Usage and Applicability of BGP Link State (BGP-LS) Shortest Path First (SPF) Routing in Data Centers

Usage and Applicability of BGP Link State (BGP-LS) Shortest Path First (SPF) Routing in Data Centers Arrcus, Inc.

2077 Gateway Pl San Jose CA United States of America 95110 keyur@arrcus.com

Arrcus, Inc.

301 Midenhall Way Cary NC United States of America 27513 acee.ietf@gmail.com

shafagh@shafagh.com

Google

Sunnyvale CA United States of America gdawra.ietf@gmail.com

Huawei Technologies

No. 156 Beiqing Road Beijing China jie.dong@huawei.com

RTG lsvr 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. The document is intended to provide simplified guidance for the deployment of BGP-LS SPF extensions.

Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

. Introduction
. Recommended Reading
. Common Deployment Scenario
. Justification for the BGP-SPF Extension
. BGP-SPF Applicability to Clos Networks
- . Usage of BGP-LS-SPF SAFI
  - . Relationship to Other BGP AFI/SAFI Tuples
- . Peering Models
  - . Sparse Peering Model
  - . Biconnected Graph Heuristic
- . BGP Spine/Leaf Topology Policy
- . BGP Peer Discovery Considerations
- . BGP Peer Discovery
  - . BGP IPv6 Simplified Peering
  - . BGP-LS-SPF Topology Visibility for Management
  - . Data Center Interconnect (DCI) Applicability
. Non-Clos / Fat Tree Topology Applicability
. Non-Transit Node Capability
. BGP Policy Applicability
. IANA Considerations
. Security Considerations
. References
- . Normative References
- . Informative References
Acknowledgements
Authors' Addresses

Introduction This document complements 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 . describes the reasons for BGP modifications for such deployments. covers the BGP SPF protocol enhancements to BGP to meet these requirements and their applicability to data center networks.

Recommended Reading This document assumes knowledge of existing data center networks and data center network topologies . This document also assumes knowledge of data center routing protocols such as BGP , BGP-LS SPF , and OSPF as well as data center Operations, Administration, and Maintenance (OAM) protocols like the Link Layer Discovery Protocol (LLDP) and Bidirectional Forwarding Detection (BFD) .

Common Deployment Scenario Within a data center, servers are commonly interconnected using the Clos topology . The Clos topology is fully non-blocking, and the topology is realized using Equal-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 in .

Illustration of the Basic Clos 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 1 is comprised of Nodes 1, 2, 3, and 4
Tier 2 is comprised of Nodes 5, 6, 7, and 8
Tier 3 is comprised of Nodes 9, 10, 11, and 12

Justification for the BGP-SPF Extension To simplify Layer 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 Clos topologies . 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 of route reflectors / route controllers as the EBGP sessions are congruent with the data path. The BGP best-path algorithm is prefix 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. 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 . Additional motivation for deploying BGP-SPF is included in .

BGP-SPF Applicability to Clos Networks With the BGP-SPF extensions , the BGP best-path computation and route computation are replaced with link-state algorithms such as those used by OSPF , both to determine whether a BGP-LS-SPF NLRI has changed and needs to be readvertised 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 . 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 , which is totally independent of the underlay address families (usually IPv4/IPv6 unicast). Furthermore, in usual 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. 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 along with BFD . 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.

Usage of BGP-LS-SPF SAFI 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 as described in 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. The BGP-LS-SPF SAFI can also coexist with BGP IP Unicast SAFI , 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 . The routes received by these SAFIs are evaluated, stored, and announced independently according to the rules of . The tiebreaking of route installation is a matter of the local policies and preferences of the network operator. Finally, as the BGP-SPF peering is done following the procedures described in , all the existing transport security mechanisms including those in are available for the BGP-LS-SPF SAFI.

Relationship to Other BGP AFI/SAFI Tuples 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/IPv6 unicast VPN, would use the BGP-SPF computed routes for next-hop resolution.

Peering Models 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 . 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.

Sparse Peering Model Alternately, BFD can be used to swiftly determine the availability of links, 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 a biconnected 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. 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 or Fat Tree topologies. Additionally, when there are multiple links, they are often aggregated using Link Aggregation Groups (LAGs) at the link layer rather than at the IP layer. Two more interesting scenarios are described below. In current data center topologies, there is often a very dense mesh of links between levels, e.g., leaf and spine, providing 32-way paths, 64-way paths, or more ECMPs. In these topologies, it is desirable not to have a BGP session on every link, and techniques such as the one described in can be used to establish sessions on some subset of northbound links. For example, in a Spine/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. 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 , 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, which is beyond the scope of this document. However, the BGP discovery mechanisms described in would be one possibility.

