< draft-ietf-lsvr-applicability-01.txt		draft-ietf-lsvr-applicability-02.txt >
	LSVR K. Patel		LSVR K. Patel
	Internet-Draft Arrcus, Inc.		Internet-Draft Arrcus, Inc.
	Intended status: Informational A. Lindem		Intended status: Informational A. Lindem
	Expires: April 25, 2019 Cisco Systems		Expires: November 2, 2019 Cisco Systems
	S. Zandi		S. Zandi
	G. Dawra		G. Dawra
	Linkedin		Linkedin
	October 22, 2018		May 1, 2019
	Usage and Applicability of Link State Vector Routing in Data Centers		Usage and Applicability of Link State Vector Routing in Data Centers
	draft-ietf-lsvr-applicability-01.txt		draft-ietf-lsvr-applicability-02.txt
	Abstract		Abstract
	This document discusses the usage and applicability of Link State		This document discusses the usage and applicability of Link State
	Vector Routing (LSVR) extensions in the CLOS architecture of Data		Vector Routing (LSVR) extensions in data center networks utilizing
	Center Networks. The document is intended to provide a simplified		CLOS or Fat-Tree topologies. The document is intended to provide a
	guide for the deployment of LSVR extensions.		simplified guide for the deployment of LSVR extensions.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on April 25, 2019.		This Internet-Draft will expire on November 2, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2019 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3		2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
	3. Recommended Reading . . . . . . . . . . . . . . . . . . . . . 3		3. Recommended Reading . . . . . . . . . . . . . . . . . . . . . 3
	4. Common Deployment Scenario . . . . . . . . . . . . . . . . . 3		4. Common Deployment Scenario . . . . . . . . . . . . . . . . . 3
	5. Justification for BGP SPF Extension . . . . . . . . . . . . . 4		5. Justification for BGP SPF Extension . . . . . . . . . . . . . 4
	6. LSVR Applicability to CLOS Networks . . . . . . . . . . . . . 5		6. LSVR Applicability to CLOS Networks . . . . . . . . . . . . . 5
	6.1. Usage of BGP-LS SAFI . . . . . . . . . . . . . . . . . . 5		6.1. Usage of BGP-LS SPF SAFI . . . . . . . . . . . . . . . . 5
	6.1.1. Relationship to Other BGP AFI/SAFI Tuples . . . . . . 6		6.1.1. Relationship to Other BGP AFI/SAFI Tuples . . . . . . 6
	6.2. Peering Models . . . . . . . . . . . . . . . . . . . . . 6		6.2. Peering Models . . . . . . . . . . . . . . . . . . . . . 6
	6.2.1. Sparse Peering Model . . . . . . . . . . . . . . . . 6		6.2.1. Sparse Peering Model . . . . . . . . . . . . . . . . 6
	6.2.2. Bi-Connected Graph Heuristic . . . . . . . . . . . . 7		6.2.2. Bi-Connected Graph Heuristic . . . . . . . . . . . . 7
	6.3. BGP Peer Discovery . . . . . . . . . . . . . . . . . . . 7		6.3. BGP Spine/Leaf Topology Policy . . . . . . . . . . . . . 7
	6.3.1. BGP Peer Discovery Requirements . . . . . . . . . . . 7		6.4. BGP Peer Discovery Requirements . . . . . . . . . . . . . 8
	6.3.2. BGP Peer Discovery Alternatives . . . . . . . . . . . 8		6.5. BGP Peer Discovery . . . . . . . . . . . . . . . . . . . 9
	6.3.3. Data Center Interconnect (DCI) Applicability . . . . 8		6.5.1. BGP Peer Discovery Alternatives . . . . . . . . . . . 9
	6.4. Non-CLOS/FAT Tree Topology Applicability . . . . . . . . 9		6.5.2. Data Center Interconnect (DCI) Applicability . . . . 9
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9		6.6. Non-CLOS/FAT Tree Topology Applicability . . . . . . . . 10
	8. Security Considerations . . . . . . . . . . . . . . . . . . . 9		7. BGP Policy Applicability . . . . . . . . . . . . . . . . . . 10
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9		8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9		9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
	10.1. Normative References . . . . . . . . . . . . . . . . . . 9		10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
	10.2. Informative References . . . . . . . . . . . . . . . . . 10		11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11		11.1. Normative References . . . . . . . . . . . . . . . . . . 11
			11.2. Informative References . . . . . . . . . . . . . . . . . 11
			Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
	1. Introduction		1. Introduction
	This document complements [I-D.ietf-lsvr-bgp-spf] by discussing the		This document complements [I-D.ietf-lsvr-bgp-spf] by discussing the
	applicability of the technology in a simple and fairly common		applicability of the technology in a simple and fairly common
	deployment scenario, which is described in Section 4.		deployment scenario, which is described in Section 4.
