BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-16
| Document | Type | Active Internet-Draft (lsvr WG) | |
|---|---|---|---|
| Authors | Keyur Patel , Acee Lindem , Shawn Zandi , Wim Henderickx | ||
| Last updated | 2022-02-15 | ||
| Replaces | draft-keyupate-lsvr-bgp-spf | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | plain text html xml htmlized pdfized bibtex | ||
| Reviews |
RTGDIR Last Call review
(of
-13)
Has Issues
RTGDIR Early review
(of
-02)
Has Issues
OPSDIR Last Call Review
Incomplete, due 2021-06-17
|
||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Victor Kuarsingh | ||
| Shepherd write-up | Show Last changed 2020-05-26 | ||
| IESG | IESG state | I-D Exists (IESG: Dead) | |
| Action Holders |
(None)
|
||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Alvaro Retana | ||
| Send notices to | Gunter Van de Velde <gunter.van_de_velde@nokia.com>, aretana.ietf@gmail.com, victor@jvknet.com |
draft-ietf-lsvr-bgp-spf-16
Network Working Group K. Patel
Internet-Draft Arrcus, Inc.
Intended status: Standards Track A. Lindem
Expires: 19 August 2022 Cisco Systems
S. Zandi
LinkedIn
W. Henderickx
Nokia
15 February 2022
BGP Link-State Shortest Path First (SPF) Routing
draft-ietf-lsvr-bgp-spf-16
Abstract
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP as their single routing protocol for both their fabric routing
and their Data Center Interconnect (DCI) routing. This document
describes extensions to BGP to use BGP Link-State distribution and
the Shortest Path First (SPF) algorithm used by Internal Gateway
Protocols (IGPs) such as OSPF. In doing this, it allows BGP to be
efficiently used as both the underlay protocol and the overlay
protocol in MSDCs.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 19 August 2022.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
Patel, et al. Expires 19 August 2022 [Page 1]
Internet-Draft BGP Link-State SPF Routing February 2022
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. BGP Shortest Path First (SPF) Motivation . . . . . . . . 4
1.3. Document Overview . . . . . . . . . . . . . . . . . . . . 6
1.4. Requirements Language . . . . . . . . . . . . . . . . . . 6
2. Base BGP Protocol Relationship . . . . . . . . . . . . . . . 6
3. BGP Link-State (BGP-LS) Relationship . . . . . . . . . . . . 7
4. BGP Peering Models . . . . . . . . . . . . . . . . . . . . . 8
4.1. BGP Single-Hop Peering on Network Node Connections . . . 8
4.2. BGP Peering Between Directly-Connected Nodes . . . . . . 8
4.3. BGP Peering in Route-Reflector or Controller Topology . . 9
5. BGP Shortest Path Routing (SPF) Protocol Extensions . . . . . 9
5.1. BGP-LS Shortest Path Routing (SPF) SAFI . . . . . . . . . 9
5.1.1. BGP-LS-SPF NLRI TLVs . . . . . . . . . . . . . . . . 9
5.1.2. BGP-LS Attribute . . . . . . . . . . . . . . . . . . 10
5.2. Extensions to BGP-LS . . . . . . . . . . . . . . . . . . 11
5.2.1. Node NLRI Usage . . . . . . . . . . . . . . . . . . . 11
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Capability
TLV . . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV . . 12
5.2.2. Link NLRI Usage . . . . . . . . . . . . . . . . . . . 13
5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length
TLVs . . . . . . . . . . . . . . . . . . . . . . . 14
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV . . 15
5.2.3. IPv4/IPv6 Prefix NLRI Usage . . . . . . . . . . . . . 16
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status
TLV . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2.4. BGP-LS Attribute Sequence-Number TLV . . . . . . . . 17
5.3. NEXT_HOP Manipulation . . . . . . . . . . . . . . . . . . 18
6. Decision Process with SPF Algorithm . . . . . . . . . . . . . 18
6.1. BGP NLRI Selection . . . . . . . . . . . . . . . . . . . 19
6.1.1. BGP Self-Originated NLRI . . . . . . . . . . . . . . 20
6.2. Dual Stack Support . . . . . . . . . . . . . . . . . . . 21
6.3. SPF Calculation based on BGP-LS-SPF NLRI . . . . . . . . 21
6.4. IPv4/IPv6 Unicast Address Family Interaction . . . . . . 26
6.5. NLRI Advertisement . . . . . . . . . . . . . . . . . . . 26
6.5.1. Link/Prefix Failure Convergence . . . . . . . . . . . 26
Patel, et al. Expires 19 August 2022 [Page 2]
Internet-Draft BGP Link-State SPF Routing February 2022
6.5.2. Node Failure Convergence . . . . . . . . . . . . . . 27
7. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 27
7.1. Processing of BGP-LS-SPF TLVs . . . . . . . . . . . . . . 27
7.2. Processing of BGP-LS-SPF NLRIs . . . . . . . . . . . . . 28
7.3. Processing of BGP-LS Attribute . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10. Management Considerations . . . . . . . . . . . . . . . . . . 32
10.1. Configuration . . . . . . . . . . . . . . . . . . . . . 32
10.1.1. Link Metric Configuration . . . . . . . . . . . . . 32
10.1.2. backoff-config . . . . . . . . . . . . . . . . . . . 32
10.2. Operational Data . . . . . . . . . . . . . . . . . . . . 33
11. Implementation Status . . . . . . . . . . . . . . . . . . . . 33
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 34
13. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . 34
14.2. Informational References . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
1. Introduction
Many Massively Scaled Data Centers (MSDCs) have converged on
simplified layer 3 routing. Furthermore, requirements for
operational simplicity have led many of these MSDCs to converge on
BGP [RFC4271] as their single routing protocol for both their fabric
routing and their Data Center Interconnect (DCI) routing [RFC7938].
This document describes an alternative solution which leverages BGP-
LS [RFC7752] and the Shortest Path First algorithm used by Internal
Gateway Protocols (IGPs) such as OSPF [RFC2328].
This document leverages both the BGP protocol [RFC4271] and the BGP-
LS [RFC7752] protocols. The relationship, as well as the scope of
changes are described respectively in Section 2 and Section 3. The
modifications to [RFC4271] for BGP SPF described herein only apply to
IPv4 and IPv6 as underlay unicast Subsequent Address Families
Identifiers (SAFIs). Operations for any other BGP SAFIs are outside
the scope of this document.
This solution avails the benefits of both BGP and SPF-based IGPs.
These include TCP based flow-control, no periodic link-state refresh,
and completely incremental NLRI advertisement. These advantages can
reduce the overhead in MSDCs where there is a high degree of Equal
Cost Multi-Path (ECMPs) and the topology is very stable.
Additionally, using an SPF-based computation can support fast
convergence and the computation of Loop-Free Alternatives (LFAs).
The SPF LFA extensions defined in [RFC5286] can be similarly applied
to BGP SPF calculations. However, the details are a matter of
Patel, et al. Expires 19 August 2022 [Page 3]
Internet-Draft BGP Link-State SPF Routing February 2022
implementation detail. Furthermore, a BGP-based solution lends
itself to multiple peering models including those incorporating
route-reflectors [RFC4456] or controllers.
1.1. Terminology
This specification reuses terms defined in section 1.1 of [RFC4271]
including BGP speaker, NLRI, and Route.
Additionally, this document introduces the following terms:
BGP SPF Routing Domain: A set of BGP routers that are under a single
administrative domain and exchange link-state information using
the BGP-LS-SPF SAFI and compute routes using BGP SPF as described
herein.
BGP-LS-SPF NLRI: This refers to BGP-LS Network Layer Reachability
Information (NLRI) that is being advertised in the BGP-LS-SPF SAFI
(Section 5.1) and is being used for BGP SPF route computation.
Dijkstra Algorithm: An algorithm for computing the shortest path
from a given node in a graph to every other node in the graph. At
each iteration of the algorithm, there is a list of candidate
vertices. Paths from the root to these vertices have been found,
but not necessarily the shortest ones. However, the paths to the
candidate vertex that is closest to the root are guaranteed to be
shortest; this vertex is added to the shortest-path tree, removed
from the candidate list, and its adjacent vertices are examined
for possible addition to/modification of the candidate list. The
algorithm then iterates again. It terminates when the candidate
list becomes empty. [RFC2328]
1.2. BGP Shortest Path First (SPF) Motivation
Given that [RFC7938] already describes how BGP could be used as the
sole routing protocol in an MSDC, one might question the motivation
for defining an alternate BGP deployment model when a mature solution
exists. For both alternatives, BGP offers the operational benefits
of a single routing protocol as opposed to the combination of an IGP
for the underlay and BGP as an overlay. However, BGP SPF offers some
unique advantages above and beyond standard BGP distance-vector
routing. With BGP SPF, the standard hop-by-hop peering model is
relaxed.
A primary advantage is that all BGP SPF speakers in the BGP SPF
routing domain will have a complete view of the topology. This will
allow support for ECMP, IP fast-reroute (e.g., Loop-Free
Alternatives), Shared Risk Link Groups (SRLGs), and other routing
Patel, et al. Expires 19 August 2022 [Page 4]
Internet-Draft BGP Link-State SPF Routing February 2022
enhancements without advertisement of additional BGP paths [RFC7911]
or other extensions. In short, the advantages of an IGP such as OSPF
[RFC2328] are availed in BGP.
With the simplified BGP decision process as defined in Section 6,
NLRI changes can be disseminated throughout the BGP routing domain
much more rapidly (equivalent to IGPs with the proper
implementation). The added advantage of BGP using TCP for reliable
transport leverages TCP's inherent flow-control and guaranteed in-
order delivery.
