be specified in this document.
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3. Advertising Peer Adjacency Segment for L2 Bundle Member in BGP-LS
BGP peering segments are generally advertised in BGP-LS from a BGP
node along with its peering topology information, in order to enable
computation of BGP-EPE policies.
When a BGP peer session is established over a Layer 2 interface
bundle, an implementation MAY allocate one or more Peer Adjacency
Segments for each member link. If so, it SHOULD advertise the Peer
Adjacency Segments of bundle members in BGP-LS, using the method
defined in this section.
In order to advertise the EPE Peer Adjacency SIDs for L2 bundle
members in BGP-LS, the L2 Bundle Member Attributes TLVs [RFC9085]
MUST also be included in the Link Attributes for the BGP-LS Link
NLRI corresponding to the BGP peering session.
Section 2.2 of [RFC9085] restricted that the L2 Bundle Member
Attributes TLV "should only be added to the BGP-LS Attribute
associated with the Link NLRI that describes the link of the IGP
node". This document updates [RFC9085] to allow the L2 Bundle Member
Attributes TLV to be added to the BGP-LS Attribute associated with
the Link NLRI of BGP peering link.
Each L2 Bundle Member Attributes TLV identifies an L2 bundle member,
and includes the EPE Peer Adjacency SID for the associated L2 bundle
member.
Note that the inclusion of a L2 Bundle Member Attributes TLV implies
that the identified link is a member of the L2 bundle and that the
member link is operationally up. If any member link fails, an
implementation MUST withdraw the L2 Bundle Member Attributes TLV in
BGP-LS, along with the Peer Adjacency Segments for the failed member
link.
3.1. SR-MPLS
For SR-MPLS, Section 5 of [RFC9086] defined the PeerAdj SID TLV and
its usage for the BGP-LS advertisement of the BGP-EPE PeerAdj SID
for L3 link. When advertising the SR-MPLS BGP-EPE Peer Adjacency
SIDs for L2 bundle members, the PeerAdj SID TLV [RFC9086] MUST be
carried in the L2 Bundle Member Attributes TLV to advertise the SR-
MPLS Peer Adjacency SID for the associated L2 bundle member. This
document updates [RFC9085] and [RFC9086] to allow the PeerAdj SID
TLV to be included as a sub-TLV of the L2 Bundle Member Attributes
TLV.
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When advertising SR-MPLS BGP-EPE Peer Adjacency SIDs for L2 bundle
members, since L2 bundle information is considered a Layer 3 link
attribute, it must be advertised in the BGP-LS Link NLRI. The
details for LINK NLRI are the same as those for the PeerAdj SID, as
described in Section 5.2 of [RFC9086]. This information mustnot be
included in the BGP-LS Link NLRI that corresponds to the PeerNode
SID, as defined in Section 5.1 of [RFC9086].
Note that for directly connected EBGP neighbors, if a BGP neighbor
is established over an L2 Bundle, an additional BGP-LS Link NLRI(as
described in Section 5.2 of [RFC9086]) must be generated to
advertise Peer Link information when generating the BGP-LS Link NLRI
(as described in Section 5.1 of [RFC9086]) corresponding to the
PeerNode SID. The L2 Bundle Member Attributes TLV should be included
under the BGP-LS Link Attribute TLVs.
The SR-MPLS BGP-EPE Peer Adjacency SIDs for L2 bundle members are
advertised with a BGP-LS Link NLRI, where:
* BGP-LS Link NLRI: as described in Section 5.2 of [RFC9086].
* Link Attribute TLVs:
- include the PeerAdj SID TLV [RFC9086] for Peer Link(Optional)
- include the L2 Bundle Member Attributes TLV.
o include the PeerAdj SID TLV [RFC9086] for each L2 Bundle
Member.