	After describing the deployment scenario, Section 5 will describe the		After describing the deployment scenario, Section 5 will describe the
	reasons for BGP modifications for such deployments.		reasons for BGP modifications for such deployments.
	skipping to change at page 3, line 23 ¶		skipping to change at page 3, line 23 ¶
	3. Recommended Reading		3. Recommended Reading
	This document assumes knowledge of existing data center networks and		This document assumes knowledge of existing data center networks and
	data center network topologies [CLOS]. This document also assumes		data center network topologies [CLOS]. This document also assumes
	knowledge of data center routing protocols like BGP [RFC4271], BGP-		knowledge of data center routing protocols like BGP [RFC4271], BGP-
	SPF [I-D.ietf-lsvr-bgp-spf], OSPF [RFC2328], as well as, data center		SPF [I-D.ietf-lsvr-bgp-spf], OSPF [RFC2328], as well as, data center
	OAM protocols like LLDP [RFC4957] and BFD [RFC5580].		OAM protocols like LLDP [RFC4957] and BFD [RFC5580].
	4. Common Deployment Scenario		4. Common Deployment Scenario
	Within a Data Center, a common network design to interconnect servers		Within a Data Center, servers are commonly interconnected the CLOS
	is done using the CLOS topology [CLOS]. The CLOS topology is fully		topology [CLOS]. The CLOS topology is fully non-blocking and the
	non-blocking and the topology is realized using Equal Cost Multipath		topology is realized using Equal Cost Multi-Path (ECMP). In a CLOS
	(ECMP). In a CLOS topology, the minimum number of parallel paths		topology, the minimum number of parallel paths between two servers is
	between two servers is determined by the width of a tier-1 stage as		determined by the width of a tier-1 stage as shown in the figure 1.
	shown in the figure 1.
	The following example illustrates multistage CLOS topology.		The following example illustrates multi-stage CLOS topology.
	Tier-1		Tier-1
	+-----+		+-----+
	|NODE |		|NODE |
	+->| 12 |--+		+->| 12 |--+
	| +-----+ |		| +-----+ |
	Tier-2 | | Tier-2		Tier-2 | | Tier-2
	+-----+ | +-----+ | +-----+		+-----+ | +-----+ | +-----+
	+------------>|NODE |--+->|NODE |--+--|NODE |-------------+		+------------>|NODE |--+->|NODE |--+--|NODE |-------------+
	| +-----| 9 |--+ | 10 | +--| 11 |-----+ |		| +-----| 9 |--+ | 10 | +--| 11 |-----+ |
	skipping to change at page 4, line 33 ¶		skipping to change at page 4, line 33 ¶
	| 1 | | 2 | | 3 | | 4 | | 5 |		| 1 | | 2 | | 3 | | 4 | | 5 |
	+-----+ +-----+ +-----+ +-----+ +-----+		+-----+ +-----+ +-----+ +-----+ +-----+
	| | | | | | | |		| | | | | | | |
	A O B O <- Servers -> Z O O O		A O B O <- Servers -> Z O O O
	Figure 1: Illustration of the basic CLOS		Figure 1: Illustration of the basic CLOS
	5. Justification for BGP SPF Extension		5. Justification for BGP SPF Extension
	In order to simplify layer-3 routing and operations [RFC7938], many		In order to simplify layer-3 routing and operations [RFC7938], many
	data centers use BGP as a routing protocol to create an overlay as		data centers use BGP as a routing protocol to create both an underlay
	well as an underlay network for their CLOS Topologies. However, BGP		and overlay network for their CLOS Topologies. However, BGP is a
	is a path-vector routing protocol. Since it does not create a fabric		path-vector routing protocol. Since it does not create a fabric
	topology, it uses hop-by-hop EBGP peering to facilitate hop-by-hop		topology, it uses hop-by-hop EBGP peering to facilitate hop-by-hop
	routing to create the underlay network and to resolve any overlay		routing to create the underlay network and to resolve any overlay
	next hops. The hop-by-hop BGP peering paradigm imposes several		next hops. The hop-by-hop BGP peering paradigm imposes several
	restrictions within a CLOS. It severely prohibits a deployment of		restrictions within a CLOS. It severely prohibits a deployment of
	Route Reflectors/Route Controllers as the EBGP sessions are congruent		Route Reflectors/Route Controllers as the EBGP sessions are congruent
	with the data path. The BGP best path algorithm is prefix-based and		with the data path. The BGP best-path algorithm is prefix-based and
	it prevents announcements of prefixes to other BGP speakers until the		it prevents announcements of prefixes to other BGP speakers until the
	best path decision process is performed for the prefix at each		best-path decision process has been performed for the prefix at each
	intermediate hop. These restrictions significantly delay the overall		intermediate hop. These restrictions significantly delay the overall
	convergence of the underlay network within a CLOS.		convergence of the underlay network within a CLOS network.