Another primary advantage is a potential reduction in NLRI
advertisement. With standard BGP distance-vector routing, a single
link failure may impact 100s or 1000s prefixes and result in the
withdrawal or re-advertisement of the attendant NLRI. With BGP SPF,
only the BGP SPF speakers corresponding to the link NLRI need to
withdraw the corresponding BGP-LS-SPF Link NLRI. Additionally, the
changed NLRI will be advertised immediately as opposed to normal BGP
where it is only advertised after the best route selection. These
advantages will afford NLRI dissemination throughout the BGP SPF
routing domain with efficiencies similar to link-state protocols.
With controller and route-reflector peering models, BGP SPF
advertisement and distributed computation require a minimal number of
sessions and copies of the NLRI since only the latest version of the
NLRI from the originator is required. Given that verification of the
adjacencies is done outside of BGP (see Section 4), each BGP SPF
speaker will only need as many sessions and copies of the NLRI as
required for redundancy (see Section 4). Additionally, a controller
could inject topology that is learned outside the BGP SPF routing
domain.
Given that controllers are already consuming BGP-LS NLRI [RFC7752],
this functionality can be reused for BGP-LS-SPF NLRI.
Another advantage of BGP SPF is that both IPv6 and IPv4 can be
supported using the BGP-LS-SPF SAFI with the same BGP-LS-SPF NLRIs.
In many MSDC fabrics, the IPv4 and IPv6 topologies are congruent,
refer to Section 5.2.2 and Section 5.2.3. Although beyond the scope
of this document, multi-topology extensions could be used to support
separate IPv4, IPv6, unicast, and multicast topologies while sharing
the same NLRI.
Finally, the BGP SPF topology can be used as an underlay for other
BGP SAFIs (using the existing model) and realize all the above
advantages.
Patel, et al. Expires 19 August 2022 [Page 5]
Internet-Draft BGP Link-State SPF Routing February 2022
1.3. Document Overview
The document begins with sections defining the precise relationship
that BGP SPF has with both the base BGP protocol [RFC4271]
(Section 2) and the BGP Link-State (BGP-LS) extensions [RFC7752]
(Section 3). This is required to dispel the notion that BGP SPF is
an independent protocol. The BGP peering models, as well as the
their respective trade-offs are then discussed in Section 4. The
remaining sections, which make up the bulk of the document, define
the protocol enhancements necessary to support BGP SPF. The BGP-LS
extensions to support BGP SPF are defined in Section 5. The
replacement of the base BGP decision process with the SPF computation
is specified in Section 6. Finally, BGP SPF error handling is
defined in Section 7
1.4. 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. Base BGP Protocol Relationship
With the exception of the decision process, the BGP SPF extensions
leverage the BGP protocol [RFC4271] without change. This includes
the BGP protocol Finite State Machine, BGP messages and their
encodings, processing of BGP messages, BGP attributes and path
attributes, BGP NLRI encodings, and any error handling defined in the
[RFC4271] and [RFC7606].
Due to the changes to the decision process, there are mechanisms and
encodings that are no longer applicable. While not necessarily
required for computation, the ORIGIN, AS_PATH, MULTI_EXIT_DISC,
LOCAL_PREF, and NEXT_HOP path attributes are mandatory and will be
validated. The ATOMIC_AGGEGATE, and AGGREGATOR are not applicable
within the context of BGP SPF and SHOULD NOT be advertised. However,
if they are advertised, they will be accepted, validated, and
propagated consistent with the BGP protocol.
Section 9 of [RFC4271] defines the decision process that is used to
select routes for subsequent advertisement by applying the policies
in the local Policy Information Base (PIB) to the routes stored in
its Adj-RIBs-In. The output of the Decision Process is the set of
routes that are announced by a BGP speaker to its peers. These
selected routes are stored by a BGP speaker in the speaker's Adj-
RIBs-Out according to policy.
Patel, et al. Expires 19 August 2022 [Page 6]
Internet-Draft BGP Link-State SPF Routing February 2022
The BGP SPF extension fundamentally changes the decision process, as
described herein, to be more like a link-state protocol (e.g., OSPF
[RFC2328]). Specifically:
1. BGP advertisements are readvertised to neighbors immediately
without waiting or dependence on the route computation as
specified in phase 3 of the base BGP decision process. Multiple
peering models are supported as specified in Section 4.
2. Determining the degree of preference for BGP routes for the SPF
calculation as described in phase 1 of the base BGP decision
process is replaced with the mechanisms in Section 6.1.
3. Phase 2 of the base BGP protocol decision process is replaced
with the Shortest Path First (SPF) algorithm, also known as the
Dijkstra algorithm Section 1.1.
3. BGP Link-State (BGP-LS) Relationship
[RFC7752] describes a mechanism by which link-state and TE
information can be collected from networks and shared with external
entities using BGP. This is achieved by defining NLRI advertised
using the BGP-LS AFI. The BGP-LS extensions defined in [RFC7752]
make use of the decision process defined in [RFC4271]. This document
reuses NLRI and TLVs defined in [RFC7752]. Rather than reusing the
BGP-LS SAFI, the BGP-LS-SPF SAFI Section 5.1 is introduced to insure
backward compatibility for the BGP-LS SAFI usage.
The BGP SPF extensions reuse the Node, Link, and Prefix NLRI defined
in [RFC7752]. The usage of the BGP-LS NLRI, attributes, and
attribute extensions is described in Section 5.2. The usage of
others BGP-LS attributes is not precluded and is, in fact, expected.
However, the details are beyond the scope of this document and will
be specified in future documents.
Support for Multiple Topology Routing (MTR) similar to the OSPF MTR
computation described in [RFC4915] is beyond the scope of this
document. Consequently, the usage of the Multi-Topology TLV as
described in section 3.2.1.5 of [RFC7752] is not specified.
The rules for setting the NLRI next-hop path attribute for the BGP-
LS-SPF SAFI will follow the BGP-LS SAFI as specified in section 3.4
of [RFC7752].
Patel, et al. Expires 19 August 2022 [Page 7]
Internet-Draft BGP Link-State SPF Routing February 2022
4. BGP Peering Models
Depending on the topology, scaling, capabilities of the BGP SPF
speakers, and redundancy requirements, various peering models are
supported. The only requirements are that all BGP SPF speakers in
the BGP SPF routing domain exchange BGP-LS-SPF NLRI, run an SPF
calculation, and update their routing table appropriately.
4.1. BGP Single-Hop Peering on Network Node Connections
The simplest peering model is the one where EBGP single-hop sessions
are established over direct point-to-point links interconnecting the
nodes in the BGP SPF routing domain. Once the single-hop BGP session
has been established and the BGP-LS-SPF AFI/SAFI capability has been
exchanged [RFC4760] for the corresponding session, then the link is
considered up from a BGP SPF perspective and the corresponding BGP-
LS-SPF Link NLRI is advertised. If the session goes down, the
corresponding Link NLRI will be withdrawn. Topologically, this would
be equivalent to the peering model in [RFC7938] where there is a BGP
session on every link in the data center switch fabric. The content
of the Link NLRI is described in Section 5.2.2.
4.2. BGP Peering Between Directly-Connected Nodes
In this model, BGP SPF speakers peer with all directly-connected
nodes but the sessions may be between loopback addresses (i.e., two-
hop sessions) and the direct connection discovery and liveliness
detection for the interconnecting links are independent of the BGP
protocol. For example, liveliness detection could be done using the
BFD protocol [RFC5880]. Precisely how discovery and liveliness
detection is accomplished is outside the scope of this document.
Consequently, there will be a single BGP session even if there are
multiple direct connections between BGP SPF speakers. BGP-LS-SPF
Link NLRI is advertised as long as a BGP session has been
established, the BGP-LS-SPF AFI/SAFI capability has been exchanged
[RFC4760], and the link is operational as determined using liveliness
detection mechanisms outside the scope of this document. This is
much like the previous peering model only peering is between loopback
addresses and the interconnecting links can be unnumbered. However,
since there are BGP sessions between every directly-connected node in
the BGP SPF routing domain, there is only a reduction in BGP sessions
when there are parallel links between nodes.
Patel, et al. Expires 19 August 2022 [Page 8]
Internet-Draft BGP Link-State SPF Routing February 2022
4.3. BGP Peering in Route-Reflector or Controller Topology
In this model, BGP SPF speakers peer solely with one or more Route
Reflectors [RFC4456] or controllers. As in the previous model,
direct connection discovery and liveliness detection for those links
in the BGP SPF routing domain are done outside of the BGP protocol.
BGP-LS-SPF Link NLRI is advertised as long as the corresponding link
is considered up as per the chosen liveness detection mechanism.
This peering model, known as sparse peering, allows for fewer BGP
sessions and, consequently, fewer instances of the same NLRI received
from multiple peers. Normally, the route-reflectors or controller
BGP sessions would be on directly-connected links to avoid dependence
on another routing protocol for session connectivity. However,
multi-hop peering is not precluded. The number of BGP sessions is
dependent on the redundancy requirements and the stability of the BGP
sessions. This is discussed in greater detail in
[I-D.ietf-lsvr-applicability].
