<?xmlversion="1.0" encoding="US-ASCII"?> <?rfc toc="yes"?> <?rfc tocompact="yes"?> <?rfc tocdepth="3"?> <?rfc tocindent="yes"?> <?rfc symrefs="yes"?> <?rfc sortrefs="yes"?> <?rfc comments="yes"?> <?rfc inline="yes"?> <?rfc compact="yes"?> <?rfc subcompact="no"?>version='1.0' encoding='UTF-8'?> <!DOCTYPE rfc [ <!ENTITY nbsp " "> <!ENTITY zwsp "​"> <!ENTITY nbhy "‑"> <!ENTITY wj "⁠"> ]> <rfc xmlns:xi="http://www.w3.org/2001/XInclude" category="info" docName="draft-ietf-lsvr-applicability-22" number="9816" ipr="trust200902" submissionType="IETF"tocDepth="5" version="2">tocDepth="3" version="3" tocInclude="true" symRefs="true" sortRefs="true" updates="" obsoletes="" consensus="true" xml:lang="en"> <front> <!--[rfced] BGP-LS and SPF a) Would you like to update the title of the document as shown below or otherwise, to more closely match how "BGP-LS" and "SPF" are handled in the title of RFC-to-be 9815? Original: Usage and Applicability of BGP Link-State Shortest Path Routing (BGP-SPF) in Data Centers Option A: Usage and Applicability of BGP Link-State Shortest Path First (SPF) Routing in Data Centers Option B: Usage and Applicability of BGP - Link State (BGP-LS) Shortest Path First (SPF) Routing in Data Centers b) To match how the companion document expands "BGP-LS" and "SPF", may we update the Abstract and Introduction as shown below for consistency? Original (Abstract): This document discusses the usage and applicability of BGP Link-State Shortest Path First (BGP-SPF) extensions in data center networks utilizing Clos or Fat-Tree topologies. Perhaps: This document discusses the usage and applicability of BGP - Link State (BGP-LS) Shortest Path First (SPF) extensions in data center networks utilizing Clos or Fat Tree topologies. ... Original (Introduction): This document complements [I-D.ietf-lsvr-bgp-spf] by discussing the applicability of the BGP-SPF technology in a simple and fairly common deployment scenario, which is described in Section 3. Perhaps: This document complements [RFC9815] by discussing the applicability of the BGP - Link State (BGP-LS) Shortest Path First (SPF) technology in a simple and fairly common deployment scenario, which is described in Section 3. c) Throughout the text, we note "BGP-SPF" vs. "BGP SPF". "BGP SPF" is used both in the companion document and the IANA registry at <https://www.iana.org/assignments/bgp-spf/>. Would you like to update each instance of "BGP-SPF" to "BGP SPF" for consistency? See one example below: Original: The document is intended to provide simplified guidance for the deployment of BGP-SPF extensions. Perhaps: The document is intended to provide simplified guidance for the deployment of BGP SPF extensions. --> <title abbrev="BGP-SPF Applicability">Usage and Applicability of BGP Link-State Shortest Path Routing (BGP-SPF) in Data Centers</title> <seriesInfo name="RFC" value="9816"/> <author fullname="Keyur Patel" initials="K" surname="Patel"> <organization>Arrcus, Inc.</organization> <address> <postal> <street>2077 Gateway Pl</street> <city>SanJose, CA</city> <country>USA</country>Jose</city><region>CA</region> <country>United States of America</country> <code>95110</code> </postal><phone/><email>keyur@arrcus.com</email> </address> </author> <author fullname="Acee Lindem" initials="A" surname="Lindem"> <organization>LabN Consulting, L.L.C.</organization> <address> <postal> <street>301 Midenhall Way</street><city>Cary, NC</city> <country>USA</country><city>Cary</city><region>NC</region> <country>United States of America</country> <code>95110</code> </postal><phone/><email>acee.ietf@gmail.com</email> </address> </author> <author fullname="Shawn Zandi" initials="S" surname="Zandi"><organization>Linkedin</organization><organization>LinkedIn</organization> <address> <postal> <street>222 2nd Street</street> <city>San Francisco</city> <region>CA</region> <code>94105</code><country>USA</country><country>United States of America</country> </postal> <email>szandi@linkedin.com</email> </address> </author> <author fullname="Gaurav Dawra" initials="G" surname="Dawra"> <organization>Linkedin</organization> <address> <postal> <street>222 2nd Street</street> <city>San Francisco</city> <region>CA</region> <code>94105</code><country>USA</country><country>United States of America</country> </postal> <email>gdawra@linkedin.com</email> </address> </author> <author fullname="Jie Dong" initials="J." surname="Dong"> <organization>Huawei Technologies</organization> <address> <postal> <street>No. 156 Beiqing Road</street> <city>Beijing</city><region/> <code/><country>China</country> </postal> <email>jie.dong@huawei.com</email> </address> </author><date/><date month="July" year="2025"/> <area>RTG</area> <workgroup>lsvr</workgroup> <!