Internet DRAFT - draft-dong-lsvr-bgp-spf-selection
draft-dong-lsvr-bgp-spf-selection
Link State Vector Routing Working Group J. Dong
Internet-Draft J. Chen
Intended status: Informational S. Fang
Expires: 25 April 2024 Huawei Technologies
23 October 2023
Proposed Update to BGP Link-State SPF NLRI Selection Rules
draft-dong-lsvr-bgp-spf-selection-00
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 update to the BGP-LS-SPF NLRI selection rules, so as to
ensure a deterministic selection result. The proposed update can
also help to mitigate some issues in BGP-LS-SPF route convergence.
This document updates the NLRI selection rules in I-D.ietf-lsvr-bgp-
spf.
Status of This Memo
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This Internet-Draft will expire on 25 April 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Network Scenarios Which Triggered This Update . . . . . . . . 3
2.1. Delayed Convergence during Link Failure . . . . . . . . . 3
2.2. Unnecessary Redundant Advertisement . . . . . . . . . . . 4
2.3. Parallal BGP-LS-SPF Peers . . . . . . . . . . . . . . . . 4
3. Update to BGP-LS-SPF Selection Rules . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
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 cases, these rules may not be enough to provide deterministic
selection result. And in some failure cases, these rules may cause
the distribution of the latest link-state information be delayed,
which would result in delayed route convergence in the network.
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This document firstly describes the network scenarios in which the
existing NLRI selection rules are considered not enough. Then some
updates to the BGP-LS-SPF NLRI selection rules are proposed.
2. Network Scenarios Which Triggered This Update
2.1. Delayed Convergence during Link Failure
Section 6.5.2 of [I-D.ietf-lsvr-bgp-spf] describes the NLRI
advertisement in case of node failures. While in some cases, route
convergence can be delayed due to the current NLRI selection rules.
+-----+ +-----+ link down +-----+ +-----+
| R1 +---------+ R2 +------X------+ R3 +--------+ R5 |
+-----+ +--\--+ +--/--+ +-----+
\ /
R1-R2: down to up \ /
\ /
\ /
\ /
\+-----+/
| R4 |
+--+--+
|
|
|
|
+--+--+
| R6 |
+-----+
As shown in the example in Figure 1, a failure of BGP session between
R2 and R3 is detected by R3, using either BFD or other detection
mechanisms. Since R2 cannot distinguish whether it is a node failure
of R2, or a link failure of R2-R3, in order to avoid unnecessary
route flaps, according to the description in Section 6.5.2 of
[I-D.ietf-lsvr-bgp-spf], R3 will hold all the NLRIs received from R1
for the period of NLRIImplicitWithdrawalDelay. During this period,
if the state of link R1-R2 change from down to up, an updated link
NLRI of R1-R2 with a greater sequence number would be originated by
R2 and advertised to its neighboring nodes. Due to the failure of
R2-R3, R3 cannot receive the updated link NLRI directly from R2,
while R3 can receive the updated link NLRI of R1-R2 with a greater
sequence number from R4. However, according to the NLRI selection
rule, R3 would prefer the link NLRI of R1-R2 directly received from
R2, thus R3 would not consider the link NLRI R1-R2 received from R4
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as the latest one. Consequently, R3 will not use the latest link
NLRI of R1-R2 for SPF computation, nor it will advertise the latest
link NLRI of R1-R2 to its neighbors. This would cause delayed
convergence of the network.
2.2. 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.
2.3. 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. To facilitate network
operation and troubleshooting, it is preferable to have a
deterministic result of NLRI selection once the network enters
relative stable state. Thus some rules to select the preferred NLRI
among parallel peering sessions is needed.
3. Update to BGP-LS-SPF Selection Rules
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. For NLRIs received from EBGP peers, the NLRI with smaller number
of AS numbers in the AS_PATH attribute SHOULD be preferred.
4. For NLRIs received from IBGP peers, the NLRI with smaller number
of Cluster IDs in the CLUSTER_LIST attributes SHOULD be
preferred.
5. The NLRI received from the BGP SPF speaker with the numerically
larger BGP Identifier SHOULD be preferred.
6. NLRI received from the BGP SPF peer with the smaller peer address
SHOULD be preferred.
The new rule 3 and 4 is to solve the duplicated advertisement problem
as described in section 2.2. The new rule 6 is to solve the
indeterministic selection problem as described in section 2.3.
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For the problem illustrated in Section 2.1, there are several options
to solve it, the details will be discussed further and documented in
a future version of this document.
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-28, 29
August 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-lsvr-bgp-spf-28>.
[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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