draft-ietf-idr-bgp-optimal-route-reflection-22.txt		draft-ietf-idr-bgp-optimal-route-reflection-23.txt
	IDR Working Group R. Raszuk, Ed.		IDR Working Group R. Raszuk, Ed.
	Internet-Draft NTT Network Innovations		Internet-Draft NTT Network Innovations
	Intended status: Standards Track C. Cassar		Intended status: Standards Track C. Cassar
	Expires: July 19, 2021 Tesla		Expires: November 13, 2021 Tesla
	E. Aman		E. Aman
	B. Decraene, Ed.		B. Decraene, Ed.
	Orange		Orange
	K. Wang		K. Wang
	Juniper Networks		Juniper Networks
	January 15, 2021		May 12, 2021
	BGP Optimal Route Reflection (BGP-ORR)		BGP Optimal Route Reflection (BGP-ORR)
	draft-ietf-idr-bgp-optimal-route-reflection-22		draft-ietf-idr-bgp-optimal-route-reflection-23
	Abstract		Abstract
	This document defines an extension to BGP route reflectors. On route		This document defines an extension to BGP route reflectors. On route
	reflectors, BGP route selection is modified in order to choose the		reflectors, BGP route selection is modified in order to choose the
	best path from the standpoint of their clients, rather than from the		best route from the standpoint of their clients, rather than from the
	standpoint of the route reflectors. Multiple types of granularity		standpoint of the route reflectors. Depending on the scaling and
	are proposed, from a per client BGP route selection or to a per peer		precision requirements, route selection can be specific for one
	group, depending on the scaling and precision requirements on route		client, common for a set of clients or common for all clients of a
	selection. This solution is particularly applicable in deployments		route reflector. This solution is particularly applicable in
	using centralized route reflectors, where choosing the best route		deployments using centralized route reflectors, where choosing the
	based on the route reflector IGP location is suboptimal. This		best route based on the route reflector's IGP location is suboptimal.
	facilitates, for example, best exit point policy (hot potato		This facilitates, for example, best exit point policy (hot potato
	routing).		routing).
	The solution relies upon all route reflectors learning all paths		The solution relies upon all route reflectors learning all paths
	which are eligible for consideration. Best path selection is		which are eligible for consideration. BGP Route Selection is
	performed in each route reflector based on the IGP cost from a		performed in the route reflectors based on the IGP cost from
	selected location in the link state IGP.		configured locations in the link state IGP.
	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 https://datatracker.ietf.org/drafts/current/.		Drafts is at https://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 July 19, 2021.		This Internet-Draft will expire on November 13, 2021.
	Copyright Notice		Copyright Notice
	Copyright (c) 2021 IETF Trust and the persons identified as the		Copyright (c) 2021 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Definitions of Terms Used in This Memo . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
	3. Modifications to BGP Best Path selection . . . . . . . . . . 5		3. Modifications to BGP Route Selection . . . . . . . . . . . . 4
	3.1. Best Path Selection from a different IGP location . . . . 6		3.1. Route Selection from a different IGP location . . . . . . 5
	3.1.1. Restriction when BGP next hop is BGP prefix . . . . . 7		3.1.1. Restriction when BGP next hop is a BGP prefix . . . . 6
	3.2. Multiple Best Path Selections . . . . . . . . . . . . . . 7		3.2. Multiple Route Selections . . . . . . . . . . . . . . . . 6
	4. Implementation considerations . . . . . . . . . . . . . . . . 7		4. Deployment Considerations . . . . . . . . . . . . . . . . . . 6
	4.1. Likely Deployments and need for backup . . . . . . . . . 7		5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	5. CPU and Memory Scalability . . . . . . . . . . . . . . . . . 8		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
	6. Advantages and Deployment Considerations . . . . . . . . . . 8		7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 9		8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10		9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
	9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10		9.1. Normative References . . . . . . . . . . . . . . . . . . 8
	10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10		9.2. Informative References . . . . . . . . . . . . . . . . . 9
	11. References . . . . . . . . . . . . . . . . . . . . . . . . . 11		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
	11.1. Normative References . . . . . . . . . . . . . . . . . . 11
	11.2. Informative References . . . . . . . . . . . . . . . . . 11
	Appendix A. Appendix: alternative solutions with limited
	applicability . . . . . . . . . . . . . . . . . . . 12
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
	1. Definitions of Terms Used in This Memo
	NLRI - Network Layer Reachability Information
	RIB - Routing Information Base
	AS - Autonomous System number
	VRF - Virtual Routing and Forwarding instance
	PE - Provider Edge router
	RR - Route Reflector
	POP - Point Of Presence
	L3VPN - Layer 3 Virtual Private Network [RFC4364]
	6PE - IPv6 Provider Edge [RFC4798]
	IGP - Interior Gateway Protocol
	SPT - Shortest Path Tree
	best path - the route chosen by the decision process detailed in
	[RFC4271] section 9.1.2 and its subsections
	best path computation - the decision process detailed in [RFC4271]
	section 9.1.2 and its subsections
	best path algorithm - the decision process detailed in [RFC4271]
	section 9.1.2 and its subsections
	best path selection - the decision process detailed in [RFC4271]
	section 9.1.2 and its subsections
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here.