-- [rfced] Please insert any keywords (beyond those that appear in the title) for use on https://www.rfc-editor.org/search. --> <abstract> <t>This document discusses the usage and applicability of BGP Link-State Shortest Path First (BGP-SPF) extensions in data center networks utilizing Clos orFat-TreeFat Tree topologies. The document is intended to provide simplified guidance for the deployment of BGP-SPF extensions.</t> </abstract> </front> <middle><section title="Introduction"><section> <name>Introduction</name> <t>This document complements <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>target="RFC9815"/> by discussing the applicability of the BGP-SPF technology in a simple and fairly common deployment scenario, which is described in <xref format="default" target="usecases"/>.</t> <t><xref format="default" target="motivation"/> describes the reasons for BGP modifications for such deployments.</t> <t><xref format="default" target="bgpspf"/> covers theBGP Link-State Shortest Path First (IGP-SPF)BGP-SPF protocol enhancements to BGP to meet these requirements and their applicability to data center <xref format="default" target="Clos"/> networks.</t> </section> <sectionanchor="reco" title="Recommended Reading">anchor="reco"> <name>Recommended Reading</name> <!-- [rfced] We updated "RFC 5580" to "RFC 5880" in the following sentence, and elsewhere in the document where "BFD" is referenced, as "BFD" is defined in RFC 5880 and not mentioned in RFC 5580. If this is not correct, please let us know. Original: This document also assumes knowledge of data center routing protocols such as BGP [RFC4271], BGP-SPF [I-D.ietf-lsvr-bgp-spf], OSPF [RFC2328] [RFC5340], as well as data center Operations, Administration, and Maintenance (OAM) protocols like Link Layer Discovery Protocol (LLDP) [RFC4957] and Bi- Directional Forwarding Detection (BFD) [RFC5580]. Current: This document also assumes knowledge of data center routing protocols such as BGP [RFC4271], BGP-SPF [RFC9815], and OSPF [RFC2328] [RFC5340] as well as data center Operations, Administration, and Maintenance (OAM) protocols like the Link Layer Discovery Protocol (LLDP) [RFC4957] and Bidirectional Forwarding Detection (BFD) [RFC5580]. --> <t>This document assumes knowledge of existing data center networks and data center network topologies <xref format="default" target="Clos"/>. This document also assumes knowledge of data center routing protocols such as BGP <xref format="default" target="RFC4271"/>, BGP-SPF <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>,target="RFC9815"/>, and OSPF <xref format="default" target="RFC2328"/> <xreftarget="RFC5340"/>,target="RFC5340"/> as well as data center Operations, Administration, and Maintenance (OAM) protocols like the Link Layer Discovery Protocol (LLDP) <xref format="default" target="RFC4957"/> andBi-DirectionalBidirectional Forwarding Detection (BFD) <xref format="default"target="RFC5580"/>.</t>target="RFC5880"/>.</t> </section> <sectionanchor="usecases" title="Commonanchor="usecases"> <name>Common DeploymentScenario">Scenario</name> <t>Within a data center, servers are commonly interconnected using the Clos topology <xref format="default" target="Clos"/>. The Clos topology is fullynon-blockingnon-blocking, and the topology is realized usingEqual Cost Multi-PathEqual-Cost Multipath (ECMP). In a multi-stage Clos topology, the minimum number of parallel paths in each tier is determined by the width of the stage as shown inthe figure 1.</t> <t><xref target="fig1"/>.</t> <figure anchor="fig1"> <name>Illustration of thebasicBasic Clos</name> <artwork align="left" alt="" name="" type=""><![CDATA[ Tier 1 +-----+ |NODE | +->| 1 |--+ | +-----+ | Tier 2 | | Tier 2 +-----+ | +-----+ | +-----+ +------------->|NODE |--+->|NODE |--+--|NODE |--------------+ | +-----| 5 |--+ | 2 | +--| 7 |-----+ | | | +-----+ +-----+ +-----+ | | | | | | | | +-----+ +-----+ +-----+ | | | +------+---->|NODE |--+ |NODE | +--|NODE |-----+------+ | | | | +---| 6 |--+->| 3 |--+--| 8 |---+ | | | | | | | +-----+ | +-----+ | +-----+ | | | | | |Tier 3| | | | | |Tier 3| | +-----+ +-----+ | +-----+ | +-----+ +-----+ |NODE | |NODE | +->|NODE |--+ |NODE | |NODE | | 9 | | 10 | | 4 | | 11 | | 12 | +-----+ +-----+ +-----+ +-----+ +-----+ | | | | | | | | | | | | <- Servers -> <- Servers ->Tier]]></artwork> </figure> <ul spacing="normal"> <li>Tier 1 is comprised of Nodes 1, 2, 3, and4 Tier4</li> <li>Tier 2 is comprised of Nodes 5, 6, 7, and8 Tier8</li> <li>Tier 3 is comprised of Nodes 9, 