Biconnected Graph Heuristic With a biconnected graph heuristic, discovery of BGP SPF peers is assumed, e.g., as described in . In this context, "biconnected" refers to the fact that there must be an advertised Link NLRI for both BGP SPF peers associated with the link before the link can be used in the BGP SPF route 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 the Top-of-Rack (ToR) 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. 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. BGP SPF sparse peering deployments not using this heuristic are possible but are not described herein and are considered out of scope.

BGP Spine/Leaf Topology Policy 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 common Autonomous 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. In the figure below, the leaves would not advertise any NLRIs with AS 64512 as the first AS in the AS path.

Spine/Leaf Topology Policy +--------+ +--------+ +--------+ AS 64512 | | | | | | for Spine | Spine 1+----+ Spine 2+- ......... -+ Spine N| Nodes at | | | | | | this Level +-+-+-+-++ ++-+-+-+-+ +-+-+-+-++ +------+ | | | | | | | | | | | | +-----|-|-|------+ | | | | | | | | | +--|-|-|--------+-|-|-----------------+ | | | | | | | | | +---+ | | | | | | | | | | | | +--|-|-------------------+ | | | | | | | | | | | | +------+ +----+ | | | | | | | | | +--------------|----------+ | | | | | | | | | +-------------+ | | | | | | | | +----|--|----------------|--|--------+ | | | | | | +------|--|--------------+ | | | | | | | | +------+ | | | | | | | | ++--+--++ +-+-+--++ ++-+--+-+ ++-+--+-+ | Leaf 1| | Leaf 2| ........ | Leaf X| | Leaf Y| +-------+ +-------+ +-------+ +-------+

BGP Peer Discovery Considerations The basic functionality of peer discovery is to discover the address of a single-hop peer in the case where the peer address is not preconfigured. This is being accomplished today by using IPv6 Router Advertisements (RAs) 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:

The AS and BGP Identifier of a potential peer.
Supported security capabilities, and for cryptographic authentication, the security capabilities and possibly a key chain for use.
A Session Policy Identifier, 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 router could have different parameters than BGP sessions between leaf and spine nodes.

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 . One mechanism, without specifying all the details, might be for the ToR routers to be identified when installed and for the other routers in the fabric to determine their level based on the distance from the closest ToR router. 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 install one or more routes toward the potential peer loopback addresses prior to BGP session establishment. 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.

BGP Peer Discovery

BGP IPv6 Simplified Peering To conserve IPv4 address space and simplify operations, BGP-SPF routers in Clos / Fat Tree deployments can use IPv6 addresses as the peer address. For IPv4 address families, IPv6 peering as specified in can be deployed to avoid configuring IPv4 addresses on router interfaces. When this is done, dynamic discovery mechanisms, as described in , can be used to learn the global or link-local IPv6 peer addresses, 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 . The Link Local/Remote Identifiers of the peering interfaces must be used in the Link NLRI as described in .

BGP-LS-SPF Topology Visibility for Management Irrespective of whether or not BGP-SPF is used for route calculation, the BGP-LS-SPF route advertisements can be used to periodically construct the Clos / Fat Tree topology. This is especially useful in deployments where an Interior Gateway Protocol (IGP) is not used and the base BGP-LS routes 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 tool that could also be used for fabric data path verification. The precise algorithms and heuristics, as well as the complete set of management applications, is beyond the scope of this document.

Data Center Interconnect (DCI) Applicability Since BGP-SPF is to be used for the routing underlay and Data Center Interconnect (DCI) gateway boxes typically have direct or very simple connectivity, BGP external sessions would typically not include the BGP-LS-SPF SAFI.

Non-Clos / Fat Tree Topology Applicability The BGP-SPF extensions can be used in other topologies and avail the inherent convergence improvements. Additionally, sparse peering techniques may be utilized as described in . However, determining whether to establish a BGP session is more complex, and the heuristic described in cannot be used. In such topologies, other techniques such as those described in 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.

Non-Transit Node Capability 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 the BGP-LS-SPF Node NLRI Attribute SPF Status TLV as described in .

BGP Policy Applicability 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 of NLRIs and will compute a different BGP local routing table. Consequently, care must be taken to assure that routing is consistent and that routes to unreachable destinations or routing loops do not ensue. However, this is no different than if classical BGP routing using the IPv4 and IPv6 address families were used.