	The LSVR SPF modifications allow BGP to overcome these limitations.		The LSVR SPF modifications allow BGP to overcome these limitations.
	Furthermore, using the BGP-LS NLRI format [RFC7752] allows the LSVR		Furthermore, using the BGP-LS NLRI format [RFC7752] allows the LSVR
	data to be advertised for nodes, links, and prefixes in the BGP		data to be advertised for nodes, links, and prefixes in the BGP
	routing domain and used for SPF computations.		routing domain and used for SPF computations.
	6. LSVR Applicability to CLOS Networks		6. LSVR Applicability to CLOS Networks
	With the BGP SPF extensions [I-D.ietf-lsvr-bgp-spf], the BGP best		With the BGP SPF extensions [I-D.ietf-lsvr-bgp-spf], the BGP best-
	path computation and route computation are replaced with OSPF-like		path computation and route computation are replaced with OSPF-like
	algorithms [RFC2328] both to determine whether an BGP-LS NLRI has		algorithms [RFC2328] both to determine whether an BGP-LS SPF NLRI has
	changed and needs to be re-advertised and to compute the routing		changed and needs to be re-advertised and to compute the BGP routes.
	table. These modifications will significantly improve convergence of		These modifications will significantly improve convergence of the
	the underlay while affording the operational benefits of a single		underlay while affording the operational benefits of a single routing
	routing protocol [RFC7938].		protocol [RFC7938].
	Data center controllers typically require visibility to the BGP		Data center controllers typically require visibility to the BGP
	topology to compute traffic-engineered paths. These controllers		topology to compute traffic-engineered paths. These controllers
	learn the topology and other relevant information via the BGP-LS		learn the topology and other relevant information via the BGP-LS
	address family [RFC7752] which is totally independent of the underlay		address family [RFC7752] which is totally independent of the underlay
	address families (usually IPv4/IPv6 unicast). Furthermore, in		address families (usually IPv4/IPv6 unicast). Furthermore, in
	traditional BGP underlays, all the BGP routers will need to advertise		traditional BGP underlays, all the BGP routers will need to advertise
	their BGP-LS information independently. With the BGP SPF extensions,		their BGP-LS information independently. With the BGP SPF extensions,
	controllers can learn the topology using the same BGP advertisements		controllers can learn the topology using the same BGP advertisements
	used to compute the underlay routes. Furthermore, these data center		used to compute the underlay routes. Furthermore, these data center
	controllers can avail the convergence advantages of the BGP SPF		controllers can avail the convergence advantages of the BGP SPF
	extensions. The placement of controllers can be outside of the		extensions. The placement of controllers can be outside of the
	forwarding path or within the forwarding path.		forwarding path or within the forwarding path.
	Alternatively, as each and every router in the BGP SPF domain will		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		have a complete view of the topology, the operator can also choose to
	configure BGP sessions in hop-by-hop peering model described in		configure BGP sessions in hop-by-hop peering model described in
	[RFC7938] along with BFD [RFC5580]. In doing so, while the hop-by-		[RFC7938] along with BFD [RFC5580]. In doing so, while the hop-by-
	hop peering model lacks inherent benefits of the controller-based		hop peering model lacks the inherent benefits of the controller-based
	model, BGP updates need not be serialized by BGP best path algorithm		model, BGP updates need not be serialized by BGP best-path algorithm
	in either of these models. This helps overall network convergence.		in either of these models. This helps overall network convergence.