10, 11, and12 ]]></artwork> </figure> </t>12</li> </ul> </section> <sectionanchor="motivation" title="Justificationanchor="motivation"> <name>Justification for the BGP-SPFExtension">Extension</name> <t>To simplifyL3Layer 3 (L3) routing and operations, many data centers use BGP as a routing protocol to create both an underlay and an overlay network for their ClosTopologiestopologies <xref format="default" target="RFC7938"/>. However, BGP is a path-vector routing protocol. Since it does not create a fabric topology, it uses hop-by-hop External BGP (EBGP) peering to facilitate hop-by-hop routing to create the underlay network and to resolve any overlay next hops. The hop-by-hop BGP peering paradigm imposes several restrictions within a Clos. It prohibits the deployment ofRoute Reflectors/Route Controllersroute reflectors / route controllers as the EBGP sessions are congruent with the data path. The BGP best-path algorithm isprefix-basedprefix based, and it prevents announcements of prefixes to other BGP speakers until the best-path decision process has been performed for the prefix at each intermediate hop. These restrictions significantly delay the overall convergence of the underlay network within a Clos network.</t><t>The<!--[rfced] "SPF" is not mentioned in RFC 9552. Should a different RFC be referenced or was "OSPF" perhaps intended? Original: The BGP-SPF modifications allow BGP to overcome these limitations. Furthermore, using the BGP-LS Network Layer Reachability Information (NLRI) format allows the BGP-SPF data to be advertised for nodes, links, and prefixes in the BGP routing domain and used forShort-Path-FirstShort- Path-First (SPF) computations [RFC9552]. --> <t>The BGP-SPF modifications allow BGP to overcome these limitations. Furthermore, using the BGP-LS Network Layer Reachability Information (NLRI) format allows the BGP-SPF data to be advertised for nodes, links, and prefixes in the BGP routing domain and used for SPF computations <xref target="RFC9552"/>.</t> <t>Additional motivation for deploying BGP-SPF is included in <xreftarget="I-D.ietf-lsvr-bgp-spf"/>.</t>target="RFC9815"/>.</t> </section> <sectionanchor="bgpspf" title="BGP-SPFanchor="bgpspf"> <name>BGP-SPF Applicability to ClosNetworks">Networks</name> <t>With the BGP-SPF extensions <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>,target="RFC9815"/>, the BGP best-path computation and route computation are replaced with link-state algorithms such as those used by OSPF <xref format="default" target="RFC2328"/>, both to determine whetherana BGP-LS-SPF NLRI has changed and needs to bere-advertisedreadvertised and to compute the BGP routes. These modifications will significantly improve convergence of the underlay while affording the operational benefits of a single routing protocol <xref format="default" target="RFC7938"/>.</t> <t>Data center controllers typically require visibility to the BGP topology to compute traffic-engineered paths. These controllers learn the topology and other relevant information via the BGP-LS address family <xref format="default"target="RFC9552"/>target="RFC9552"/>, which is totally independent of the underlay address families (usually IPv4/IPv6 unicast). Furthermore, in traditional BGP underlays, all the BGP routers will need to advertise their BGP-LS information independently. With the BGP-SPF extensions, controllers can learn the topology using the same BGP advertisements used to compute the underlay routes. Furthermore, these data center controllers can avail the convergence advantages of the BGP-SPF extensions. The placement of controllers can be outside of the forwarding path or within the forwarding path.</t> <t>Alternatively, as each and every router in the BGP-SPF domain will have a complete view of the topology, the operator can also choose to configure BGP sessions in the hop-by-hop peering model described in <xref format="default" target="RFC7938"/> along with BFD <xref format="default" target="RFC5580"/>. In doing so, while the hop-by-hop peering model lacks the inherent benefits of the controller-based model, BGP updates need not be serialized by the BGP best-path algorithm in either of these models. This helps overall network convergence.</t> <sectionanchor="lsvrsafi" title="Usageanchor="lsvrsafi"> <name>Usage ofBGP-LS SPF SAFI"> <t>Section 5.1 of <xrefBGP-LS-SPF SAFI</name> <t><xref section="5.1" sectionFormat="of" format="default"target="I-D.ietf-lsvr-bgp-spf"/>target="RFC9815"/> defines a new BGP-LS-SPF SAFI for announcement of the BGP-SPF link-state. The NLRI format and its associated attributes follow the format of BGP-LS for node, link, and prefix announcements. Whether the peering model within a Clos follows hop-by-hop peering described in <xref format="default" target="RFC7938"/> or any controller-based or route-reflector peering, an operator can exchange BGP-LS-SPF SAFI routes over the BGP peering by simply configuring BGP-LS-SPF SAFI between the necessary BGP speakers.