5. BGP Shortest Path Routing (SPF) Protocol Extensions
5.1. BGP-LS Shortest Path Routing (SPF) SAFI
In order to replace the existing BGP decision process with an SPF-
based decision process in a backward compatible manner by not
impacting the BGP-LS SAFI, this document introduces the BGP-LS-SPF
SAFI. The BGP-LS-SPF (AFI 16388 / SAFI 80) [RFC4760] is allocated by
IANA as specified in the Section 8. In order for two BGP SPF
speakers to exchange BGP SPF NLRI, they MUST exchange the
Multiprotocol Extensions Capability [RFC5492] [RFC4760] to ensure
that they are both capable of properly processing such NLRI. This is
done with AFI 16388 / SAFI 80 for BGP-LS-SPF advertised within the
BGP SPF Routing Domain. The BGP-LS-SPF SAFI is used to carry IPv4
and IPv6 prefix information in a format facilitating an SPF-based
decision process.
5.1.1. BGP-LS-SPF NLRI TLVs
The NLRI format of BGP-LS-SPF SAFI uses exactly same format as the
BGP-LS AFI [RFC7752]. In other words, all the TLVs used in BGP-LS
AFI are applicable and used for the BGP-LS-SPF SAFI. These TLVs
within BGP-LS-SPF NLRI advertise information that describes links,
nodes, and prefixes comprising IGP link-state information.
In order to compare the NLRI efficiently, it is REQUIRED that all the
TLVs within the given NLRI must be ordered in ascending order by the
TLV type. For multiple TLVs of same type within a single NLRI, it is
REQUIRED that these TLVs are ordered in ascending order by the TLV
Patel, et al. Expires 19 August 2022 [Page 9]
Internet-Draft BGP Link-State SPF Routing February 2022
value field. Comparison of the value fields is performed by treating
the entire value field as a hexadecimal string. NLRIs having TLVs
which do not follow the ordering rules MUST be considered as
malformed and discarded with appropriate error logging.
[RFC7752] defines certain NLRI TLVs as a mandatory TLVs. These TLVs
are considered mandatory for the BGP-LS-SPF SAFI as well. All the
other TLVs are considered as an optional TLVs.
Given that there is a single BGP-LS Attribute for all the BGP-LS-SPF
NLRI in a BGP Update, Section 3.3, [RFC7752], a BGP Update will
normally contain a single BGP-LS-SPF NLRI since advertising multiple
NLRI would imply identical attributes.
5.1.2. BGP-LS Attribute
The BGP-LS attribute of the BGP-LS-SPF SAFI uses exactly same format
of the BGP-LS AFI [RFC7752]. In other words, all the TLVs used in
BGP-LS attribute of the BGP-LS AFI are applicable and used for the
BGP-LS attribute of the BGP-LS-SPF SAFI. This attribute is an
optional, non-transitive BGP attribute that is used to carry link,
node, and prefix properties and attributes. The BGP-LS attribute is
a set of TLVs.
The BGP-LS attribute may potentially grow large in size depending on
the amount of link-state information associated with a single Link-
State NLRI. The BGP specification [RFC4271] mandates a maximum BGP
message size of 4096 octets. It is RECOMMENDED that an
implementation support [RFC8654] in order to accommodate larger size
of information within the BGP-LS Attribute. BGP SPF speakers MUST
ensure that they limit the TLVs included in the BGP-LS Attribute to
ensure that a BGP update message for a single Link-State NLRI does
not cross the maximum limit for a BGP message. The determination of
the types of TLVs to be included by the BGP SPF speaker originating
the attribute is outside the scope of this document. When a BGP SPF
speaker finds that it is exceeding the maximum BGP message size due
to addition or update of some other BGP Attribute (e.g., AS_PATH), it
MUST consider the BGP-LS Attribute to be malformed and the attribute
discard handling of [RFC7606] applies.
In order to compare the BGP-LS attribute efficiently, it is REQUIRED
that all the TLVs within the given attribute must be ordered in
ascending order by the TLV type. For multiple TLVs of same type
within a single attribute, it is REQUIRED that these TLVs are ordered
in ascending order by the TLV value field. Comparison of the value
fields is performed by treating the entire value field as a
hexadecimal string. Attributes having TLVs which do not follow the
ordering rules MUST NOT be considered as malformed.
Patel, et al. Expires 19 August 2022 [Page 10]
Internet-Draft BGP Link-State SPF Routing February 2022
All TLVs within the BGP-LS Attribute are considered optional unless
specified otherwise.
5.2. Extensions to BGP-LS
[RFC7752] describes a mechanism by which link-state and TE
information can be collected from IGPs and shared with external
components using the BGP protocol. It describes both the definition
of the BGP-LS NLRI that advertise links, nodes, and prefixes
comprising IGP link-state information and the definition of a BGP
path attribute (BGP-LS attribute) that carries link, node, and prefix
properties and attributes, such as the link and prefix metric or
auxiliary Router-IDs of nodes, etc. This document extends the usage
of BGP-LS NLRI for the purpose of BGP SPF calculation via
advertisement in the BGP-LS-SPF SAFI.
The protocol identifier specified in the Protocol-ID field [RFC7752]
will represent the origin of the advertised NLRI. For Node NLRI and
Link NLRI, this MUST be the direct protocol (4). Node or Link NLRI
with a Protocol-ID other than direct will be considered malformed.
For Prefix NLRI, the specified Protocol-ID MUST be the origin of the
prefix. The local and remote node descriptors for all NLRI MUST
include the BGP Identifier (TLV 516) and the AS Number (TLV 512)
[RFC7752]. The BGP Confederation Member (TLV 517) [RFC7752] is not
appliable and SHOULD not be included. If TLV 517 is included, it
will be ignored.
5.2.1. Node NLRI Usage
The Node NLRI MUST be advertised unconditionally by all routers in
the BGP SPF routing domain.
5.2.1.1. BGP-LS-SPF Node NLRI Attribute SPF Capability TLV
The SPF capability is an additional Node Attribute TLV. This
attribute TLV MUST be included with the BGP-LS-SPF SAFI and SHOULD
NOT be used for other SAFIs. The TLV type 1180 will be assigned by
IANA. The Node Attribute TLV will contain a single-octet SPF
algorithm as defined in [RFC8665].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1180) | Length - (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Algorithm |
+-+-+-+-+-+-+-+-+
Patel, et al. Expires 19 August 2022 [Page 11]
Internet-Draft BGP Link-State SPF Routing February 2022
The SPF algorithm inherits the values from the IGP Algorithm Types
registry [RFC8665]. Algorithm 0, (Shortest Path Algorithm (SPF)
based on link metric, is supported and described in Section 6.3.
Support for other algorithm types is beyond the scope of this
specification.
When computing the SPF for a given BGP routing domain, only BGP nodes
advertising the SPF capability TLV with same SPF algorithm will be
included in the Shortest Path Tree (SPT) Section 6.3. An
implementation MAY optionally log detection of a BGP node that has
either not advertised the SPF capability TLV or is advertising the
SPF capability TLV with an algorithm type other than 0.
5.2.1.2. BGP-LS-SPF Node NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF Node NLRI is defined to
indicate the status of the node with respect to the BGP SPF
calculation. This will be used to rapidly take a node out of service
Section 6.5.2 or to indicate the node is not to be used for transit
(i.e., non-local) traffic Section 6.3. If the SPF Status TLV is not
included with the Node NLRI, the node is considered to be up and is
available for transit traffic. The SPF status is acted upon with the
execution of the next SPF calculation Section 6.3. A single TLV type
will be shared by the BGP-LS-SPF Node, Link, and Prefix NLRI. The
TLV type 1184 will be assigned by IANA.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Node Unreachable with respect to BGP SPF
2 - Node does not support transit with respect
to BGP SPF
3-254 - Undefined
255 - Reserved
Patel, et al. Expires 19 August 2022 [Page 12]
Internet-Draft BGP Link-State SPF Routing February 2022
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184). This implies that a BGP Update cannot contain multiple NLRI
with differing status. If the BGP-LS-SPF Status TLV is advertised
and the advertised value is not defined for all NLRI included in the
BGP update, then the SPF Status TLV is ignored and not used in SPF
computation but is still announced to other BGP SPF speakers. An
implementation MAY log an error for further analysis.
If a BGP SPF speaker received the Node NLRI but the SPF Status TLV is
not received, then any previously received information is considered
as implicitly withdrawn and the update is propagated to other BGP SPF
speakers. A BGP SPF speaker receiving a BGP Update containing a SPF
Status TLV in the BGP-LS attribute [RFC7752] with a value that is
outside the range of defined values SHOULD be processed and announced
to other BGP SPF speakers. However, a BGP SPF speaker MUST NOT use
the Status TLV in its SPF computation. An implementation MAY log
this condition for further analysis.
5.2.2. Link NLRI Usage
The criteria for advertisement of Link NLRI are discussed in
Section 4.
Link NLRI is advertised with unique local and remote node descriptors
dependent on the IP addressing. For IPv4 links, the link's local
IPv4 (TLV 259) and remote IPv4 (TLV 260) addresses will be used. For
IPv6 links, the local IPv6 (TLV 261) and remote IPv6 (TLV 262)
addresses will be used. For unnumbered links, the link local/remote
identifiers (TLV 258) will be used. For links supporting having both
IPv4 and IPv6 addresses, both sets of descriptors MAY be included in
the same Link NLRI. The link identifiers are described in table 5 of
[RFC7752].
For a link to be used in Shortest Path Tree (SPT) for a given address
family, i.e., IPv4 or IPv6, both routers connecting the link MUST
have an address in the same subnet for that address family. However,
an IPv4 or IPv6 prefix associated with the link MAY be installed
without the corresponding address on the other side of link.