	6.1. Usage of BGP-LS SAFI		6.1. Usage of BGP-LS SPF SAFI
	The BGP SPF extensions [I-D.ietf-lsvr-bgp-spf] define a new BGP-LS		The BGP SPF extensions [I-D.ietf-lsvr-bgp-spf] define a new BGP-LS
	SAFI for announcement of BGP SPF link-state. The NLRI format and its		SPF SAFI for announcement of BGP SPF link-state. The NLRI format and
	associated attributes follow the format of BGP-LS for node, link, and		its associated attributes follow the format of BGP-LS for node, link,
	prefix announcements. Whether the peering model within a CLOS		and prefix announcements. Whether the peering model within a CLOS
	follows hop-by-hop peering described in [RFC7938] or any controller-		follows hop-by-hop peering described in [RFC7938] or any controller-
	based or route-reflector peering, an operator can exchange BGP SPF		based or route-reflector peering, an operator can exchange BGP SPF
	SAFI routes over the BGP peering by simply configuring BGP SPF SAFI		SAFI routes over the BGP peering by simply configuring BGP SPF SAFI
	between the necessary BGP speakers.		between the necessary BGP speakers.
	The BGP-LS SPF SAFI can also co-exist with BGP IP Unicast SAFI which		The BGP-LS SPF SAFI can also co-exist with BGP IP Unicast SAFI which
	could exchange overlapping IP routes. The routes received by these		could exchange overlapping IP routes. The routes received by these
	SAFIs are evaluated, stored, and announced separately according to		SAFIs are evaluated, stored, and announced independently according to
	the rules of [RFC4760]. The tie-breaking of route installation is a		the rules of [RFC4760]. The tie-breaking of route installation is a
	matter of the local policies and preferences of the network operator.		matter of the local policies and preferences of the network operator.
	Finally, as the BGP SPF peering is done following the procedures		Finally, as the BGP SPF peering is done following the procedures
	described in [RFC4271], all the existing transport security		described in [RFC4271], all the existing transport security
	mechanisms including [RFC5925] are available for the BGP-LS SPF SAFI.		mechanisms including [RFC5925] are available for the BGP-LS SPF SAFI.
	6.1.1. Relationship to Other BGP AFI/SAFI Tuples		6.1.1. Relationship to Other BGP AFI/SAFI Tuples
	Normally, the BGP-LS AFI/SAFI is used solely to compute the underlay		Normally, the BGP-LS AFI/SAFI is used solely to compute the underlay
	and is given preference over other AFI/SAFIs. Other BGP SAFIs, e.g.,		and is given preference over other AFI/SAFIs. Other BGP SAFIs, e.g.,
	IPv6/IPv6 Unicast VPN would use the BGP-SPF computed routes for next		IPv6/IPv6 Unicast VPN would use the BGP-SPF computed routes for next
	hop resolution. However, if BGP-LS NLRI is also being advertised for		hop resolution. However, if BGP-LS NLRI is also being advertised for
	controller consumption, there is no need to replicate the Node, Link,		controller consumption, there is no need to replicate the Node, Link,
	and Prefix NLRI in BGP-NLRI. Rather, additional NLRI attributes can		and Prefix NLRI in BGP-NLRI. Rather, additional NLRI attributes can
	be advertised in the BGP-LS SPF AFI/SAFI as required.		be advertised in the BGP-LS SPF AFI/SAFI as required.
	6.2. Peering Models		6.2. Peering Models
	As previously stated, BGP SPF can be deployed using the existing		As previously stated, BGP SPF can be deployed using the existing
	peering model where there is a single hop BGP session on each and		peering model where there is a single-hop BGP session on each and
	every link in the data center fabric [RFC7938]. This provides for		every link in the data center fabric [RFC7938]. This provides for
	both the advertisement of routes and the determination of link and		both the advertisement of routes and the determination of link and
	neighboring switch availability. With BGP SPF, the underlay will		neighboring switch availability. With BGP SPF, the underlay will
	converge faster due to changes in the decision process which will		converge faster due to changes to the decision process that will
	allow NLRI changes to be advertised faster after detecting a change.		allow NLRI changes to be advertised faster after detecting a change.
	6.2.1. Sparse Peering Model		6.2.1. Sparse Peering Model
	Alternately, BFD [RFC5580] can be used to swiftly determine the		Alternately, BFD [RFC5580] can be used to swiftly determine the
	availability of links and the BGP peering model can be significantly		availability of links and the BGP peering model can be significantly
	sparser than the data center fabric. BGP SPF sessions then only be		sparser than the data center fabric. BGP SPF sessions only need to
	established with enough peers to provide a bi-connected graph. If		be established with enough peers to provide a bi-connected graph. If
	IEBGP is used, then the BGP routers at tier N-1 will act as route-		IEBGP is used, then the BGP routers at tier N-1 will act as route-
	reflectors for the routers at tier N.		reflectors for the routers at tier N.
