Link State Vector Routing Working Group J. Dong
Internet-Draft J. Chen
Intended status: Informational S. Fang
Expires: 4 September 2025 Huawei Technologies
3 March 2025
Proposed Update to BGP Link-State SPF NLRI Selection Rules
draft-dong-lsvr-bgp-spf-selection-02
Abstract
For network scenarios such as Massively Scaled Data Centers (MSDCs),
BGP is extended for Link-State (LS) distribution and the Shortest
Path First (SPF) algorithm based calculation. BGP-LS-SPF leverages
the mechanisms of both BGP protocol and BGP-LS protocol extensions,
with new selection rules defined for BGP-LS-SPF NLRI. This document
proposes some updates to the BGP-LS-SPF NLRI selection rules, so as
to improve the route updates and convergence, while consistent SPF
computation result can still be achieved. This document updates the
NLRI selection rules in I-D.ietf-lsvr-bgp-spf.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Network Scenarios Which Triggered This Update . . . . . . . . 3
2.1. Unnecessary Redundant Advertisement . . . . . . . . . . . 3
2.2. Parallal BGP-LS-SPF Peers . . . . . . . . . . . . . . . . 3
3. Update to BGP-LS-SPF Selection Rules . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
For network scenarios such as Massively Scaled Data Centers (MSDCs),
BGP is extended for Link-State (LS) distribution and the Shortest
Path First (SPF) algorithm based calculation. BGP-LS-SPF leverages
the mechanisms of both BGP protocol and BGP-LS protocol extensions,
with new selection rules for BGP-LS-SPF NLRI defined in
[I-D.ietf-lsvr-bgp-spf]. For all BGP-LS-SPF NLRIs, the NLRI
selection rules are defined as below:
1. NLRI originated by directly connected BGP SPF peers are
preferred.
2. The NLRI with the most recent Sequence Number TLV, i.e., highest
sequence number is selected.
3. The NLRI received from the BGP SPF speaker with the numerically
larger BGP Identifier is preferred.
In some network scenarios, these rules may not be enough to provide
optimized route convergence. This document proposes some updates to
the BGP-LS-SPF NLRI selection rules, so as to improve the route
convergence while consistent SPF computation result can still be
achieved.
This document firstly describes the network scenarios in which the
existing NLRI selection rules are considered not enough. Then it
provides suggested updates to the BGP-LS-SPF NLRI selection rules.
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2. Network Scenarios Which Triggered This Update
2.1. Unnecessary Redundant Advertisement
According to the rules in [I-D.ietf-lsvr-bgp-spf], for the BGP-LS-SPF
NLRIs with the same sequence number, the NLRI received from the
numerically larger BGP ID is preferred. While in some cases, this
may cause unnecessary redundant advertisement of the same NLRI.
+----+ new +----+ +----+ +----+
| R6 +-------+ R1 +---------+ R2 +-------+ R5 |
+----+ +-+--+ +-+--+ +----+
| |
| |
| |
| |
| |
+-+--+ +-+--+
| R3 +---------+ R4 |
+----+ +----+
As shown in the example in Figure 2, a new BGP session is established
between R1 and R6, and R1 advertise the link NLRI of R1-R6 to its
neighboring nodes (R2 and R3). R2 firstly receives the link NLRI
R1-R6 from R1 directly, and advertise it further to its neighbors (R4
and R5). R4 receives the link NLRI of R1-R6 with the same sequence
number from both R3 and R2, and according to the NLRI selection
rules, R4 would prefer the NLRI received from R3 according to the
rule of numerically larger BGP ID, then R4 advertises this link NLRI
of R1-R6 to R2. R2 would also prefer the NLRI received from R4
according to the rule of numerically larger BGP ID, and further
advertises this link NLRI to R5, which is a redundant advertisement
of its previous advertisement of the same link NLRI.
2.2. Parallal BGP-LS-SPF Peers
In some scenarios, BGP single-hop peering model is used between
directly connected BGP nodes. When two or more parallel links exists
between the BGP nodes, multiple BGP sessions are established between
the peering nodes, and each session will be used for the distribution
of BGP-LS-SPF NLRIs.
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parallel BGP sessions
+----+ +----+ +----+ +----+
| | | +---------+ | | |
| R3 +-------+ R1 +---------+ R2 +-------+ R4 |
+----+ +-+--+ +-+--+ +----+
As shown in the example of Figure 3, there are two parallel links
between R1 and R2, and a separate BGP session is established on each
link. Based on the existing BGP-LS-SPF NLRI selection rules, from
R2's perspective, for the same NLRI with the same sequence number,
either the route received from peer R1.1, or the route received from
peer R1.2 may be selected as the best. No matter what tie-breaking
rule is used, depending on the order of the routes received from R1,
R2 may need to advertise duplicated NLRIs to R4.
3. Update to BGP-LS-SPF Selection Rules
As the BGP-LS-SPF NLRIs are used to distribute the link-state
information, which are then used for the SPF computation, BGP
attributes which are used in BGP best path selection (e.g. AS_PATH)
for BGP address families other than BGP-LS are not considered in the
computation of BGP-LS-SPF. The consistency of BGP-LS-SPF computation
result only relies on the sequence number associated with the BGP-LS-
SPF NLRIs. For network scenarios where optimized route convergence
is more desirable, route updates due to the changes in BGP attributes
which are not considered in the SPF computation, although may help to
achieve deterministic NLRI selection, is considered not quite
necessary.
Thus this document proposes to update the selection rules for all
BGP-LS-SPF NLRI as follows:
1. NLRI originated by directly connected BGP SPF peers SHOULD be
preferred.
2. The NLRI with the most recent Sequence Number TLV, i.e., highest
sequence number SHOULD be selected.
3. NLRI received from a BGP-LS-SPF peer with the same sequence
number as the one of the current selected NLRI SHOULD not be
selected.
The new rule 3 can help to solve the duplicated advertisement problem
as described in section 2.
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4. IANA Considerations
This document makes no request of IANA.
5. Security Considerations
The mechanism described in this document provide updates to the NLRI
selection rules for BGP-LS-SPF. It does not introduce any additional
security considerations than those described in [RFC4271] and
[RFC4272].
6. Acknowledgements
The authors would like to thank Haibo Wang, Jun Ge and Li Zhang for
the valuable discussion and suggestions.
7. References
7.1. Normative References
[I-D.ietf-lsvr-bgp-spf]
Patel, K., Lindem, A., Zandi, S., and W. Henderickx, "BGP
Link-State Shortest Path First (SPF) Routing", Work in
Progress, Internet-Draft, draft-ietf-lsvr-bgp-spf-51, 23
January 2025, <https://datatracker.ietf.org/doc/html/
draft-ietf-lsvr-bgp-spf-51>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
7.2. Informative References
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
Authors' Addresses
Jie Dong
Huawei Technologies
China
Email: jie.dong@huawei.com
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Jinqiang Chen
Huawei Technologies
China
Email: chenjinqiang@huawei.com
Sheng Fang
Huawei Technologies
China
Email: fangsheng@huawei.com
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