The link IGP metric attribute TLV (TLV 1095) MUST be advertised. If
a BGP SPF speaker receives a Link NLRI without an IGP metric
attribute TLV, then it SHOULD consider the received NLRI as a
malformed and the receiving BGP SPF speaker MUST handle such
malformed NLRI as 'Treat-as-withdraw' [RFC7606]. The BGP SPF metric
length is 4 octets. Like OSPF [RFC2328], a cost is associated with
the output side of each router interface. This cost is configurable
by the system administrator. The lower the cost, the more likely the
Patel, et al. Expires 19 August 2022 [Page 13]
Internet-Draft BGP Link-State SPF Routing February 2022
interface is to be used to forward data traffic. One possible
default for metric would be to give each interface a cost of 1 making
it effectively a hop count. Algorithms such as setting the metric
inversely to the link speed as supported in the OSPF MIB [RFC4750]
MAY be supported. However, this is beyond the scope of this
document. Refer to Section 10.1.1 for operational guidance.
The usage of other link attribute TLVs is beyond the scope of this
document.
5.2.2.1. BGP-LS-SPF Link NLRI Attribute Prefix-Length TLVs
Two BGP-LS Attribute TLVs of the BGP-LS-SPF Link NLRI are defined to
advertise the prefix length associated with the IPv4 and IPv6 link
prefixes derived from the link descriptor addresses. The prefix
length is used for the optional installation of prefixes
corresponding to Link NLRI as defined in Section 6.3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|IPv4 (1182) or IPv6 Type (1183)| Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix-Length |
+-+-+-+-+-+-+-+-+
Prefix-length - A one-octet length restricted to 1-32 for IPv4
Link NLRI endpoint prefixes and 1-128 for IPv6
Link NLRI endpoint prefixes.
The Prefix-Length TLV is only relevant to Link NLRIs. The Prefix-
Length TLVs MUST be discarded as an error and not passed to other BGP
peers as specified in [RFC7606] when received with any NLRIs other
than Link NRLIs. An implementation MAY log an error for further
analysis.
The maximum prefix-length for IPv4 Prefix-Length TLV is 32 bits. A
prefix-length field indicating a larger value than 32 bits MUST be
discarded as an error and the received TLV is not passed to other BGP
peers as specified in [RFC7606]. The corresponding Link NLRI is
considered as malformed and MUST be handled as 'Treat-as-withdraw'.
An implementation MAY log an error for further analysis.
Patel, et al. Expires 19 August 2022 [Page 14]
Internet-Draft BGP Link-State SPF Routing February 2022
The maximum prefix-length for IPv6 Prefix-Length Type is 128 bits. A
prefix-length field indicating a larger value than 128 bits MUST be
discarded as an error and the received TLV is not passed to other BGP
peers as specified in [RFC7606]. The corresponding Link NLRI is
considered as malformed and MUST be handled as 'Treat-as-withdraw'.
An implementation MAY log an error for further analysis.
5.2.2.2. BGP-LS-SPF Link NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF Link NLRI is defined to
indicate the status of the link with respect to the BGP SPF
calculation. This will be used to expedite convergence for link
failures as discussed in Section 6.5.1. If the SPF Status TLV is not
included with the Link NLRI, the link is considered up and available.
The SPF status is acted upon with the execution of the next SPF
calculation Section 6.3. A single TLV type will be shared by the
Node, Link, and Prefix NLRI. The TLV type 1184 will be assigned by
IANA.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Link Unreachable with respect to BGP SPF
2-254 - Undefined
255 - Reserved
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184). This implies that a BGP Update cannot contain multiple NLRI
with differing status. If the BGP-LS-SPF Status TLV is advertised
and the advertised value is not defined for all NLRI included in the
BGP update, then the SPF Status TLV is ignored and not used in SPF
computation but is still announced to other BGP SPF speakers. An
implementation MAY log an error for further analysis.
Patel, et al. Expires 19 August 2022 [Page 15]
Internet-Draft BGP Link-State SPF Routing February 2022
If a BGP SPF speaker received the Link NLRI but the SPF Status TLV is
not received, then any previously received information is considered
as implicitly withdrawn and the update is propagated to other BGP SPF
speakers. A BGP SPF speaker receiving a BGP Update containing an SPF
Status TLV in the BGP-LS attribute [RFC7752] with a value that is
outside the range of defined values SHOULD be processed and announced
to other BGP SPF speakers. However, a BGP SPF speaker MUST NOT use
the Status TLV in its SPF computation. An implementation MAY log
this information for further analysis.
5.2.3. IPv4/IPv6 Prefix NLRI Usage
IPv4/IPv6 Prefix NLRI is advertised with a Local Node Descriptor and
the prefix and length. The Prefix Descriptors field includes the IP
Reachability Information TLV (TLV 265) as described in [RFC7752].
The Prefix Metric attribute TLV (TLV 1155) MUST be advertised. The
IGP Route Tag TLV (TLV 1153) MAY be advertised. The usage of other
attribute TLVs is beyond the scope of this document. For loopback
prefixes, the metric should be 0. For non-loopback prefixes, the
setting of the metric is a local matter and beyond the scope of this
document.
5.2.3.1. BGP-LS-SPF Prefix NLRI Attribute SPF Status TLV
A BGP-LS Attribute TLV to BGP-LS-SPF Prefix NLRI is defined to
indicate the status of the prefix with respect to the BGP SPF
calculation. This will be used to expedite convergence for prefix
unreachability as discussed in Section 6.5.1. If the SPF Status TLV
is not included with the Prefix NLRI, the prefix is considered
reachable. A single TLV type will be shared by the Node, Link, and
Prefix NLRI. The TLV type 1184 will be assigned by IANA.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1184) | Length (1 Octet) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPF Status |
+-+-+-+-+-+-+-+-+
BGP Status Values: 0 - Reserved
1 - Prefix Unreachable with respect to SPF
2-254 - Undefined
255 - Reserved
The BGP-LS-SPF Node Attribute SPF Status TLV, Link Attribute SPF
Status TLV, and Prefix Attribute SPF Status TLV use the same TLV Type
(1184). This implies that a BGP Update cannot contain multiple NLRI
Patel, et al. Expires 19 August 2022 [Page 16]
Internet-Draft BGP Link-State SPF Routing February 2022
with differing status. If the BGP-LS-SPF Status TLV is advertised
and the advertised value is not defined for all NLRI included in the
BGP update, then the SPF Status TLV is ignored and not used in SPF
computation but is still announced to other BGP SPF speakers. An
implementation MAY log an error for further analysis.
If a BGP SPF speaker received the Prefix NLRI but the SPF Status TLV
is not received, then any previously received information is
considered as implicitly withdrawn and the update is propagated to
other BGP SPF speakers. A BGP SPF speaker receiving a BGP Update
containing an SPF Status TLV in the BGP-LS attribute [RFC7752] with a
value that is outside the range of defined values SHOULD be processed
and announced to other BGP SPF speakers. However, a BGP SPF speaker
MUST NOT use the Status TLV in its SPF computation. An
implementation MAY log this information for further analysis.
5.2.4. BGP-LS Attribute Sequence-Number TLV
A BGP-LS Attribute TLV of the BGP-LS-SPF NLRI types is defined to
assure the most recent version of a given NLRI is used in the SPF
computation. The Sequence-Number TLV is mandatory for BGP-LS-SPF
NLRI. The TLV type 1181 has been assigned by IANA. The BGP-LS
Attribute TLV will contain an 8-octet sequence number. The usage of
the Sequence Number TLV is described in Section 6.1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type (1181) | Length (8 Octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (High-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (Low-Order 32 Bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Sequence Number The 64-bit strictly-increasing sequence number MUST
be incremented for every self-originated version of BGP-LS-SPF NLRI.
BGP SPF speakers implementing this specification MUST use available
mechanisms to preserve the sequence number's strictly increasing
property for the deployed life of the BGP SPF speaker (including cold
restarts). One mechanism for accomplishing this would be to use the
high-order 32 bits of the sequence number as a wrap/boot count that
is incremented any time the BGP router loses its sequence number
state or the low-order 32 bits wrap.
When incrementing the sequence number for each self-originated NLRI,
the sequence number should be treated as an unsigned 64-bit value.
If the lower-order 32-bit value wraps, the higher-order 32-bit value
Patel, et al. Expires 19 August 2022 [Page 17]
Internet-Draft BGP Link-State SPF Routing February 2022
should be incremented and saved in non-volatile storage. If a BGP
SPF speaker completely loses its sequence number state (e.g., the BGP
SPF speaker hardware is replaced or experiences a cold-start), the
BGP NLRI selection rules (see Section 6.1) will insure convergence,
albeit not immediately.
The Sequence-Number TLV is mandatory for BGP-LS-SPF NLRI. If the
Sequence-Number TLV is not received then the corresponding Link NLRI
is considered as malformed and MUST be handled as 'Treat-as-
withdraw'. An implementation MAY log an error for further analysis.
5.3. NEXT_HOP Manipulation
All BGP peers that support SPF extensions would locally compute the
LOC-RIB Next-Hop as a result of the SPF process. Consequently, the
Next-Hop is always ignored on receipt. The Next-Hop address MUST be
encoded as described in [RFC4760]. BGP SPF speakers MUST interpret
the Next-Hop address of MP_REACH_NLRI attribute as an IPv4 address
whenever the length of the Next-Hop address is 4 octets, and as a
IPv6 address whenever the length of the Next-Hop address is 16
octets.
[RFC4760] modifies the rules of NEXT_HOP attribute whenever the
multiprotocol extensions for BGP-4 are enabled. BGP SPF speakers
MUST set the NEXT_HOP attribute according to the rules specified in
[RFC4760] as the BGP-LS-SPF routing information is carried within the
multiprotocol extensions for BGP-4.