	2. Introduction		1. Introduction
	There are three types of BGP deployments within Autonomous Systems		There are three types of BGP deployments within Autonomous Systems
	today: full mesh, confederations and route reflection. BGP route		today: full mesh, confederations and route reflection. BGP route
	reflection [RFC4456] is the most popular way to distribute BGP routes		reflection [RFC4456] is the most popular way to distribute BGP routes
	between BGP speakers belonging to the same Autonomous System.		between BGP speakers belonging to the same Autonomous System.
	However, in some situations, this method suffers from non-optimal		However, in some situations, this method suffers from non-optimal
	path selection.		path selection.
	[RFC4456] asserts that, because the IGP cost to a given point in the		[RFC4456] asserts that, because the IGP cost to a given point in the
	network will vary across routers, "the route reflection approach may		network will vary across routers, "the route reflection approach may
	skipping to change at page 4, line 18 ¶		skipping to change at page 3, line 22 ¶
	dictates otherwise. As a consequence of the route reflection method,		dictates otherwise. As a consequence of the route reflection method,
	the choice of exit point for a route reflector and its clients will		the choice of exit point for a route reflector and its clients will
	be the exit point that is optimal for the route reflector - not		be the exit point that is optimal for the route reflector - not
	necessarily the one that is optimal for its clients.		necessarily the one that is optimal for its clients.
	Section 11 of [RFC4456] describes a deployment approach and a set of		Section 11 of [RFC4456] describes a deployment approach and a set of
	constraints which, if satisfied, would result in the deployment of		constraints which, if satisfied, would result in the deployment of
	route reflection yielding the same results as the IBGP full mesh		route reflection yielding the same results as the IBGP full mesh
	approach. This deployment approach makes route reflection compatible		approach. This deployment approach makes route reflection compatible
	with the application of hot potato routing policy. In accordance		with the application of hot potato routing policy. In accordance
	with these design rules, route reflectors have traditionally often		with these design rules, route reflectors have often been deployed in
	been deployed in the forwarding path and carefully placed on the POP		the forwarding path and carefully placed on the POP to core
	to core boundaries.		boundaries.
	The evolving model of intra-domain network design has enabled		The evolving model of intra-domain network design has enabled
	deployments of route reflectors outside of the forwarding path.		deployments of route reflectors outside of the forwarding path.
	Initially this model was only employed for new address families, e.g.		Initially this model was only employed for new services, e.g. IP
	L3VPNs and L2VPNs, however it has been gradually extended to other		VPNs [RFC4364], however it has been gradually extended to other BGP
	BGP address families including IPv4 and IPv6 Internet using either		services including IPv4 and IPv6 Internet. In such environments, hot
	native routing or 6PE. In such environments, hot potato routing		potato routing policy remains desirable.
	policy remains desirable.
	Route reflectors outside of the forwarding path can be placed on the		Route reflectors outside of the forwarding path can be placed on the
	POP to core boundaries, but they are often placed in arbitrary		POP to core boundaries, but they are often placed in arbitrary
	locations in the core of large networks.		locations in the core of large networks.