3.2. SRv6
For SRv6, according to Section 4.1 of [RFC9514], the SRv6 End.X SID
TLV is used for the advertisement of L3 link BGP EPE Peer Adjacency
SID. When advertising the SRv6 BGP-EPE Peer Adjacency SIDs for L2
bundle members, the SRv6 End.X SID TLV [RFC9514] MUST be carried in
the L2 Bundle Member Attributes TLV to advertise the SRv6 Peer
Adjacency SID for the associated L2 bundle member.
Note Appendix A of [RFC 9514], SRv6 BGP PeerNode is no longer
advertised as BGP LINK NLRI. When advertising SRv6 BGP-EPE Peer
Adjacency SIDs for L2 bundle members, since L2 bundle information is
considered a Layer 3 link attribute, it must be advertised in the
BGP-LS Link NLRI. The details for LINK NLRI are the same as those
for the Peer Adjacency SID, as described in Section 5.2 of
[RFC9086].
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The SRv6 BGP-EPE Peer Adjacency SIDs for L2 bundle members are
advertised with a BGP-LS Link NLRI, where:
* BGP-LS Link NLRI: as described in Section 5.2 of [RFC9086].
* Link Attribute TLV:
- include the SRv6 End.X SID TLV [RFC9514] for Peer
Link (Optional).
- include the L2 Bundle Member Attributes TLV.
o include the SRv6 End.X SID TLV [RFC9514] for each L2 Bundle
Member.
4. Manageability Considerations
The manageability considerations described in [RFC9552] and
[RFC9086] also apply to this document.
The operator MUST be provided with the options of configuring,
enabling, and disabling the advertisement of Peer Adjacency Segment
for L2 Bundle member links, as well as control of which information
is advertised to which internal or external peer.
5. MC-LAG Bundles Considerations
In environments where MC-LAG (Multi-Chassis Link Aggregation Group)
bundles are deployed across multiple devices, it is critical to
implement mechanisms to prevent Broadcast, Unknown Unicast, and
Multicast (BUM) traffic from looping and ensure a loop-free network.
The following loop prevention mechanisms are included:
o Split Horizon Forwarding: Each MC-LAG device maintains a split
horizon rule where it does not forward BUM traffic received from
one MC-LAG member port to another MC-LAG member port. This
prevents BUM frames from being forwarded back into the MC-LAG,
creating loops.
o Designated Forwarder Election: In a typical MC-LAG configuration,
one device is elected as the designated forwarder for BUM
traffic. This ensures that only one device is responsible for
forwarding BUM frames, preventing the possibility of multiple
devices forwarding the same frame simultaneously and causing a
loop.
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o Consistent Hashing Algorithms: MC-LAG devices employ consistent
hashing algorithms to ensure that traffic distribution across
member links is stable and predictable. This minimizes the risk
of reordering and helps in effective loop prevention.
By incorporating these mechanisms, MC-LAG deployments can
effectively prevent BUM traffic from looping and ensure a stable,
loop-free network.
6. Security Considerations
The security considerations described in [RFC9552] and [RFC9086]
also apply to this document.
This document does not introduce any new security consideration.
7. IANA Considerations
This document has no IANA actions.
8. References
8.1. 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>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[RFC9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085, DOI
10.17487/RFC9085, August 2021, <https://www.rfc-
editor.org/info/rfc9085>.
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[RFC9086] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Patel, K.,
Ray, S., and J. Dong, "Border Gateway Protocol - Link
State (BGP-LS) Extensions for Segment Routing BGP Egress
Peer Engineering", RFC 9086, DOI 10.17487/RFC9086, August
2021, <https://www.rfc-editor.org/info/rfc9086>.
[RFC9514] Dawra, G., Filsfils, C., Talaulikar, K., Ed., Chen, M.,
Bernier, D., and B. Decraene, "Border Gateway Protocol -
Link State (BGP-LS) Extensions for Segment Routing over
IPv6 (SRv6)", RFC 9514, DOI 10.17487/RFC9514, December
2023, <https://www.rfc-editor.org/info/rfc9514>.