	The obvious usage of sparse peering is to avoid parallel sessions		The obvious usage of sparse peering is to avoid parallel sessions on
	between the same two BGP speakers in the data center fabric.		links between the same two BGP speakers in the data center fabric.
	However, this use case is not very useful since parallel layer-3		However, this use case is not very useful since parallel layer-3
	links between the same two BGP routers are rare in CLOS or Fat-Tree		links between the same two BGP routers are rare in CLOS or Fat-Tree
	topologies. Two more interesting scenarios are described below.		topologies. Two more interesting scenarios are described below.
	In current Data Center topologies, there is often a very dense mesh		In current data center topologies, there is often a very dense mesh
	of links between levels, e.g., leaf and spine, providing 32-way,		of links between levels, e.g., leaf and spine, providing 32-way,
	64-way, or more Equal-Cost Multi-Path (ECMP) paths. In these		64-way, or more Equal-Cost Multi-Path (ECMP) paths. In these
	topologies, it is desirable not to have a BGP session on every link		topologies, it is desirable not to have a BGP session on every link
	and techniques such as the one described below Section 6.2.2 can be		and techniques such as the one described in Section 6.2.2 can be used
	used establish sessions on some subset of northbound links.		establish sessions on some subset of northbound links.
	Alternately, controller-based data center topologies are envisioned		Alternately, controller-based data center topologies are envisioned
	where BGP speakers within the data center only establish BGP sessions		where BGP speakers within the data center only establish BGP sessions
	with two or more controllers. In these topologies, fabric nodes		with two or more controllers. In these topologies, fabric nodes
	below the first tier (using [RFC7938] hierarchy) will establish BGP		below the first tier (using [RFC7938] hierarchy) will establish BGP
	multi-hop sessions with the controllers. For the multi-hop sessions,		multi-hop sessions with the controllers. For the multi-hop sessions,
	determining the route to the controllers without depending on BGP		determining the route to the controllers without depending on BGP
	would need to be through some other means beyond the scope of this		would need to be through some other means beyond the scope of this
	document. However, the BGP discovery mechanisms Section 6.3 would be		document. However, the BGP discovery mechanisms described in
	one possibility.		Section 6.5 would be one possibility.
	6.2.2. Bi-Connected Graph Heuristic		6.2.2. Bi-Connected Graph Heuristic
	With this heuristic, discovery of BGP peers is assumed Section 6.3.		With this heuristic, discovery of BGP peers is assumed, e.g., as
	Additionally, it assumed that the direction of the peering can be		described in Section 6.5. Additionally, it assumed that the
	ascertained. In the context of a data center fabric, direction is		direction of the peering can be ascertained. In the context of a
	either northbound (toward the spine), southbound (toward the Top-Of-		data center fabric, direction is either northbound (toward the
	Rack (TOR) switches) or east-west (same level in hierarchy. The		spine), southbound (toward the Top-Of-Rack (TOR) switches) or east-
	determination of the direction is beyond the scope of this document.		west (same level in hierarchy. The determination of the direction is
	However, it would be reasonable to assume a technique where the TOR		beyond the scope of this document. However, it would be reasonable
	switches can be identified and the number of hops to the TOR is used		to assume a technique where the TOR switches can be identified and
	to determine the direction.		the number of hops to the TOR is used to determine the direction.
	In this heuristic, BGP speakers allow passive session establishment		In this heuristic, BGP speakers allow passive session establishment
	for southbound BGP sessions. For northbound sessions, BGP speakers		for southbound BGP sessions. For northbound sessions, BGP speakers
	will attempt to maintain two northbound BGP sessions with different		will attempt to maintain two northbound BGP sessions with different
	switches (in data center fabrics there is normally a single layer-3		switches (in data center fabrics there is normally a single layer-3
	connection anyway). For east-west sessions, passive BGP session		connection anyway). For east-west sessions, passive BGP session
	establishment is allowed. However, BGP speaker will never actively		establishment is allowed. However, BGP speaker will never actively
	establish an east-west BGP session unless it can't establish two		establish an east-west BGP session unless it can't establish two
	northbound BGP sessions.		northbound BGP sessions.
	6.3. BGP Peer Discovery		6.3. BGP Spine/Leaf Topology Policy
	6.3.1. BGP Peer Discovery Requirements		One of the advantages of using BGP SPF as the underlay protocol is
			that BGP policy can be applied at any level. In Spine/Leaf
			topologies, it is not necessary to advertise BGP-LS NLRI received by
			leaves northbound to the spine nodes at the level above. If a common
			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 NLRI with AS
			64512 as the first AS in the AS path.