	Such deployments suffer from a critical drawback in the context of		Such deployments suffer from a critical drawback in the context of
	best path selection: A route reflector with knowledge of multiple		BGP Route Selection: A route reflector with knowledge of multiple
	paths for a given prefix will typically pick its best path and only		paths for a given prefix will typically pick its best path and only
	advertise that best path to its clients. If the best path for a		advertise that best path to its clients. If the best path for a
	prefix is selected on the basis of an IGP tie-break, the path		prefix is selected on the basis of an IGP tie-break, the path
	advertised will be the exit point closest to the route reflector.		advertised will be the exit point closest to the route reflector.
	However, the clients are in a different place in the network topology		However, the clients are in a different place in the network topology
	than the route reflector. In networks where the route reflectors are		than the route reflector. In networks where the route reflectors are
	not in the forwarding path, this difference will be even more acute.		not in the forwarding path, this difference will be even more acute.
	In addition, there are deployment scenarios where service providers		In addition, there are deployment scenarios where service providers
	want to have more control in choosing the exit points for clients		want to have more control in choosing the exit points for clients
	based on other factors, such as traffic type, traffic load, etc.		based on other factors, such as traffic type, traffic load, etc.
	This further complicates the issue and makes it less likely for the		This further complicates the issue and makes it less likely for the
	route reflector to select the best path from the client's		route reflector to select the best path from the client's
	perspective. It follows that the best path chosen by the route		perspective. It follows that the best path chosen by the route
	reflector is not necessarily the same as the path which would have		reflector is not necessarily the same as the path which would have
	been chosen by the client if the client had considered the same set		been chosen by the client if the client had considered the same set
	of candidate paths as the route reflector.		of candidate paths as the route reflector.
	3. Modifications to BGP Best Path selection		2. Terminology
			This memo makes use of the terms defined in [RFC4271] and [RFC4456].
			The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
			"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
			"OPTIONAL" in this document are to be interpreted as described in BCP
			14 [RFC2119] [RFC8174] when, and only when, they appear in all
			capitals, as shown here.
			3. Modifications to BGP Route Selection
	The core of this solution is the ability for an operator to specify		The core of this solution is the ability for an operator to specify
	the IGP location for which the route reflector should calculate		the IGP location for which the route reflector calculates interior
	routes. This can be done on a per route reflector basis, per peer/		cost for the NEXT_HOP. The IGP location is defined as a node in the
	update group basis, or per peer basis. This ability enables the		IGP topology and may be configured on a per route reflector basis,
			per set of clients, or per client basis. This ability enables the
	route reflector to send to a given set of clients routes with		route reflector to send to a given set of clients routes with
	shortest distance to the next hops from the position of the selected		shortest distance to the next hops from the position of the selected
	IGP location. This provides for freedom of route reflector physical		IGP location. This provides for freedom of route reflector physical
	location, and allows transient or permanent migration of this network		location, and allows transient or permanent migration of this network
	control plane function to an arbitrary location.		control plane function to an arbitrary location.
	The choice of specific granularity (route reflector, peer/update		The choice of specific granularity (route reflector, set of clients,
	group, or peer) is configured by the network operator. An		or client) is configured by the network operator. An implementation
	implementation is considered compliant with this document if it		is considered compliant with this document if it supports at least
	supports at least one listed grouping of IGP location.		one listed grouping of IGP location.
	For purposes of route selection, the perspective of a client can		For purposes of route selection, the perspective of a client can
	differ from that of a route reflector or another client in two		differ from that of a route reflector or another client in two
	distinct ways:		distinct ways:
	o it can, and usually will, have a different position in the IGP		o it has a different position in the IGP topology, and
	topology, and
	o it can have a different routing policy.		o it can have a different routing policy.
	These factors correspond to the issues described earlier.		These factors correspond to the issues described earlier.