IANA Considerations This document has no IANA actions.

Security Considerations This document introduces no new security considerations above and beyond those already specified in and .

References Normative References BGP Link State (BGP-LS) Shortest Path First (SPF) Routing Arrcus, Inc. LabN Consulting, LLC LinkedIn Nokia Informative References A Study of Non-Blocking Switching Networks The Bell System Technical Journal, vol. 32, no. 2, pp. 406-424 IEEE Standard for Local and Metropolitan Area Networks--Link Aggregation IEEE OSPF Version 2 This memo documents version 2 of the OSPF protocol. OSPF is a link- state routing protocol. [STANDARDS-TRACK] A Border Gateway Protocol 4 (BGP-4) This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol. The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability from which routing loops may be pruned, and, at the AS level, some policy decisions may be enforced. BGP-4 provides a set of mechanisms for supporting Classless Inter-Domain Routing (CIDR). These mechanisms include support for advertising a set of destinations as an IP prefix, and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths. This document obsoletes RFC 1771. [STANDARDS-TRACK] BGP/MPLS IP Virtual Private Networks (VPNs) This document describes a method by which a Service Provider may use an IP backbone to provide IP Virtual Private Networks (VPNs) for its customers. This method uses a "peer model", in which the customers' edge routers (CE routers) send their routes to the Service Provider's edge routers (PE routers); there is no "overlay" visible to the customer's routing algorithm, and CE routers at different sites do not peer with each other. Data packets are tunneled through the backbone, so that the core routers do not need to know the VPN routes. [STANDARDS-TRACK] Multiprotocol Extensions for BGP-4 This document defines extensions to BGP-4 to enable it to carry routing information for multiple Network Layer protocols (e.g., IPv6, IPX, L3VPN, etc.). The extensions are backward compatible - a router that supports the extensions can interoperate with a router that doesn't support the extensions. [STANDARDS-TRACK] Neighbor Discovery for IP version 6 (IPv6) This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence, to determine each other's link-layer addresses, to find routers, and to maintain reachability information about the paths to active neighbors. [STANDARDS-TRACK] Link-Layer Event Notifications for Detecting Network Attachments Certain network access technologies are capable of providing various types of link-layer status information to IP. Link-layer event notifications can help IP expeditiously detect configuration changes. This document provides a non-exhaustive catalogue of information available from well-known access technologies. This memo provides information for the Internet community. OSPF for IPv6 This document describes the modifications to OSPF to support version 6 of the Internet Protocol (IPv6). The fundamental mechanisms of OSPF (flooding, Designated Router (DR) election, area support, Short Path First (SPF) calculations, etc.) remain unchanged. However, some changes have been necessary, either due to changes in protocol semantics between IPv4 and IPv6, or simply to handle the increased address size of IPv6. These modifications will necessitate incrementing the protocol version from version 2 to version 3. OSPF for IPv6 is also referred to as OSPF version 3 (OSPFv3). Changes between OSPF for IPv4, OSPF Version 2, and OSPF for IPv6 as described herein include the following. Addressing semantics have been removed from OSPF packets and the basic Link State Advertisements (LSAs). New LSAs have been created to carry IPv6 addresses and prefixes. OSPF now runs on a per-link basis rather than on a per-IP-subnet basis. Flooding scope for LSAs has been generalized. Authentication has been removed from the OSPF protocol and instead relies on IPv6's Authentication Header and Encapsulating Security Payload (ESP). Even with larger IPv6 addresses, most packets in OSPF for IPv6 are almost as compact as those in OSPF for IPv4. Most fields and packet- size limitations present in OSPF for IPv4 have been relaxed. In addition, option handling has been made more flexible. All of OSPF for IPv4's optional capabilities, including demand circuit support and Not-So-Stubby Areas (NSSAs), are also supported in OSPF for IPv6. [STANDARDS-TRACK] Bidirectional Forwarding Detection (BFD) This document describes a protocol intended to detect faults in the bidirectional path between two forwarding engines, including interfaces, data link(s), and to the extent possible the forwarding engines themselves, with potentially very low latency. It operates independently of media, data protocols, and routing protocols. [STANDARDS-TRACK] The TCP Authentication Option This document specifies the TCP Authentication Option (TCP-AO), which obsoletes the TCP MD5 Signature option of RFC 2385 (TCP MD5). TCP-AO specifies the use of stronger Message Authentication Codes (MACs), protects against replays even for long-lived TCP connections, and provides more details on the association of security with TCP connections than TCP MD5. TCP-AO