			+--------+ +--------+ +--------+
			AS 64512 | | | | | |
			for Spine | Spine1 +----+ Spine2 +- ......... -+ SpineN |
			Nodes at | | | | | |
			this Level +-+-+-+-++ ++-+-+-+-+ +-+-+-+-++
			+------+ | | | | | | | | | | |
			| +-----|-|-|------+ | | | | | | |
			| | +--|-|-|--------+-|-|-----------------+ | | |
			| | | | | | +---+ | | | | |
			| | | | | | | +--|-|-------------------+ | |
			| | | | | | | | | | +------+ +----+
			| | | | | | | | | +--------------|----------+ |
			| | | | | | | | +-------------+ | | |
			| | | | | +----|--|----------------|--|--------+ | |
			| | | | +------|--|--------------+ | | | | |
			| | | +------+ | | | | | | | |
			++--+--++ +-+-+--++ ++-+--+-+ ++-+--+-+
			| Leaf1 |~~~~~~| Leaf2 | ........ | LeafX | | LeafY |
			+-------+ +-------+ +-------+ +-------+
			Figure 2: Spine/Leaf Topology Policy
			6.4. BGP Peer Discovery Requirements
	The most basic requirement is to be able to discover the address of a		The most basic requirement is to be able to discover the address of a
	single-hop peer without pre-configuration. This is being		single-hop peer without pre-configuration. This is being
	accomplished today with using IPv6 Router Advertisements (RA)		accomplished today with using IPv6 Router Advertisements (RA)
	[RFC4861] and assuming that a BGP sessions is desired with any		[RFC4861] and assuming that a BGP sessions is desired with any
	discovered peer. Beyond the basic requirement, it is useful to have		discovered peer. Beyond the basic requirement, it is useful to have
	to following information relating to the BGP session:		to following information relating to the BGP session:
	o Autonomous System (AS) and BGP Identifier of a potential peer.		o Autonomous System (AS) and BGP Identifier of a potential peer.
	The latter can be used for debugging and to decrease the		The latter can be used for debugging and to decrease the
	skipping to change at page 8, line 12 ¶		skipping to change at page 9, line 4 ¶
	authentication, the security capabilities and possibly a key-chain		authentication, the security capabilities and possibly a key-chain
	[RFC8177] to be used.		[RFC8177] to be used.
	o Session Policy Identifier - A group number or name used to		o Session Policy Identifier - A group number or name used to
	associate common session parameters with the peer. For example,		associate common session parameters with the peer. For example,
	in a data center, BGP sessions with a Top of Rack (ToR) device		in a data center, BGP sessions with a Top of Rack (ToR) device
	could have parameters than BGP sessions between leaf and spine.		could have parameters than BGP sessions between leaf and spine.
	In a data center fabric, it is often useful to know whether a peer is		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		southbound (towards the servers) or northbound (towards the spine or
	super-spine) Section 6.2.2. A potential requirement would also be to		super-spine), e.g., Section 6.2.2. A potential requirement would be
	determine this dynamically. One mechanism, without specifying all		to determine this dynamically. One mechanism, without specifying all
	the details, might be for the ToRs to be identified when installed		the details, might be for the ToRs to be identified when installed
	and for the others switches in the fabric to determine their level		and for the others switches in the fabric to determine their level
	based on the distance from the closest ToR.		based on the distance from the closest ToR.
	If there are multiple links between BGP speakers or the links between		If there are multiple links between BGP speakers or the links between
	BGP speakers are unnumbered, it is also useful to be able to		BGP speakers are unnumbered, it is also useful to be able to
	establish multi-hop sessions using the loopback addresses. This will		establish multi-hop sessions using the loopback addresses. This will
	often require the discovery protocol to install route(s) toward the		often require the discovery protocol to install route(s) toward the
	potential peer loopback addresses prior to BGP session establishment.		potential peer loopback addresses prior to BGP session establishment.
	Finally, a simple BGP discovery protocol could also be used to		Finally, a simple BGP discovery protocol could also be used to
	establish a multi-hop session with one or more controllers by		establish a multi-hop session with one or more controllers by
	advertising connectivity to one or more controllers. However, once		advertising connectivity to one or more controllers. However, once
	the multi-hop session actually traverses multiple nodes, it is		the multi-hop session actually traverses multiple nodes, it is
	bordering a distance-vector routing protocol and possibly this is not		bordering a distance-vector routing protocol and possibly this is not
	a good requirement for the discovery protocol.		a good requirement for the discovery protocol.