	This document defines, on BGP Route Reflectors [RFC4456], two changes		This document defines, for BGP Route Reflectors [RFC4456], two
	to the BGP Best Path selection algorithm:		changes to the BGP Route Selection algorithm:
	o The first change, introduced in Section 3.1, is related to the IGP		o The first change, introduced in Section 3.1, is related to the IGP
	cost to the BGP Next Hop in the BGP decision process. The change		cost to the BGP Next Hop in the BGP decision process. The change
	consists in using the IGP cost from a different IGP location than		consists in using the IGP cost from a different IGP location than
	the route reflector itself.		the route reflector itself.
	o The second change, introduced in Section 3.2, is to extend the		o The second change, introduced in Section 3.2, is to extend the
	granularity of the BGP decision process, to allow for running		granularity of the BGP decision process, to allow for running
	multiple decisions process using different perspective or		multiple decisions processes using different perspective or
	policies.		policies.
	A route reflector can implement either or both of the modifications
	in order to allow it to choose the best path for its clients that the
	clients themselves would have chosen given the same set of candidate
	paths.
	A significant advantage of these approaches is that the route		A significant advantage of these approaches is that the route
	reflector clients do not need to run new software or hardware.		reflector clients do not need to be modified.
	3.1. Best Path Selection from a different IGP location		3.1. Route Selection from a different IGP location
	In this approach, optimal refers to the decision made during best		In this approach, optimal refers to the decision where the interior
	path selection at the IGP metric to BGP next hop comparison step. It		cost of a route is determined during step e) of [RFC4271] section
	does not apply to path selection preference based on other policy		9.1.2.2 "Breaking Ties (Phase 2)". It does not apply to path
	steps and provisions.		selection preference based on other policy steps and provisions.
	In addition to the change specified in [RFC4456] section 9, the BGP		In addition to the change specified in [RFC4456] section 9, [RFC4271]
	Decision Process tie-breaking rules ([RFC4271] section 9.1.2.2) are		section 9.1.2.2 is modified as follows.
	modified as follows.
	The below text in step e)		The below text in step e)
	e) Remove from consideration any routes with less-preferred		e) Remove from consideration any routes with less-preferred
	interior cost. The interior cost of a route is determined by		interior cost. The interior cost of a route is determined by
	calculating the metric to the NEXT_HOP for the route using the		calculating the metric to the NEXT_HOP for the route using the
	Routing Table.		Routing Table.
	...is replaced by this new text:		...is replaced by this new text:
	skipping to change at page 6, line 40 ¶		skipping to change at page 5, line 46 ¶
	calculating the metric from the selected IGP location to the		calculating the metric from the selected IGP location to the
	NEXT_HOP for the route using the shortest IGP path tree rooted at		NEXT_HOP for the route using the shortest IGP path tree rooted at
	the selected IGP location.		the selected IGP location.
	In order to be able to compute the shortest path tree rooted at the		In order to be able to compute the shortest path tree rooted at the
	selected IGP locations, knowledge of the IGP topology for the area/		selected IGP locations, knowledge of the IGP topology for the area/
	level that includes each of those locations is needed. This		level that includes each of those locations is needed. This
	knowledge can be gained with the use of the link state IGP such as		knowledge can be gained with the use of the link state IGP such as
	IS-IS [ISO10589] or OSPF [RFC2328] [RFC5340] or via BGP-LS [RFC7752].		IS-IS [ISO10589] or OSPF [RFC2328] [RFC5340] or via BGP-LS [RFC7752].
	The configuration of the IGP location is outside of the scope of this		The way the IGP location is configured is outside the scope of this
	document. The operator may configure it manually, an implementation		document. The operator may configure it manually, an implementation
	may automate it based on heuristics, or it can be computed centrally		may automate it based on heuristics, or it can be computed centrally
	and configured by an external system.		and configured by an external system. One or more backup locations
			SHOULD be allowed to be specified for redundancy.
	This solution does not require any change (BGP or IGP) on the
	clients, as all required changes are limited to the route reflector.
	This solution applies to NLRIs of all address families that can be
	route reflected.