</t> <t>The BGP-LS-SPF SAFI can alsoco-existcoexist with BGP IP Unicast SAFI <xreftarget="RFC4760"/>target="RFC4760"/>, which could exchange overlapping IP routes. One use case for this is where BGP-LS-SPF routes are used for the underlay and BGP IP Unicast routes for VPNs are advertised in the overlay as described in <xref target="RFC4364"/>. The routes received by these SAFIs are evaluated, stored, and announced independently according to the rules of <xref format="default" target="RFC4760"/>. Thetie-breakingtiebreaking of route installation is a matter of the local policies and preferences of the network operator.</t> <t>Finally, as the BGP-SPF peering is done following the procedures described in <xref format="default" target="RFC4271"/>, all the existing transport security mechanisms including those in <xref format="default" target="RFC5925"/> are available for the BGP-LS-SPF SAFI.</t> <sectionanchor="other-safi" title="Relationshipanchor="other-safi"> <name>Relationship to Other BGP AFI/SAFITuples">Tuples</name> <t>Normally, the BGP-LS-SPF AFI/SAFI is used solely to compute the underlay and is given precedence over other AFI/SAFIs in route processing. Other BGP SAFIs, e.g., IPv6/IPv6Unicast VPNunicast VPN, would use the BGP-SPF computed routes fornext hopnext-hop resolution.</t> </section> </section> <sectionanchor="peering" title="Peering Models">anchor="peering"> <name>Peering Models</name> <t>As previously stated, BGP-SPF can be deployed using the existing peering model where there is a single-hop BGP session on each and every link in the data center fabric <xref format="default" target="RFC7938"/>. This provides for both the advertisement of routes and the determination of link and neighboring router availability. With BGP-SPF, the underlay will converge faster due to changes to the decision process that will allow NLRI changes to be advertised faster after detecting a change.</t> <sectionanchor="sparse-peering" title="Sparseanchor="sparse-peering"> <name>Sparse PeeringModel">Model</name> <t>Alternately, BFD <xref format="default" target="RFC5580"/> can be used to swiftly determine the availability oflinkslinks, and the BGP peering model can be significantly sparser than the data center fabric. BGP-SPF sessions only need to be established with enough peers to provide abi-connectedbiconnected graph. If Internal BGP (IBGP) is used, then the BGP routers at tier N-1 will act as route-reflectors for the routers at tier N.</t> <t>The obvious usage of sparse peering is to avoid parallel BGP sessions on links between the same two routers in the data center fabric. However, this use case is not very useful since parallel L3 links between the same two BGP routers are rare in Clos orFat-TreeFat Tree topologies. Additionally, when there are multiple links, they are often aggregatedat the link layerusing Link Aggregation Groups (LAGs) at the link layer <xref target="IEEE.802.1AX"/> rather than at the IP layer. Two more interesting scenarios are described below.</t> <t>In current data center topologies, there is often a very dense mesh of links between levels, e.g., leaf and spine, providing32-way, 64-way,32-way paths, 64-way paths, or moreEqual-Cost Multi-Path (ECMP) paths.ECMPs. In these topologies, it is desirable not to have a BGP session on everylinklink, and techniques such as the one described in <xref format="default" target="bi-connected"/> can be used to establish sessions on some subset of northbound links. For example, in aSpine-LeafSpine/Leaf topology, each leaf router would only peer with a subset of the spines dependent on the flooding redundancy required to be reasonably certain that every node within the BGP-SPF routing domain has the complete topology.</t> <t>Alternately, controller-based data center topologies are envisioned where BGP speakers within the data center only establish BGP sessions with two or more controllers. In these topologies, fabric nodes below the first tier, as shown in Figure 1 of <xref format="default" target="RFC7938"/>, will establish BGP multi-hop sessions with the controllers. For the multi-hop sessions, determining the route to the controllers without depending on BGP would need to be through some other means beyond the scope of this document. However, the BGP discovery mechanisms described in <xref format="default" target="bgp-discovery"/> would be one possibility.