6. Decision Process with SPF Algorithm
The Decision Process described in [RFC4271] takes place in three
distinct phases. The Phase 1 decision function of the Decision
Process is responsible for calculating the degree of preference for
each route received from a BGP SPF speaker's peer. The Phase 2
decision function is invoked on completion of the Phase 1 decision
function and is responsible for choosing the best route out of all
those available for each distinct destination, and for installing
each chosen route into the LOC-RIB. The combination of the Phase 1
and 2 decision functions is characterized as a Path Vector algorithm.
The SPF based Decision process replaces the BGP Decision process
described in [RFC4271]. This process starts with selecting only
those Node NLRI whose SPF capability TLV matches with the local BGP
SPF speaker's SPF capability TLV value. Since Link-State NLRI always
contains the local node descriptor Section 5.2, each NLRI is uniquely
originated by a single BGP SPF speaker in the BGP SPF routing domain
(the BGP node matching the NLRI's Node Descriptors). Instances of
the same NLRI originated by multiple BGP SPF speakers would be
Patel, et al. Expires 19 August 2022 [Page 18]
Internet-Draft BGP Link-State SPF Routing February 2022
indicative of a configuration error or a masquerading attack
(Section 9). These selected Node NLRI and their Link/Prefix NLRI are
used to build a directed graph during the SPF computation as
described below. The best routes for BGP prefixes are installed in
the RIB as a result of the SPF process.
When BGP-LS-SPF NLRI is received, all that is required is to
determine whether it is the most recent by examining the Node-ID and
sequence number as described in Section 6.1. If the received NLRI
has changed, it will be advertised to other BGP-LS-SPF peers. If the
attributes have changed (other than the sequence number), a BGP SPF
calculation will be triggered. However, a changed NLRI MAY be
advertised immediately to other peers and prior to any SPF
calculation. Note that the BGP MinRouteAdvertisementIntervalTimer
and MinASOriginationIntervalTimer [RFC4271] timers are not applicable
to the BGP-LS-SPF SAFI. The scheduling of the SPF calculation, as
described in Section 6.3, is an implementation issue. Scheduling MAY
be dampened consistent with the SPF back-off algorithm specified in
[RFC8405].
The Phase 3 decision function of the Decision Process [RFC4271] is
also simplified since under normal SPF operation, a BGP SPF speaker
MUST advertise the changed NLRIs to all BGP peers with the BGP-LS-SPF
AFI/SAFI and install the changed routes in the Global RIB. The only
exception are unchanged NLRIs or stale NLRIs, i.e., NLRI received
with a less recent (numerically smaller) sequence number.
6.1. BGP NLRI Selection
The rules for all BGP-LS-SPF NLRIs selection for phase 1 of the BGP
decision process, section 9.1.1 [RFC4271], no longer apply.
1. Routes originated by directly connected BGP SPF peers are
preferred. This condition can be determined by comparing the BGP
Identifiers in the received Local Node Descriptor and OPEN
message. This rule will assure that stale NLRI is updated even
if a BGP-LS router loses its sequence number state due to a cold-
start.
2. The NLRI with the most recent Sequence Number TLV, i.e., highest
sequence number is selected.
3. The route received from the BGP SPF speaker with the numerically
larger BGP Identifier is preferred.
When a BGP SPF speaker completely loses its sequence number state,
i.e., due to a cold start, or in the unlikely possibility that 64-bit
sequence number wraps, the BGP routing domain will still converge.
Patel, et al. Expires 19 August 2022 [Page 19]
Internet-Draft BGP Link-State SPF Routing February 2022
This is due to the fact that BGP SPF speakers adjacent to the router
will always accept self-originated NLRI from the associated speaker
as more recent (rule # 1). When a BGP SPF speaker reestablishes a
connection with its peers, any existing session will be taken down
and stale NLRI will be replaced. The adjacent BGP SPF speaker will
update their NLRI advertisements, hop by hop, until the BGP routing
domain has converged.
The modified SPF Decision Process performs an SPF calculation rooted
at the BGP SPF speaker using the metrics from the Link Attribute IGP
Metric TLV (1095) and the Prefix Attribute Prefix Metric TLV (1155)
[RFC7752]. As a result, any other BGP attributes that would
influence the BGP decision process defined in [RFC4271] including
ORIGIN, MULTI_EXIT_DISC, and LOCAL_PREF attributes are ignored by the
SPF algorithm. The NEXT_HOP attribute is discussed in Section 5.3.
The AS_PATH and AS4_PATH [RFC6793] attributes are preserved and used
for loop detection [RFC4271]. They are ignored during the SPF
computation for BGP-LS-SPF NRLIs.
6.1.1. BGP Self-Originated NLRI
Node, Link, or Prefix NLRI with Node Descriptors matching the local
BGP SPF speaker are considered self-originated. When self-originated
NLRI is received and it doesn't match the local node's NLRI content
(including sequence number), special processing is required.
* If a self-originated NLRI is received and the sequence number is
more recent (i.e., greater than the local node's sequence number
for the NLRI), the NLRI sequence number will be advanced to one
greater than the received sequence number and the NLRI will be
readvertised to all peers.
* If self-originated NLRI is received and the sequence number is the
same as the local node's sequence number but the attributes
differ, the NLRI sequence number will be advanced to one greater
than the received sequence number and the NLRI will be
readvertised to all peers.
* If self-originated Link or Prefix NLRI is received and the Link or
Prefix NLRI is no longer being advertised by the local node, the
NLRI will be withdrawn.
Patel, et al. Expires 19 August 2022 [Page 20]
Internet-Draft BGP Link-State SPF Routing February 2022
The above actions are performed immediately when the first instance
of a newer self-originated NLRI is received. In this case, the newer
instance is considered to be a stale instance that was advertised by
the local node prior to a restart where the NLRI state is lost.
However, if subsequent newer self-originated NLRI is received for the
same Node, Link, or Prefix NLRI, the readvertisement or withdrawal is
delayed by 5 seconds since it is likely being advertised by a
misconfigured or rogue BGP SPF speaker Section 9.
6.2. Dual Stack Support
The SPF-based decision process operates on Node, Link, and Prefix
NLRIs that support both IPv4 and IPv6 addresses. Whether to run a
single SPF computation or multiple SPF computations for separate AFs
is an implementation matter. Normally, IPv4 next-hops are calculated
for IPv4 prefixes and IPv6 next-hops are calculated for IPv6
prefixes.
6.3. SPF Calculation based on BGP-LS-SPF NLRI
This section details the BGP-LS-SPF local routing information base
(RIB) calculation. The router will use BGP-LS-SPF Node, Link, and
Prefix NLRI to compute routes using the following algorithm. This
calculation yields the set of routes associated with the BGP SPF
Routing Domain. A router calculates the shortest-path tree using
itself as the root. Optimizations to the BGP-LS-SPF algorithm are
possible but MUST yield the same set of routes. The algorithm below
supports Equal Cost Multi-Path (ECMP) routes. Weighted Unequal Cost
Multi-Path routes are out of scope. The organization of this section
owes heavily to section 16 of [RFC2328].
The following abstract data structures are defined in order to
specify the algorithm.
* Local Route Information Base (LOC-RIB) - This routing table
contains reachability information (i.e., next hops) for all
prefixes (both IPv4 and IPv6) as well as BGP-LS-SPF node
reachability. Implementations may choose to implement this with
separate RIBs for each address family and/or Prefix versus Node
reachability. It is synonymous with the Loc-RIB specified in
[RFC4271].
* Global Routing Information Base (GLOBAL-RIB) - This is Routing
Information Base (RIB) containing the current routes that are
installed in the router's forwarding plane. This is commonly
referred to in networking parlance as "the RIB".
Patel, et al. Expires 19 August 2022 [Page 21]
Internet-Draft BGP Link-State SPF Routing February 2022
* Link State NLRI Database (LSNDB) - Database of BGP-LS-SPF NLRI
that facilitates access to all Node, Link, and Prefix NLRI.
* Candidate List (CAN-LIST) - This is a list of candidate Node NLRIs
used during the BGP SPF calculation Section 6.3. The list is
sorted by the cost to reach the Node NLRI with the Node NLRI with
the lowest reachability cost at the head of the list. This
facilitates execution of the Dijkstra algorithm Section 1.1 where
the shortest paths between the local node and other nodes in graph
area computed. The CAN-LIST is typically implemented as a heap
but other data structures have been used.
The algorithm is comprised of the steps below:
1. The current LOC-RIB is invalidated, and the CAN-LIST is
initialized to empty. The LOC-RIB is rebuilt during the course
of the SPF computation. The existing routing entries are
preserved for comparison to determine changes that need to be
made to the GLOBAL-RIB in step 6.
2. The computing router's Node NLRI is updated in the LOC-RIB with a
cost of 0 and the Node NLRI is also added to the CAN-LIST. The
next-hop list is set to the internal loopback next-hop.
3. The Node NLRI with the lowest cost is removed from the candidate
list for processing. If the BGP-LS Node attribute doesn't
include an SPF Capability TLV (Section 5.2.1.1, the Node NLRI is
ignored and the next lowest cost Node NLRI is selected from
candidate list. The If the BGP-LS Node attribute includes an SPF
Status TLV (Section 5.2.1.1) indicating the node is unreachable,
the Node NLRI is ignored and the next lowest cost Node NLRI is
selected from candidate list. The Node corresponding to this
NLRI will be referred to as the Current-Node. If the candidate
list is empty, the SPF calculation has completed and the
algorithm proceeds to step 6.