is compatible with either a static Master Key Tuple (MKT) configuration or an external, out-of-band MKT management mechanism; in either case, TCP-AO also protects connections when using the same MKT across repeated instances of a connection, using traffic keys derived from the MKT, and coordinates MKT changes between endpoints. The result is intended to support current infrastructure uses of TCP MD5, such as to protect long-lived connections (as used, e.g., in BGP and LDP), and to support a larger set of MACs with minimal other system and operational changes. TCP-AO uses a different option identifier than TCP MD5, even though TCP-AO and TCP MD5 are never permitted to be used simultaneously. TCP-AO supports IPv6, and is fully compatible with the proposed requirements for the replacement of TCP MD5. [STANDARDS-TRACK] Using Only Link-Local Addressing inside an IPv6 Network In an IPv6 network, it is possible to use only link-local addresses on infrastructure links between routers. This document discusses the advantages and disadvantages of this approach to facilitate the decision process for a given network. Use of BGP for Routing in Large-Scale Data Centers Some network operators build and operate data centers that support over one hundred thousand servers. In this document, such data centers are referred to as "large-scale" to differentiate them from smaller infrastructures. Environments of this scale have a unique set of network requirements with an emphasis on operational simplicity and network stability. This document summarizes operational experience in designing and operating large-scale data centers using BGP as the only routing protocol. The intent is to report on a proven and stable routing design that could be leveraged by others in the industry. YANG Data Model for Key Chains This document describes the key chain YANG data model. Key chains are commonly used for routing protocol authentication and other applications requiring symmetric keys. A key chain is a list containing one or more elements containing a Key ID, key string, send/accept lifetimes, and the associated authentication or encryption algorithm. By properly overlapping the send and accept lifetimes of multiple key chain elements, key strings and algorithms may be gracefully updated. By representing them in a YANG data model, key distribution can be automated. Advertising IPv4 Network Layer Reachability Information (NLRI) with an IPv6 Next Hop Multiprotocol BGP (MP-BGP) specifies that the set of usable next-hop address families is determined by the Address Family Identifier (AFI) and the Subsequent Address Family Identifier (SAFI). The AFI/SAFI definitions for the IPv4 address family only have provisions for advertising a next-hop address that belongs to the IPv4 protocol when advertising IPv4 Network Layer Reachability Information (NLRI) or VPN-IPv4 NLRI. This document specifies the extensions necessary to allow the advertising of IPv4 NLRI or VPN-IPv4 NLRI with a next-hop address that belongs to the IPv6 protocol. This comprises an extension of the AFI/SAFI definitions to allow the address of the next hop for IPv4 NLRI or VPN-IPv4 NLRI to also belong to the IPv6 protocol, the encoding of the next hop to determine which of the protocols the address actually belongs to, and a BGP Capability allowing MP-BGP peers to dynamically discover whether they can exchange IPv4 NLRI and VPN-IPv4 NLRI with an IPv6 next hop. This document obsoletes RFC 5549. Distribution of Link-State and Traffic Engineering Information Using BGP In many environments, a component external to a network is called upon to perform computations based on the network topology and the current state of the connections within the network, including Traffic Engineering (TE) information. This is information typically distributed by IGP routing protocols within the network. This document describes a mechanism by which link-state and TE information can be collected from networks and shared with external components using the BGP routing protocol. This is achieved using a BGP Network Layer Reachability Information (NLRI) encoding format. The mechanism applies to physical and virtual (e.g., tunnel) IGP links. The mechanism described is subject to policy control. Applications of this technique include Application-Layer Traffic Optimization (ALTO) servers and Path Computation Elements (PCEs). This document obsoletes RFC 7752 by completely replacing that document. It makes some small changes and clarifications to the previous specification. This document also obsoletes RFC 9029 by incorporating the updates that it made to RFC 7752. Dynamic Flooding on Dense Graphs Routing with link-state protocols in dense network topologies can result in suboptimal convergence times due to the overhead associated with flooding. This can be addressed by decreasing the flooding topology so that it is less dense. This document discusses the problem in some depth and an architectural solution. Specific protocol changes for IS-IS, OSPFv2, and OSPFv3 are described in this document.

Acknowledgements The authors would like to thank , , , , , , , , , and for their reviews and comments.

Authors' Addresses Arrcus, Inc.