	6.3.2. BGP Peer Discovery Alternatives		6.5. BGP Peer Discovery
			6.5.1. BGP Peer Discovery Alternatives
	While BGP peer discovery is not part of [I-D.ietf-lsvr-bgp-spf],		While BGP peer discovery is not part of [I-D.ietf-lsvr-bgp-spf],
	there are, at least, three proposals for BGP peer discovery. At		there are, at least, three proposals for BGP peer discovery. At
	least one of these mechanisms will be adopted and will be applicable		least one of these mechanisms will be adopted and will be applicable
	to deployments other than the data center. It is strongly		to deployments other than the data center. It is strongly
	RECOMMENDED that the accepted mechanism be used in conjunction with		RECOMMENDED that the accepted mechanism be used in conjunction with
	BGP SPF in data centers. The BGP discovery mechanism should		BGP SPF in data centers. The BGP discovery mechanism should
	discovery both peer addresses and endpoints for BFD discovery.		discovery both peer addresses and endpoints for BFD discovery.
	Additionally, it would be great if there were a heuristic for		Additionally, it would be great if there were a heuristic for
	determining whether the peer is at a tier above or below the		determining whether the peer is at a tier above or below the
	discovering BGP speaker (refer to Section 6.2.2).		discovering BGP speaker (refer to Section 6.2.2).
	The BGP discovery mechanisms under consideration are		The BGP discovery mechanisms under consideration are
	[I-D.acee-idr-lldp-peer-discovery],		[I-D.acee-idr-lldp-peer-discovery],
	[I-D.xu-idr-neighbor-autodiscovery], and [I-D.ymbk-lsvr-lsoe].		[I-D.xu-idr-neighbor-autodiscovery], and [I-D.ietf-lsvr-l3dl].
	6.3.3. Data Center Interconnect (DCI) Applicability		6.5.2. Data Center Interconnect (DCI) Applicability
	Since BGP SPF is to be used for the routing underlay and DCI gateway		Since BGP SPF is to be used for the routing underlay and DCI gateway
	boxes typically have direct or very simple connectivity, BGP external		boxes typically have direct or very simple connectivity, BGP external
	sessions would typically not include the BGP SPF SAFI.		sessions would typically not include the BGP SPF SAFI.
	6.4. Non-CLOS/FAT Tree Topology Applicability		6.6. Non-CLOS/FAT Tree Topology Applicability
	The BGP SPF extensions [I-D.ietf-lsvr-bgp-spf] can be used in other		The BGP SPF extensions [I-D.ietf-lsvr-bgp-spf] can be used in other
	topologies and avail the inherent convergence improvements.		topologies and avail the inherent convergence improvements.
	Additionally, sparse peering techniques may be utilized Section 6.2.		Additionally, sparse peering techniques may be utilized Section 6.2.
	However, determining whether or to establish a BGP session is more		However, determining whether or to establish a BGP session is more
	complex and the heuristic described in Section 6.2.2 cannot be used.		complex and the heuristic described in Section 6.2.2 cannot be used.
	In such topologies, other techniques such as those described in		In such topologies, other techniques such as those described in
	[I-D.li-lsr-dynamic-flooding] may be employed. One potential		[I-D.ietf-lsr-dynamic-flooding] may be employed. One potential
	deployment would be the underlay for a Service Provider (SP) backbone		deployment would be the underlay for a Service Provider (SP) backbone
	where usage of a single protocol, i.e., BGP, is desired.		where usage of a single protocol, i.e., BGP, is desired.
	7. IANA Considerations		7. BGP Policy Applicability
			Existing BGP policy including aggregation and prefix filtering may be
			used in conjunction with the BGP-LS SPF SAFI. When aggregation
			policy is used, BGP-LS SPF prefix NLRI will be originated for the
			aggregate prefix and BGP-LS SPF prefix NLRI for components will be
			filtered. Additionally, link and node NLRI may be filtered and the
			abstracted by the prefix NLRI.
			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 NLRI 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
			tradition BGP routing using the IPv4 and IPv6 address families were
			used.
			8. IANA Considerations
	No IANA updates are requested by this document.		No IANA updates are requested by this document.
	8. Security Considerations		9. Security Considerations
	This document introduces no new security considerations above and		This document introduces no new security considerations above and
	beyond those already specified in the [RFC4271] and		beyond those already specified in the [RFC4271] and
	[I-D.ietf-lsvr-bgp-spf].		[I-D.ietf-lsvr-bgp-spf].