4. All the Prefix NLRI with the same Node Identifiers as the
Current-Node will be considered for installation. The next-
hop(s) for these Prefix NLRI are inherited from the Current-Node.
The cost for each prefix is the metric advertised in the Prefix
Attribute Prefix Metric TLV (1155) added to the cost to reach the
Current-Node. The following will be done for each Prefix NLRI
(referred to as the Current-Prefix):
* If the BGP-LS Prefix attribute includes an SPF Status TLV
indicating the prefix is unreachable, the Current-Prefix is
considered unreachable and the next Prefix NLRI is examined in
Step 4.
Patel, et al. Expires 19 August 2022 [Page 22]
Internet-Draft BGP Link-State SPF Routing February 2022
* If the Current-Prefix's corresponding prefix is in the LOC-RIB
and the LOC-RIB cost is less than the Current-Prefix's metric,
the Current-Prefix does not contribute to the route and the
next Prefix NLRI is examined in Step 4.
* If the Current-Prefix's corresponding prefix is not in the
LOC-RIB, the prefix is installed with the Current-Node's next-
hops installed as the LOC-RIB route's next-hops and the metric
being updated. If the IGP Route Tag TLV (1153) is included in
the Current-Prefix's NLRI Attribute, the tag(s) are installed
in the current LOC-RIB route's tag(s).
* If the Current-Prefix's corresponding prefix is in the LOC-RIB
and the cost is less than the LOC-RIB route's metric, the
prefix is installed with the Current-Node's next-hops
replacing the LOC-RIB route's next-hops and the metric being
updated and any route tags removed. If the IGP Route Tag TLV
(1153) is included in the Current-Prefix's NLRI Attribute, the
tag(s) are installed in the current LOC-RIB route's tag(s).
* If the Current-Prefix's corresponding prefix is in the LOC-RIB
and the cost is the same as the LOC-RIB route's metric, the
Current-Node's next-hops will be merged with LOC-RIB route's
next-hops. If the number of merged next-hops exceeds the
Equal-Cost Multi-Path (ECMP) limit, the number of next-hops is
reduced with next-hops on numbered links preferred over next-
hops on unnumbered links. Among next-hops on numbered links,
the next-hops with the highest IPv4 or IPv6 addresses are
preferred. Among next-hops on unnumbered links, the next-hops
with the highest Remote Identifiers are preferred [RFC5307].
If the IGP Route Tag TLV (1153) is included in the Current-
Prefix's NLRI Attribute, the tag(s) are merged into the LOC-
RIB route's current tags.
5. All the Link NLRI with the same Node Identifiers as the Current-
Node will be considered for installation. Each link will be
examined and will be referred to in the following text as the
Current-Link. The cost of the Current-Link is the advertised IGP
Metric TLV (1095) from the Link NLRI BGP-LS attribute added to
the cost to reach the Current-Node. If the Current-Node is for
the local BGP Router, the next-hop for the link will be a direct
next-hop pointing to the corresponding local interface. For any
other Current-Node, the next-hop(s) for the Current-Link will be
inherited from the Current-Node. The following will be done for
each link:
Patel, et al. Expires 19 August 2022 [Page 23]
Internet-Draft BGP Link-State SPF Routing February 2022
a. The prefix(es) associated with the Current-Link are installed
into the LOC-RIB using the same rules as were used for Prefix
NLRI in the previous steps. Optionally, in deployments where
BGP-SPF routers have limited routing table capacity,
installation of these subnets can be suppressed. Suppression
will have an operational impact as the IPv4/IPv6 link
endpoint addresses will not be reachable and tools such as
traceroute will display addresses that are not reachable.
b. If the Current-Node NLRI attributes includes the SPF status
TLV (Section 5.2.1.2) and the status indicates that the Node
doesn't support transit, the next link for the Current-Node
is processed in Step 5.
c. If the Current-Link's NLRI attribute includes an SPF Status
TLV indicating the link is down, the BGP-LS-SPF Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.
d. The Current-Link's Remote Node NLRI is accessed (i.e., the
Node NLRI with the same Node identifiers as the Current-
Link's Remote Node Descriptors). If it exists, it will be
referred to as the Remote-Node and the algorithm will proceed
as follows:
* If the Remote-Node's NLRI attribute includes an SPF Status
TLV indicating the node is unreachable, the next link for
the Current-Node is examined in Step 5.
* All the Link NLRI corresponding the Remote-Node will be
searched for a Link NLRI pointing to the Current-Node.
Each Link NLRI is examined for Remote Node Descriptors
matching the Current-Node and Link Descriptors matching
the Current-Link. For numbered links to match, the Link
Descriptors MUST share a common IPv4 or IPv6 subnet. For
unnumbered links to match, the Current Link's Local
Identifier MUST match the Remote Node Link's Remote
Identifier and the Current Link's Remote Identifier MUST
the Remote Node Link's Local Identifier [RFC5307]. If
these conditions are satisfied for one of the Remote-
Node's links, the bi-directional connectivity check
succeeds and the Remote-Node may be processed further.
The Remote-Node's Link NLRI providing bi-directional
connectivity will be referred to as the Remote-Link. If
no Remote-Link is found, the next link for the Current-
Node is examined in Step 5.
Patel, et al. Expires 19 August 2022 [Page 24]
Internet-Draft BGP Link-State SPF Routing February 2022
* If the Remote-Link NLRI attribute includes an SPF Status
TLV indicating the link is down, the Remote-Link NLRI is
considered down and the next link for the Current-Node is
examined in Step 5.
* If the Remote-Node is not on the CAN-LIST, it is inserted
based on the cost. The Remote Node's cost is the cost of
Current-Node added the Current-Link's IGP Metric TLV
(1095). The next-hop(s) for the Remote-Node are inherited
from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with a
higher cost, it must be removed and reinserted with the
Remote-Node cost based on the Current-Link (as calculated
in the previous step). The next-hop(s) for the Remote-
Node are inherited from the Current-Link.
* If the Remote-Node NLRI is already on the CAN-LIST with
the same cost, it need not be reinserted on the CAN-LIST.
However, the Current-Link's next-hop(s) must be merged
into the current set of next-hops for the Remote-Node.
* If the Remote-Node NLRI is already on the CAN-LIST with a
lower cost, it need not be reinserted on the CAN-LIST.
e. Return to step 3 to process the next lowest cost Node NLRI on
the CAN-LIST.
6. The LOC-RIB is examined and changes (adds, deletes,
modifications) are installed into the GLOBAL-RIB. For each route
in the LOC-RIB:
* If the route was added during the current BGP SPF computation,
install the route into the GLOBAL-RIB.
* If the route modified during the current BGP SPF computation
(e.g., metric, tags, or next-hops), update the route in the
GLOBAL-RIB.
* If the route was not installed during the current BGP SPF
computation, remove the route from both the GLOBAL-RIB and the
LOC-RIB.
Patel, et al. Expires 19 August 2022 [Page 25]
Internet-Draft BGP Link-State SPF Routing February 2022
6.4. IPv4/IPv6 Unicast Address Family Interaction
While the BGP-LS-SPF address family and the IPv4/IPv6 unicast address
families MAY install routes into the same device routing tables, they
will operate independently much the same as OSPF and IS-IS would
operate today (i.e., "Ships-in-the-Night" mode). There is no
implicit route redistribution between the BGP address families.
It is RECOMMENDED that BGP-LS-SPF IPv4/IPv6 route computation and
installation be given scheduling priority by default over other BGP
address families as these address families are considered as underlay
SAFIs. Similarly, it is RECOMMENDED that the route preference or
administrative distance give active route installation preference to
BGP-LS-SPF IPv4/IPv6 routes over BGP routes from other AFI/SAFIs.
However, this preference MAY be overridden by an operator-configured
policy.
6.5. NLRI Advertisement
6.5.1. Link/Prefix Failure Convergence
A local failure will prevent a link from being used in the SPF
calculation due to the IGP bi-directional connectivity requirement.
Consequently, local link failures SHOULD always be given priority
over updates (e.g., withdrawing all routes learned on a session) in
order to ensure the highest priority propagation and optimal
convergence.
An IGP such as OSPF [RFC2328] will stop using the link as soon as the
Router-LSA for one side of the link is received. With a BGP
advertisement, the link would continue to be used until the last copy
of the BGP-LS-SPF Link NLRI is withdrawn. In order to avoid this
delay, the originator of the Link NLRI SHOULD advertise a more recent
version with an increased Sequence Number TLV for the BGP-LS-SPF Link
NLRI including the SPF Status TLV (Section 5.2.2.2) indicating the
link is down with respect to BGP SPF. The configurable
LinkStatusDownAdvertise timer controls the interval that the BGP-LS-
LINK NLRI is advertised with SPF Status indicating the link is down
prior to withdrawal. If the link becomes available in that period,
the originator of the BGP-LS-SPF LINK NLRI SHOULD advertise a more
recent version of the BGP-LS-SPF Link NLRI without the SPF Status TLV
in the BGP-LS Link Attributes. The suggested default value for the
LinkStatusDownAdvertise timer is 2 seconds.