</t> </section> <sectionanchor="bi-connected" title="Bi-Connectedanchor="bi-connected"> <name>Biconnected GraphHeuristic">Heuristic</name> <t>Withthisa biconnected graph heuristic, discovery of BGP SPF peers is assumed, e.g., as described in <xref format="default" target="bgp-discovery"/>. In this context,"bi-connected""biconnected" refers to the fact that there must be anadverised linkadvertised Link NLRI for both BGP and SPF peers associated with the link before the link can be used in the BGP SPF routecalcuation.calculation. Additionally, it is assumed that the direction of the peering can be ascertained. In the context of a data center fabric, the direction is either northbound (toward the spine), southbound (toward theTop-Of-RackTop-of-Rack (ToR)routers)routers), or east-west (same level in the hierarchy). The determination of the direction is beyond the scope of this document. However, it would be reasonable to assume a technique where the ToR routers can be identified and the number of hops to the ToR is used to determine the direction.</t> <t>In this heuristic, BGP speakers allow passive session establishment for southbound BGP sessions. For northbound sessions, BGP speakers will attempt to maintain two northbound BGP sessions with different routers. For east-west sessions, passive BGP session establishment is allowed. However, a BGP speaker will never actively establish an east-west BGP session unless it cannot establish two northbound BGP sessions.</t> <t>BGP SPF sparse peering deployments not using thishueristicheuristic are possible but are not described herein and are considered out of scope.</t> </section> </section> <sectionanchor="bgp-policy" title="BGPanchor="bgp-policy"> <name>BGP Spine/Leaf TopologyPolicy">Policy</name> <t>One of the advantages of using BGP-SPF as the underlay protocol is that BGP policy can be applied at any level. For example, depending on the topology, it may be possible to aggregate or filter prefix advertisements using the existing BGP policy. In Spine/Leaf topologies, it is not necessary to advertise a BGP-LS Prefix NLRI received by leaf nodes from the spine back to other spine nodes. If a commonASAutonomous System (AS) is used for the spine nodes, this can easily be accomplished with EBGP and a simple policy to filter advertisements from the leaves to the spine if the first AS in the AS path is the spine AS.</t> <t>In the figure below, the leaves would not advertise anyNLRINLRIs with AS 64512 as the first AS in the AS path.</t><t><figure anchor="fig2"> <name>Spine/Leaf Topology Policy</name> <artwork align="left" alt="" name="" type=""><![CDATA[ +--------+ +--------+ +--------+ AS 64512 | | | | | | for Spine | Spine 1+----+ Spine 2+- ......... -+ Spine N| Nodes at | | | | | | this Level +-+-+-+-++ ++-+-+-+-+ +-+-+-+-++ +------+ | | | | | | | | | | | | +-----|-|-|------+ | | | | | | | | | +--|-|-|--------+-|-|-----------------+ | | | | | | | | | +---+ | | | | | | | | | | | | +--|-|-------------------+ | | | | | | | | | | | | +------+ +----+ | | | | | | | | | +--------------|----------+ | | | | | | | | | +-------------+ | | | | | | | | +----|--|----------------|--|--------+ | | | | | | +------|--|--------------+ | | | | | | | | +------+ | | | | | | | | ++--+--++ +-+-+--++ ++-+--+-+ ++-+--+-+ | Leaf 1| | Leaf 2| ........ | Leaf X| | Leaf Y| +-------+ +-------+ +-------+ +-------+ ]]></artwork> </figure></t></section> <sectionanchor="bgp-discovery-con" title="BGPanchor="bgp-discovery-con"> <name>BGP Peer DiscoveryConsiderations">Considerations</name> <t>The basic functionality of peer discovery is tobediscover the address of a single-hop peer in the case where the peer address is notpre-configured.preconfigured. This is being accomplished today by using IPv6 Router Advertisements(RA)(RAs) <xref format="default" target="RFC4861"/> and assuming that a BGP session is desired with any discovered peer. Beyond the basic functionality, it may be useful to have the following information relating to the BGP session:</t><list style="symbols"> <t>Autonomous System (AS)<ul spacing="normal"> <li> <t>The AS and BGP Identifier of a potential peer.</t><t>Security capabilities supported</li> <li> <t>Supported security capabilities, and for cryptographic authentication, the security capabilities and possibly akey-chainkey chain <xref format="default" target="RFC8177"/>to be used.</t> <t>Sessionfor use.