[RFC9552] K. Talaulikar, "Distribution of Link-State and Traffic
Engineering Information Using BGP", RFC 9552, DOI
10.17487/RFC9552, December 2023, <https://www.rfc-
editor.org/info/rfc9552>.
8.2. Informative References
[IEEE802.1AX] IEEE, "IEEE Standard for Local and metropolitan area
networks -- Link Aggregation", IEEE 802.1AX,
<https://ieeexplore.ieee.org/document/7055197>.
[RFC8668] Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
M., and E. Aries, "Advertising Layer 2 Bundle Member Link
Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
December 2019, <https://www.rfc-editor.org/info/rfc8668>.
[RFC9087] Filsfils, C., Ed., Previdi, S., Dawra, G., Ed., Aries, E.,
and D. Afanasiev, "Segment Routing Centralized BGP Egress
Peer Engineering", RFC 9087, DOI 10.17487/RFC9087, August
2021, <https://www.rfc-editor.org/info/rfc9087>.
Appendix A. Example
This section shows an example of how Node B in Figure 1 allocates
and advertises Peer Adjacency Segments for L2 bundle members.
B allocates a PeerAdj SID for the Layer 2 interface bundle to peer
C, along with a PeerAdj SID for each member link. B programs its
forwarding table accordingly:
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+===============================+====================+
| PeerAdj SID | Outgoing Interface |
+---------------+---------------+ |
| IF on SR-MPLS | IF on SRv6 | |
| Data Plane | Data Plane | |
+===============+===============+====================+
| 1010 | A::A0 | L2 Bundle to C |
+---------------+---------------+--------------------+
| 1011 | A::A1 | Member link 1 to C |
+---------------+---------------+--------------------+
| 1012 | A::A2 | Member link 2 to C |
+---------------+---------------+--------------------+
| 1013 | A::A3 | Member link 3 to C |
+---------------+---------------+--------------------+
B signals the related BGP-LS Link NLRI and Link Attributes including
the PeerAdj SID for L3 parent link to the BGP-EPE controller, as
specified in Section 5.2 of [RFC9086]. In addition, B also
advertises L2 Bundle Member Attribute TLVs carrying the PeerAdj SIDs
for L2 bundle members.
For SR-MPLS, the Link Attributes are as follows:
o PeerAdj SID TLV (Label-1010)
o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 1)
* PeerAdj SID TLV (Label-1011)
o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 2)
* PeerAdj SID TLV (Label-1012)
o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 3)
* PeerAdj SID TLV (Label-1013)
For SRv6, the Link Attributes are as follows:
o SRv6 End.X SID TLV (SID-A::A0)
o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 1)
* SRv6 End.X SID TLV (SID-A::A1)
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o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 2)
* SRv6 End.X SID TLV (SID-A::A2)
o L2 Bundle Member Attribute TLV (Link Local Identifier describing
the member link 3)
* SRv6 End.X SID TLV (SID-A::A3)
Acknowledgements
Many thanks to Sasha Vainshtein, Acee Lindem, Chen Ran, Liyan Gong,
Yongqing Zhu, Lan cheng, Wisdom Tan, Yisong Liu, Libin Liu, Liu Yao,
Hongwei Li, Allan Michael, Huo Pengfei, Gyan Mishra, Dong Jie, Meng
Liu, etc. for their valuable comments on this document.
Authors' Addresses
Changwang Lin
New H3C Technologies
China
Email: linchangwang.04414@h3c.com
Zhenqiang Li
China Mobile
China
Email: lizhenqiang@chinamobile.com
Ran Pang
China Unicom
China
Email: pangran@chinaunicom.cn
Ketan Talaulikar
Cisco Systems
India
Email: ketant.ietf@gmail.com
Mengxiao Chen
New H3C Technologies
China
Email: chen.mengxiao@h3c.com
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