Similarly, when a prefix becomes unreachable, a more recent version
of the BGP-LS-SPF Prefix NLRI SHOULD be advertised with the SPF
Status TLV (Section 5.2.3.1) indicating the prefix is unreachable in
the BGP-LS Prefix Attributes and the prefix will be considered
Patel, et al. Expires 19 August 2022 [Page 26]
Internet-Draft BGP Link-State SPF Routing February 2022
unreachable with respect to BGP SPF. The configurable
PrefixStatusDownAdvertise timer controls the interval that the BGP-
LS-Prefix NLRI is advertised with SPF Status indicating the prefix is
unreachable prior to withdrawal. If the prefix becomes reachable in
that period, the originator of the BGP-LS-SPF Prefix NLRI SHOULD
advertise a more recent version of the BGP-LS-SPF Prefix NLRI without
the SPF Status TLV in the BGP-LS Prefix Attributes. The suggested
default value for the PrefixStatusDownAdvertise timer is 2 seconds.
6.5.2. Node Failure Convergence
With BGP without graceful restart [RFC4724], all the NLRI advertised
by a node are implicitly withdrawn when a session failure is
detected. If fast failure detection such as BFD is utilized, and the
node is on the fastest converging path, the most recent versions of
BGP-LS-SPF NLRI may be withdrawn. This will result into an older
version of the NLRI being used until the new versions arrive and,
potentially, unnecessary route flaps. For the BGP-LS-SPF SAFI, NLRI
SHOULD NOT be implicitly withdrawn immediately to prevent such
unnecessary route flaps. The configurable
NLRIImplicitWithdrawalDelay timer controls the interval that NLRI is
retained prior to implicit withdrawal after a BGP SPF speaker has
transitioned out of Established state. This will not delay
convergence since the adjacent nodes will detect the link failure and
advertise a more recent NLRI indicating the link is down with respect
to BGP SPF (Section 6.5.1) and the BGP SPF calculation will fail the
bi-directional connectivity check Section 6.3. The suggested default
value for the NLRIImplicitWithdrawalDelay timer is 2 seconds.
7. Error Handling
This section describes the Error Handling actions, as described in
[RFC7606], that are specific to SAFI BGP-LS-SPF BGP Update message
processing.
7.1. Processing of BGP-LS-SPF TLVs
When a BGP SPF speaker receives a BGP Update containing a malformed
Node NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in [RFC7606]. When discarding an associated Node NLRI with
a malformed TLV, a BGP SPF speaker SHOULD log an error for further
analysis.
Patel, et al. Expires 19 August 2022 [Page 27]
Internet-Draft BGP Link-State SPF Routing February 2022
When a BGP SPF speaker receives a BGP Update containing a malformed
Link NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in [RFC7606]. When discarding an associated Link NLRI with
a malformed TLV, a BGP SPF speaker SHOULD log an error for further
analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
Prefix NLRI SPF Status TLV in the BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and MUST NOT pass it to other BGP peers as
specified in [RFC7606]. When discarding an associated Prefix NLRI
with a malformed TLV, a BGP SPF speaker SHOULD log an error for
further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
SPF Capability TLV in the Node NLRI BGP-LS Attribute [RFC7752], it
MUST ignore the received TLV and the Node NLRI and MUST NOT pass it
to other BGP peers as specified in [RFC7606]. When discarding a Node
NLRI with a malformed TLV, a BGP SPF speaker SHOULD log an error for
further analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
IPv4 Prefix-Length TLV in the Link NLRI BGP-LS Attribute [RFC7752],
it MUST ignore the received TLV and the Node NLRI and MUST NOT pass
it to other BGP peers as specified in [RFC7606]. The corresponding
Link NLRI is considered as malformed and MUST be handled as 'Treat-
as-withdraw'. An implementation MAY log an error for further
analysis.
When a BGP SPF speaker receives a BGP Update containing a malformed
IPv6 Prefix-Length TLV in the Link NLRI BGP-LS Attribute [RFC7752],
it MUST ignore the received TLV and the Node NLRI and MUST NOT pass
it to other BGP peers as specified in [RFC7606]. The corresponding
Link NLRI is considered as malformed and MUST be handled as 'Treat-
as-withdraw'. An implementation MAY log an error for further
analysis.
7.2. Processing of BGP-LS-SPF NLRIs
A Link-State NLRI MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS-SPF NLRI to determine if it is malformed.
Patel, et al. Expires 19 August 2022 [Page 28]
Internet-Draft BGP Link-State SPF Routing February 2022
1. Does the sum of all TLVs found in the BGP MP_REACH_NLRI attribute
correspond to the BGP MP_REACH_NLRI length?
2. Does the sum of all TLVs found in the BGP MP_UNREACH_NLRI
attribute correspond to the BGP MP_UNREACH_NLRI length?
3. Does the sum of all TLVs found in a BGP-LS-SPF NLRI correspond to
the Total NLRI Length field of all its Descriptors?
4. When an NLRI TLV is recognized, is the length of the TLV and its
sub-TLVs valid?
5. Has the syntactic correctness of the NLRI fields been verified as
per [RFC7606]?
6. Has the rule regarding ordering of TLVs been followed as
described in Section 5.1.1?
When the error determined allows for the router to skip the malformed
NLRI(s) and continue processing of the rest of the update message
(e.g., when the TLV ordering rule is violated), then it MUST handle
such malformed NLRIs as 'Treat-as-withdraw'. In other cases, where
the error in the NLRI encoding results in the inability to process
the BGP update message (e.g., length related encoding errors), then
the router SHOULD handle such malformed NLRIs as 'AFI/SAFI disable'
when other AFI/SAFI besides BGP-LS are being advertised over the same
session. Alternately, the router MUST perform 'session reset' when
the session is only being used for BGP-LS-SPF or when its 'AFI/SAFI
disable' action is not possible.
7.3. Processing of BGP-LS Attribute
A BGP-LS Attribute MUST NOT be considered as malformed or invalid
based on the inclusion/exclusion of TLVs or contents of the TLV
fields (i.e., semantic errors), as described in Section 5.1 and
Section 5.1.1.
A BGP-LS-SPF Speaker MUST perform the following syntactic validation
of the BGP-LS Attribute to determine if it is malformed.
1. Does the sum of all TLVs found in the BGP-LS-SPF Attribute
correspond to the BGP-LS Attribute length?
2. Has the syntactic correctness of the Attributes (including BGP-LS
Attribute) been verified as per [RFC7606]?
3. Is the length of each TLV and, when the TLV is recognized then,
its sub-TLVs in the BGP-LS Attribute valid?
Patel, et al. Expires 19 August 2022 [Page 29]
Internet-Draft BGP Link-State SPF Routing February 2022
When the detected error allows for the router to skip the malformed
BGP-LS Attribute and continue processing of the rest of the update
message (e.g., when the BGP-LS Attribute length and the total Path
Attribute Length are correct but some TLV/sub-TLV length within the
BGP-LS Attribute is invalid), then it MUST handle such malformed BGP-
LS Attribute as 'Attribute Discard'. In other cases, when the error
in the BGP-LS Attribute encoding results in the inability to process
the BGP update message, then the handling is the same as described
above for malformed NLRI.
Note that the 'Attribute Discard' action results in the loss of all
TLVs in the BGP-LS Attribute and not the removal of a specific
malformed TLV. The removal of specific malformed TLVs may give a
wrong indication to a BGP SPF speaker that the specific information
is being deleted or is not available.
When a BGP SPF speaker receives an update message with Link-State
NLRI(s) in the MP_REACH_NLRI but without the BGP-LS-SPF Attribute, it
is most likely an indication that a BGP SPF speaker preceding it has
performed the 'Attribute Discard' fault handling. An implementation
SHOULD preserve and propagate the Link-State NLRIs in such an update
message so that the BGP SPF speaker can detect the loss of link-state
information for that object and not assume its deletion/withdrawal.
This also makes it possible for a network operator to trace back to
the BGP SPF speaker which actually detected a problem with the BGP-LS
Attribute.
An implementation SHOULD log an error for further analysis for
problems detected during syntax validation.
When a BGP SPF speaker receives a BGP Update containing a malformed
IGP metric TLV in the Link NLRI BGP-LS Attribute [RFC7752], it MUST
ignore the received TLV and the Link NLRI and MUST NOT pass it to
other BGP peers as specified in [RFC7606]. When discarding a Link
NLRI with a malformed TLV, a BGP SPF speaker SHOULD log an error for
further analysis.
8. IANA Considerations
This document defines the use of SAFI (80) for BGP SPF operation
Section 5.1, and requests IANA to assign the value from the First
Come First Serve (FCFS) range in the Subsequent Address Family
Identifiers (SAFI) Parameters registry.
Patel, et al. Expires 19 August 2022 [Page 30]
Internet-Draft BGP Link-State SPF Routing February 2022
This document also defines five attribute TLVs of BGP-LS-SPF NLRI.
We request IANA to assign types for the SPF capability TLV, Sequence
Number TLV, IPv4 Link Prefix-Length TLV, IPv6 Link Prefix-Length TLV,
and SPF Status TLV from the "BGP-LS Node Descriptor, Link Descriptor,
Prefix Descriptor, and Attribute TLVs" Registry.