</t> </li> <li> <t>A Session PolicyIdentifier - AIdentifier, which is a group number or name used to associate common session parameters with the peer. For example, in a data center, BGP sessions with a ToR device could have different parameters than BGP sessions between leaf and spine.</t></list></li> </ul> <t>In a data center fabric, it is often useful to know whether a peer is southbound (towards the servers) or northbound (towards the spine or super-spine), e.g., see <xref format="default" target="bi-connected"/>. One mechanism, without specifying all the details, might be for the ToR routers to be identified when installed and for theothersother routers in the fabric to determine their level based on the distance from the closest ToR router.</t> <t>If there are multiple links between BGP speakers or the links between BGP speakers are unnumbered, it is also useful to be able to establish multi-hop sessions using the loopback addresses. This will often require the discovery protocol to installroute(s)one or more routes toward the potential peer loopback addresses prior to BGP session establishment.</t> <t>Finally, a simple BGP discovery protocol may be used to establish a multi-hop session with one or more controllers by advertising connectivity to one or more controllers.</t> </section> <sectionanchor="bgp-discovery" title="BGPanchor="bgp-discovery"> <name>BGP PeerDiscovery">Discovery</name> <sectionanchor="bgp-ipv6-peering" title="BGPanchor="bgp-ipv6-peering"> <name>BGP IPv6 SimplifiedPeering">Peering</name> <t>To conserve IPv4 address space and simplify operations, BGP-SPF routers inClos/FatClos / Fat Tree deployments can use IPv6 addresses as the peer address. For IPv4 address families, IPv6 peering as specified in <xref format="default" target="RFC8950"/> can be deployed to avoid configuring IPv4 addresses on router interfaces. When this is done, dynamic discovery mechanisms, as described in <xref format="default" target="bgp-discovery"/>, can be used to learn the global or link-local IPv6 peeraddressesaddresses, and IPv4 addresses need not be configured on these interfaces. If IPv6 link-local peering is used, then configuration of IPv6 global addresses is also not required <xreftarget="RFC7404"/> .target="RFC7404"/>. <!-- [rfced] We see that the approved I-D (v22) contains one instance of a keyword ("MUST" in Section 5.5.1). Is this intentional? If so, we will add the typical paragraph from BCP 14 regarding use of keywords after the Introduction and add RFCs 2119 and 8174 to the Normative References section. Otherwise, we will update "MUST" to "must". Original: The Link Local/Remote Identifiers of the peering interfaces MUST be used in the link NLRI as described in section 5.2.2 of [I-D.ietf-lsvr-bgp-spf]. --> The Link Local/Remote Identifiers of the peering interfaces MUST be used in the Link NLRI as described in <xreftarget="I-D.ietf-lsvr-bgp-spf"/>.</t>section="5.2.2" sectionFormat="of" target="RFC9815"/>.</t> </section> <sectionanchor="config-checking" title="BGP-LSanchor="config-checking"> <!--[rfced] Is "BGP-LS SPF Topology" correct or should it perhaps be "BGP-LS-SPF Topology" for consistency? Original: 5.5.2 BGP-LS SPF Topology Visibility forManagement">Management Perhaps: 5.5.2 BGP-LS-SPF Topology Visibility for Management --> <name>BGP-LS SPF Topology Visibility for Management</name> <t>Irrespective of whether or not BGP-SPF is used for route calculation, the BGP-LS-SPF route advertisements can be used to periodically construct theClos/FatClos / Fat Tree topology. This is especially useful in deployments where an Interior Gateway Protocol (IGP) is not used and the base BGP-LS routes <xref format="default" target="RFC9552"/> are not available. The resultant topology visibility can then be used for troubleshooting and consistency checking. This would normally be done on a central controller or other management toolwhichthat could also be used for fabric data path verification. The precise algorithms and heuristics, as well as the complete set of managementapplicationsapplications, is beyond the scope of this document.</t> </section> <sectionanchor="dci" title="Dataanchor="dci"> <name>Data Center Interconnect (DCI)Applicability">Applicability</name> <t>Since BGP-SPF is to be used for the routing underlay andDCIData Center Interconnect (DCI) gateway boxes typically have direct or very simple connectivity, BGP external sessions would typically not include the BGP-LS-SPF SAFI.</t> </section> </section> </section> <!