	3.1.1. Restriction when BGP next hop is BGP prefix		3.1.1. Restriction when BGP next hop is a BGP prefix
	In situations where the BGP next hop is a BGP prefix itself, the IGP		In situations where the BGP next hop is a BGP prefix itself, the IGP
	metric of a route used for its resolution SHOULD be the final IGP		metric of a route used for its resolution SHOULD be the final IGP
	cost to reach such next hop. Implementations which can not inform		cost to reach such next hop. Implementations which can not inform
	BGP of the final IGP metric to a recursive next hop SHOULD treat such		BGP of the final IGP metric to a recursive next hop MUST treat such
	paths as least preferred during next hop metric comparison. However		paths as least preferred during next hop metric comparison. However
	such paths SHOULD still be considered valid for best path selection.		such paths MUST still be considered valid for BGP Phase 2 Route
			Selection.
	3.2. Multiple Best Path Selections		3.2. Multiple Route Selections
	BGP Route Reflector as per [RFC4456] runs a single best path		BGP Route Reflector as per [RFC4456] runs a single BGP Decision
	selection. Optimal route reflection may require calculation of		Process. Optimal route reflection may require multiple BGP Decision
	multiple best path selections or subsets of best path selection in		Processes or subsets of the Decision Process in order to consider
	order to consider different IGP locations or BGP policies for		different IGP locations or BGP policies for different sets of
	different sets of clients.		clients.
	If the required routing optimization is limited to the IGP cost to		If the required routing optimization is limited to the IGP cost to
	the BGP Next-Hop, only step e) as defined [RFC4271] section 9.1.2.2,		the BGP Next-Hop, only step e) and below as defined [RFC4271] section
	needs to be duplicated.		9.1.2.2, needs to be run multiple times.
	If the routing optimization requires the use of different BGP		If the routing optimization requires the use of different BGP
	policies for different sets of clients, a larger part of the decision		policies for different sets of clients, a larger part of the decision
	process needs to be duplicated, up to the whole decision process as		process needs to be run multiple times, up to the whole decision
	defined in section 9.1 of [RFC4271]. This is for example the case		process as defined in section 9.1 of [RFC4271]. This is for example
	when there is a need to use different policies to compute different		the case when there is a need to use different policies to compute
	degree of preference during Phase 1. This is needed for use cases		different degree of preference during Phase 1. This is needed for
	involving traffic engineering or dedicating certain exit points for		use cases involving traffic engineering or dedicating certain exit
	certain clients. In the latter case, the user MAY specify and apply		points for certain clients. In the latter case, the user may specify
	a general policy on the route reflector for a set of clients. For a		and apply a general policy on the route reflector for a set of
	given set of clients, the policy SHOULD in that case allow the		clients. Regular path selection, including IGP perspective for a set
	operator to select different candidate exit points for different		of clients as per Section 3.1, is then applied to the candidate paths
	address families. Regular path selection, including IGP perspective		to select the final paths to advertise to the clients.
	for a set of clients as per Section 3.1, is then applied to the
	candidate paths to select the final paths to advertise to the
	clients.
	4. Implementation considerations
	4.1. Likely Deployments and need for backup
	With IGP based optimal route reflection, even though the IGP location
	could be specified on a per route reflector basis or per peer/update
	group basis or per peer basis, in reality, it's most likely to be
	specified per peer/update group basis. All clients with the same or
	similar IGP location can be grouped into the same peer/update group.
	An IGP location is then specified for the peer/update group. The
	location is usually specified as the location of one of the clients
	from the peer group or an ABR to the area where clients are located.
	Also, one or more backup locations SHOULD be allowed to be specified
	for redundancy. Implementations may wish to take advantage of peer
	group mechanisms in order to provide for better scalability of
	optimal route reflector client groups with similar properties.
	5. CPU and Memory Scalability
	For IGP based optimal route reflection, determining the shortest path
	and associated cost between any two arbitrary points in a network
	based on the IGP topology learned by a router is expected to add some
	extra cost in terms of CPU resources. However, current SPF tree
	generation code is implemented efficiently in a number of
	implementations, and therefore this is not expected to be a major
	drawback. The number of SPTs computed is expected to be of the order
	of the number of clients of a route reflector whenever a topology
	change is detected. It is expected to be higher but comparable to
	some existing deployed features such as (Remote) Loop Free Alternate
	which computes a (r)SPT per IGP neighbor.