	9. Acknowledgements		10. Acknowledgements
	The authors would like to thank Alvaro Retana and Yan Filyurin for		The authors would like to thank Alvaro Retana and Yan Filyurin for
	the review and comments.		the review and comments.
	10. References		11. References
	10.1. Normative References		11.1. Normative References
	[I-D.ietf-lsvr-bgp-spf]		[I-D.ietf-lsvr-bgp-spf]
	Patel, K., Lindem, A., Zandi, S., and W. Henderickx,		Patel, K., Lindem, A., Zandi, S., and W. Henderickx,
	"Shortest Path Routing Extensions for BGP Protocol",		"Shortest Path Routing Extensions for BGP Protocol",
	draft-ietf-lsvr-bgp-spf-03 (work in progress), September		draft-ietf-lsvr-bgp-spf-04 (work in progress), December
	2018.		2018.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997, <https://www.rfc-		DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
	editor.org/info/rfc2119>.		editor.org/info/rfc2119>.
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	10.2. Informative References		11.2. Informative References
	[CLOS] "A Study of Non-Blocking Switching Networks", The Bell		[CLOS] "A Study of Non-Blocking Switching Networks", The Bell
	System Technical Journal, Vol. 32(2), DOI		System Technical Journal, Vol. 32(2), DOI
	10.1002/j.1538-7305.1953.tb01433.x, March 1953.		10.1002/j.1538-7305.1953.tb01433.x, March 1953.
	[I-D.acee-idr-lldp-peer-discovery]		[I-D.acee-idr-lldp-peer-discovery]
	Lindem, A., Patel, K., Zandi, S., Haas, J., and X. Xu,		Lindem, A., Patel, K., Zandi, S., Haas, J., and X. Xu,
	"BGP Logical Link Discovery Protocol (LLDP) Peer		"BGP Logical Link Discovery Protocol (LLDP) Peer
	Discovery", draft-acee-idr-lldp-peer-discovery-03 (work in		Discovery", draft-acee-idr-lldp-peer-discovery-04 (work in
	progress), June 2018.		progress), December 2018.
	[I-D.li-lsr-dynamic-flooding]		[I-D.ietf-lsr-dynamic-flooding]
	Li, T., Psenak, P., Ginsberg, L., Przygienda, T., and D.		Li, T., Psenak, P., Ginsberg, L., Przygienda, T., Cooper,
	Cooper, "Dynamic Flooding on Dense Graphs", draft-li-lsr-		D., Jalil, L., and S. Dontula, "Dynamic Flooding on Dense
	dynamic-flooding-01 (work in progress), October 2018.		Graphs", draft-ietf-lsr-dynamic-flooding-00 (work in
			progress), February 2019.
			[I-D.ietf-lsvr-l3dl]
			Bush, R., Austein, R., and K. Patel, "Layer 3 Discovery
			and Liveness", draft-ietf-lsvr-l3dl-00 (work in progress),
			April 2019.
	[I-D.xu-idr-neighbor-autodiscovery]		[I-D.xu-idr-neighbor-autodiscovery]
	Xu, X., Talaulikar, K., Bi, K., Tantsura, J., and N.		Xu, X., Talaulikar, K., Bi, K., Tantsura, J., and N.
	Triantafillis, "BGP Neighbor Discovery", draft-xu-idr-		Triantafillis, "BGP Neighbor Discovery", draft-xu-idr-
	neighbor-autodiscovery-10 (work in progress), October		neighbor-autodiscovery-11 (work in progress), April 2019.
	2018.
	[I-D.ymbk-lsvr-lsoe]
	Bush, R. and K. Patel, "Link State Over Ethernet", draft-
	ymbk-lsvr-lsoe-01 (work in progress), July 2018.
	[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,		[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
	DOI 10.17487/RFC2328, April 1998, <https://www.rfc-		DOI 10.17487/RFC2328, April 1998, <https://www.rfc-
	editor.org/info/rfc2328>.		editor.org/info/rfc2328>.
	[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A		[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
	Border Gateway Protocol 4 (BGP-4)", RFC 4271,		Border Gateway Protocol 4 (BGP-4)", RFC 4271,
	DOI 10.17487/RFC4271, January 2006, <https://www.rfc-		DOI 10.17487/RFC4271, January 2006, <https://www.rfc-
	editor.org/info/rfc4271>.		editor.org/info/rfc4271>.
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