--[rfced] Would repeating "Non" make this section title more clear? The meaning seems to be applicability to topologies that are neither Clos nor Fat Tree topologies. Original: 6. Non-CLOS/FAT Tree Topology Applicability Current: 6. Non-Clos / Fat Tree Topology Applicability Perhaps: 6. Non-Clos / Non-Fat-Tree Topology Applicability --> <sectionanchor="other-env" title="Non-CLOS/FATanchor="other-env"> <name>Non-Clos / Fat Tree TopologyApplicability">Applicability</name> <t>The BGP-SPF extensions <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>target="RFC9815"/> can be used in other topologies and avail the inherent convergence improvements. Additionally, sparse peering techniques may be utilized <xref format="default" target="peering"/>. However, determining whether to establish a BGP session is morecomplexcomplex, and the heuristic described in <xref format="default" target="bi-connected"/> cannot be used. In such topologies, other techniques such as those described in <xref format="default" target="RFC9667"/> may be employed. One potential deployment would be the underlay for a Service Provider (SP) backbone where usage of a single protocol, i.e., BGP, is desired.</t> </section> <sectionanchor="non-transit-node" title="Non-Transitanchor="non-transit-node"> <name>Non-Transit NodeCapability">Capability</name> <t>In certain scenarios, a BGP node wishes to participate in the BGP-SPF topology but never be used for transit traffic. These include situations where a server wants to make application services available to clients homed at subnets throughout the BGP-SPF domain but does not ever want to be used as a router (i.e., carry transit traffic). Another specific instance is where a controller is resident on a server and direct connectivity to the controller is required throughout the entire domain. This can readily be accomplished using theBGP-LSBGP-LS-SPF Node NLRI Attribute SPF Status TLV as described in <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>.</t>target="RFC9815"/>.</t> </section> <sectionanchor="policy" title="BGPanchor="policy"> <name>BGP PolicyApplicability">Applicability</name> <t>Existing BGP policy such as prefix filtering may be used in conjunction with the BGP-LS-SPF SAFI. When BGP policy is used with the BGP-LS-SPF SAFI, BGP speakers in the BGP-LS-SPF routing domain will not all have the same set ofNLRINLRIs and will compute a different BGP local routing table. Consequently, care must be taken to assure routing is consistent and blackholes or routing loops do not ensue. However, this is no different than if traditional BGP routing using the IPv4 and IPv6 address families were used.</t> </section> <sectionanchor="IANA" title="IANA Considerations"> <t>Noanchor="IANA"> <name>IANA Considerations</name> <t>This document has no IANAupdates are requested by this document.</t>actions.</t> </section> <sectionanchor="Security" title="Security Considerations">anchor="Security"> <name>Security Considerations</name> <t>This document introduces no new security considerations above and beyond those already specified inthe<xref format="default" target="RFC4271"/> and <xref format="default"target="I-D.ietf-lsvr-bgp-spf"/>.</t> </section> <section anchor="Acknowledgements" title="Acknowledgements"> <t>The authors would like to thank Alvaro Retana, Yan Filyurin, Boris Hassanov, Stig Venaas, Ron Bonica, Mallory Knodel, Dhruv Dhody, Erik Kline, Eric Vyncke, and John Scudder for their review and comments.</t>target="RFC9815"/>.</t> </section> </middle> <back><references title="Normative References"> <?rfc include='reference.I-D.ietf-lsvr-bgp-spf.xml'?><references> <name>References</name> <references> <name>Normative References</name> <!--[RFC9815] draft-ietf-lsvr-bgp-spf-51 is RFC-to-be 9815. Note: will need an update to the title based on author reply --> <reference anchor="RFC9815" target="https://www.rfc-editor.org/info/rfc9815"> <front> <title>BGP Link-State Shortest Path First (SPF) Routing</title> <author initials="K." surname="Patel" fullname="Keyur Patel"> <organization>Arrcus, Inc.</organization> </author> <author initials="A." surname="Lindem" fullname="Acee Lindem"> <organization>LabN Consulting, LLC</organization> </author> <author initials="S." surname="Zandi" fullname="Shawn Zandi"> <organization>LinkedIn</organization> </author> <author initials="W." surname="Henderickx" fullname="Wim Henderickx"> <organization>Nokia</organization> </author> <date month="July" year="2025" /> </front> <seriesInfo name="RFC" value="9815"/> <seriesInfo name="DOI" value="10.17487/RFC9815"/> </reference> </references><references title="Informative References"> <?rfc include='reference.RFC.2328.xml'?> <?rfc include='reference.RFC.4271.xml'?> <?rfc include='reference.RFC.4364.xml'?> <?rfc include='reference.RFC.4760.xml'?> <?rfc include='reference.RFC.4861.xml'?