+=========================+=================+====================+
| Attribute TLV | Suggested Value | NLRI Applicability |
+=========================+=================+====================+
| SPF Capability | 1180 | Node |
+-------------------------+-----------------+--------------------+
| SPF Status | 1184 | Node, Link, Prefix |
+-------------------------+-----------------+--------------------+
| IPv4 Link Prefix Length | 1182 | Link |
+-------------------------+-----------------+--------------------+
| IPv6 Link Prefix Length | 1183 | Link |
+-------------------------+-----------------+--------------------+
| Sequence Number | 1181 | Node, Link, Prefix |
+-------------------------+-----------------+--------------------+
Table 1: NLRI Attribute TLVs
9. Security Considerations
This document defines a BGP SAFI, i.e., the BGP-LS-SPF SAFI. This
document does not change the underlying security issues inherent in
the BGP protocol [RFC4271]. The Security Considerations discussed in
[RFC4271] apply to the BGP SPF functionality as well. The analysis
of the security issues for BGP mentioned in [RFC4272] and [RFC6952]
also applies to this document. The analysis of Generic Threats to
Routing Protocols done in [RFC4593] is also worth noting. As the
modifications described in this document for BGP SPF apply to IPv4
Unicast and IPv6 Unicast as undelay SAFIs in a single BGP SPF Routing
Domain, the BGP security solutions described in [RFC6811] and
[RFC8205] are somewhat constricted as they are meant to apply for
inter-domain BGP where multiple BGP Routing Domains are typically
involved. The BGP-LS-SPF SAFI NLRI described in this document are
typically advertised between EBGP or IBGP speakers under a single
administrative domain.
In the context of the BGP peering associated with this document, a
BGP speaker MUST NOT accept updates from a peer that is not within
any administrative control of an operator. That is, a participating
BGP speaker SHOULD be aware of the nature of its peering
relationships. Such protection can be achieved by manual
configuration of peers at the BGP speaker.
Patel, et al. Expires 19 August 2022 [Page 31]
Internet-Draft BGP Link-State SPF Routing February 2022
In order to mitigate the risk of peering with BGP speakers
masquerading as legitimate authorized BGP speakers, it is recommended
that the TCP Authentication Option (TCP-AO) [RFC5925] be used to
authenticate BGP sessions. If an authorized BGP peer is compromised,
that BGP peer could advertise modified Node, Link, or Prefix NLRI
will result in misrouting, repeating origination of NLRI, and/or
excessive SPF calculations. When a BGP speaker detects that its
self-originated NLRI is being originated by another BGP speaker, an
appropriate error should be logged so that the operator can take
corrective action.
10. Management Considerations
This section includes unique management considerations for the BGP-
LS-SPF address family.
10.1. Configuration
All routers in BGP SPF Routing Domain are under a single
administrative domain allowing for consistent configuration.
10.1.1. Link Metric Configuration
Within a BGP SPF Routing Domain, the IGP metrics for all advertised
links SHOULD be configured or defaulted consistently. For example,
if a default metric is used for one router's links, then a similar
metric should be used for all router's links. Similarly, if the link
cost is derived from using the inverse of the link bandwidth on one
router, then this SHOULD be done for all routers and the same
reference bandwidth should be used to derive the inversely
proportional metric. Failure to do so will not result in correct
routing based on link metric.
10.1.2. backoff-config
In addition to configuration of the BGP-LS-SPF address family,
implementations SHOULD support the "Shortest Path First (SPF) Back-
Off Delay Algorithm for Link-State IGPs" [RFC8405]. If supported,
configuration of the INITIAL_SPF_DELAY, SHORT_SPF_DELAY,
LONG_SPF_DELAY, TIME_TO_LEARN, and HOLDDOWN_INTERVAL MUST be
supported [RFC8405]. Section 6 of [RFC8405] recommends consistent
configuration of these values throughout the IGP routing domain and
this also applies to the BGP SPF Routing Domain.
Patel, et al. Expires 19 August 2022 [Page 32]
Internet-Draft BGP Link-State SPF Routing February 2022
10.2. Operational Data
In order to troubleshoot SPF issues, implementations SHOULD support
an SPF log including entries for previous SPF computations. Each SPF
log entry would include the BGP-LS-SPF NLRI SPF triggering the SPF,
SPF scheduled time, SPF start time, SPF end time, and SPF type if
different types of SPF are supported. Since the size of the log will
be finite, implementations SHOULD also maintain counters for the
total number of SPF computations and the total number of SPF
triggering events. Additionally, to troubleshoot SPF scheduling and
back-off [RFC8405], the current SPF back-off state, remaining time-
to-learn, remaining holddown, last trigger event time, last SPF time,
and next SPF time should be available.
11. Implementation Status
Note RFC Editor: Please remove this section and the associated
references prior to publication.
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft and is based on a proposal described in [RFC7942].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to RFC 7942, "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
The BGP-LS-SPF implementation status is documented in
[I-D.psarkar-lsvr-bgp-spf-impl].
Patel, et al. Expires 19 August 2022 [Page 33]
Internet-Draft BGP Link-State SPF Routing February 2022
12. Acknowledgements
The authors would like to thank Sue Hares, Jorge Rabadan, Boris
Hassanov, Dan Frost, Matt Anderson, Fred Baker, Lukas Krattiger,
Yingzhen Qu, and Haibo Wang for their review and comments. Thanks to
Pushpasis Sarkar for discussions on preventing a BGP SPF Router from
being used for non-local traffic (i.e., transit traffic).
The authors extend special thanks to Eric Rosen for fruitful
discussions on BGP-LS-SPF convergence as compared to IGPs.
13. Contributors
In addition to the authors listed on the front page, the following
co-authors have contributed to the document.
Derek Yeung
Arrcus, Inc.
derek@arrcus.com
Gunter Van De Velde
Nokia
gunter.van_de_velde@nokia.com
Abhay Roy
Arrcus, Inc.
abhay@arrcus.com
Venu Venugopal
Cisco Systems
venuv@cisco.com
Chaitanya Yadlapalli
AT&T
cy098d@att.com
14. References
14.1. 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>.
Patel, et al. Expires 19 August 2022 [Page 34]
Internet-Draft BGP Link-State SPF Routing February 2022
[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>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC4593] Barbir, A., Murphy, S., and Y. Yang, "Generic Threats to
Routing Protocols", RFC 4593, DOI 10.17487/RFC4593,
October 2006, <https://www.rfc-editor.org/info/rfc4593>.
[RFC4750] Joyal, D., Ed., Galecki, P., Ed., Giacalone, S., Ed.,
Coltun, R., and F. Baker, "OSPF Version 2 Management
Information Base", RFC 4750, DOI 10.17487/RFC4750,
December 2006, <https://www.rfc-editor.org/info/rfc4750>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<https://www.rfc-editor.org/info/rfc6793>.
[RFC6811] Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
Patel, et al. Expires 19 August 2022 [Page 35]
Internet-Draft BGP Link-State SPF Routing February 2022
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[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>.
[RFC8205] Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
Specification", RFC 8205, DOI 10.17487/RFC8205, September
2017, <https://www.rfc-editor.org/info/rfc8205>.
[RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
Francois, P., and C. Bowers, "Shortest Path First (SPF)
Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
DOI 10.17487/RFC8405, June 2018,
<https://www.rfc-editor.org/info/rfc8405>.
[RFC8654] Bush, R., Patel, K., and D. Ward, "Extended Message
Support for BGP", RFC 8654, DOI 10.17487/RFC8654, October
2019, <https://www.rfc-editor.org/info/rfc8654>.
[RFC8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC8665, December 2019,
<https://www.rfc-editor.org/info/rfc8665>.
14.2. Informational References
[I-D.ietf-lsvr-applicability]
Patel, K., Lindem, A., Zandi, S., and G. Dawra, "Usage and
Applicability of Link State Vector Routing in Data
Centers", Work in Progress, Internet-Draft, draft-ietf-
lsvr-applicability-05, 24 March 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-lsvr-
applicability-05.txt>.
[I-D.psarkar-lsvr-bgp-spf-impl]
Sarkar, P., Patel, K., Pallagatti, S., and s.
sajibasil@gmail.com, "BGP Shortest Path Routing Extension
Implementation Report", Work in Progress, Internet-Draft,
draft-psarkar-lsvr-bgp-spf-impl-00, 2 June 2020,
<http://www.ietf.org/internet-drafts/draft-psarkar-lsvr-
bgp-spf-impl-00.txt>.
Patel, et al. Expires 19 August 2022 [Page 36]
Internet-Draft BGP Link-State SPF Routing February 2022
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<https://www.rfc-editor.org/info/rfc2328>.
[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
Reflection: An Alternative to Full Mesh Internal BGP
(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
<https://www.rfc-editor.org/info/rfc4456>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, DOI 10.17487/RFC4915, June 2007,
<https://www.rfc-editor.org/info/rfc4915>.
[RFC5286] Atlas, A., Ed. and A. Zinin, Ed., "Basic Specification for
IP Fast Reroute: Loop-Free Alternates", RFC 5286,
DOI 10.17487/RFC5286, September 2008,
<https://www.rfc-editor.org/info/rfc5286>.
[RFC5307] Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions
in Support of Generalized Multi-Protocol Label Switching
(GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008,
<https://www.rfc-editor.org/info/rfc5307>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<https://www.rfc-editor.org/info/rfc5880>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
Patel, et al. Expires 19 August 2022 [Page 37]
Internet-Draft BGP Link-State SPF Routing February 2022
[RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", BCP 205,
RFC 7942, DOI 10.17487/RFC7942, July 2016,
<https://www.rfc-editor.org/info/rfc7942>.
Authors' Addresses
Keyur Patel
Arrcus, Inc.
Email: keyur@arrcus.com
Acee Lindem
Cisco Systems
301 Midenhall Way
Cary, NC 27513
United States of America
Email: acee@cisco.com
Shawn Zandi
LinkedIn
222 2nd Street
San Francisco, CA 94105
United States of America
Email: szandi@linkedin.com
Wim Henderickx
Nokia
Antwerp
Belgium
Email: wim.henderickx@nokia.com
Patel, et al. Expires 19 August 2022 [Page 38]