> <?rfc include='reference.RFC.4957.xml'?> <?rfc include='reference.RFC.5340.xml'?> <?rfc include='reference.RFC.5580.xml'?> <?rfc include='reference.RFC.5925.xml'?> <?rfc include='reference.RFC.7404.xml'?> <?rfc include='reference.RFC.7938.xml'?> <?rfc include='reference.RFC.8177.xml'?> <?rfc include='reference.RFC.8950.xml'?> <?rfc include='reference.RFC.9552.xml'?> <?rfc include='reference.RFC.9667.xml'?><references> <name>Informative References</name> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.2328.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4271.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4364.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4760.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4861.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.4957.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5340.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5580.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5880.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.5925.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7404.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.7938.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8177.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.8950.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9552.xml"/> <xi:include href="https://bib.ietf.org/public/rfc/bibxml/reference.RFC.9667.xml"/> <referenceanchor="IEEE.802.1AX" target="https://standards.ieee.org/standard/802_1AX-2020.html">anchor="IEEE.802.1AX"> <front> <title>IEEE Standard for Local and Metropolitan AreaNetworks: LinkNetworks--Link Aggregation</title> <author> <organization>IEEE</organization> </author> <date month="May" year="2020"/> </front> <seriesInfo name="IEEE Std" value="802.1AX-2020"/> <seriesInfo name="DOI" value="10.1109/IEEESTD.2020.9105034"/> </reference> <reference anchor="Clos" target=""> <front> <title>A Study of Non-Blocking Switching Networks</title> <authorinitials="" surname="">initials="C." surname="Clos"> <organization/> </author> <date month="March" year="1953"/> </front><seriesInfo name="" value="The<refcontent>The Bell System Technical Journal,Vol. 32(2), DOI 10.1002/j.1538-7305.1953.tb01433.x"/>vol. 32, no. 2, pp. 406-424</refcontent> <seriesInfo name="DOI" value="10.1002/j.1538-7305.1953.tb01433.x"/> </reference> </references> </references> <section anchor="Acknowledgements" numbered="false"> <name>Acknowledgements</name> <t>The authors would like to thank <contact fullname="Alvaro Retana"/>, <contact fullname="Yan Filyurin"/>, <contact fullname="Boris Hassanov"/>, <contact fullname="Stig Venaas"/>, <contact fullname="Ron Bonica"/>, <contact fullname="Mallory Knodel"/>, <contact fullname="Dhruv Dhody"/>, <contact fullname="Erik Kline"/>, <contact fullname="Éric Vyncke"/>, and <contact fullname="John Scudder"/> for their reviews and comments.</t> </section> <!-- [rfced] FYI - We updated the following terms for consistency. Please let us know of any objections. BGP-LS Node NLRI Attribute SPF Status TLV -> BGP-LS-SPF Node NLRI Attribute SPF Status TLV (per the companion doc) BGP-LS SPF SAFI -> BGP-LS-SPF SAFI (per the companion doc) Clos Topologies -> Clos topologies Fat-Tree -> Fat Tree (per use in the RFC Series) link NLRI -> Link NLRI (per RFC 9552) Route Controllers -> route controllers (per companion document) Route Reflectors -> route reflectors (per companion document) Spine Nodes -> spine nodes Unicast -> unicast --> <!-- [rfced] FYI - We have updated the expansions for the following abbreviations to reflect the form on the right for consistency with the companion document and/or RFC Series. Bi-Directional Forwarding Detection (BFD) -> Bidirectional Forwarding Detection (BFD) Equal-Cost Multi-Path (ECMP) -> Equal-Cost Multipath (ECMP) Top-Of-Rack (ToR) -> Top-of-Rack (ToR) --> <!-- [rfced] Please review the "Inclusive Language" portion of the online Style Guide <https://www.rfc-editor.org/styleguide/part2/#inclusive_language> and let us know if any changes are needed. Updates of this nature typically result in more precise language, which is helpful for readers. For example, please consider whether the following should be updated: - blackhole In addition, please consider whether "traditional" should be updated for clarity. While the NIST website <https://web.archive.org/web/20250214092458/https://www.nist.gov/ nist-research-library/nist-technical-series-publications-author-instructions#table1> indicates that this term is potentially biased, it is also ambiguous. "Tradition" is a subjective term, as it is not the same for everyone. --> </back> </rfc>