	For policy based optimal route reflection, there will be some
	overhead to apply the policy to select the candidate paths. This
	overhead is comparable to existing BGP export policies and therefore
	should be manageable.
	By the nature of route reflection, the number of clients can be split
	arbitrarily by the deployment of more route reflectors for a given
	number of clients. While this is not expected to be necessary in
	existing networks with best in class route reflectors available
	today, this avenue to scaling up the route reflection infrastructure
	is available.
	If we consider the overall network wide cost/benefit factor, the only
	alternative to achieve the same level of optimality would require
	significantly increasing state on the edges of the network. This
	will consume CPU and memory resources on all BGP speakers in the
	network. Building this client perspective into the route reflectors
	seems appropriate.
	6. Advantages and Deployment Considerations		A route reflector can implement either or both of the modifications
			in order to allow it to choose the best path for its clients that the
			clients themselves would have chosen given the same set of candidate
			paths.
	The solutions described provide a model for integrating the client		4. Deployment Considerations
	perspective into the best path computation for route reflectors.
	More specifically, the choice of BGP path factors in either the IGP
	cost between the client and the nexthop (rather than the IGP cost
	from the route reflector to the nexthop) or other user configured
	policies.
	The achievement of optimal routing relies upon all route reflectors		BGP Optimal Route Reflection provides a model for integrating the
	learning all paths that are eligible for consideration. In order to		client perspective into the BGP Route Selection decision function for
	satisfy this requirement, path diversity enhancing mechanisms such as		route reflectors. More specifically, the choice of BGP path factors
	BGP add-path [RFC7911] may need to be deployed between route		in either the IGP cost between the client and the NEXT_HOP (rather
	reflectors.		than the IGP cost from the route reflector to the NEXT_HOP) or other
			user configured policies.
	Implementations considered compliant with this document allow the		The achievement of optimal routing between clients of different
	configuration of a logical location from which the best path will be		clusters relies upon all route reflectors learning all paths that are
	computed, on the basis of either a peer, a peer group, or an entire		eligible for consideration. In order to satisfy this requirement,
	routing instance.		BGP add-path [RFC7911] needs to be deployed between route reflectors.
	These solutions can be deployed in traditional hop-by-hop forwarding		This solution can be deployed in traditional hop-by-hop forwarding
	networks as well as in end-to-end tunneled environments. In networks		networks as well as in end-to-end tunneled environments. In networks
	where there are multiple route reflectors and hop-by-hop forwarding		where there are multiple route reflectors and hop-by-hop forwarding
	without encapsulation, such optimizations SHOULD be enabled in a		without encapsulation, such optimizations SHOULD be enabled in a
	consistent way on all route reflectors. Otherwise, clients may		consistent way on all route reflectors. Otherwise, clients may
	receive an inconsistent view of the network, in turn leading to		receive an inconsistent view of the network, in turn leading to
	intra-domain forwarding loops.		intra-domain forwarding loops.
			As discussed in section 11 of [RFC4456], the IGP locations of BGP
			route reflectors is important and has routing implications. This
			equally applies to the choice of the IGP locations configured on
			optimal route reflectors. After selecting suitable IGP locations, an
			operator may let one or multiple route reflectors handle route
			selection for all of them. The operator may alternatively deploy one
			or multiple route reflector for each IGP location or create any
			design in between. This choice may depend on operational model
			(centralized vs per region), acceptable blast radius in case of
			failure, acceptable number of IBGP sessions for the mesh between the
			route reflectors, performance and configuration granularity of the
			equipment.
	With this approach, an ISP can effect a hot potato routing policy		With this approach, an ISP can effect a hot potato routing policy
	even if route reflection has been moved out of the forwarding plane,		even if route reflection has been moved out of the forwarding plane,
	and hop-by-hop switching has been replaced by end-to-end MPLS or IP		and hop-by-hop switching has been replaced by end-to-end MPLS or IP
	encapsulation.		encapsulation. Compared with a deployment of ADD-PATH on all
			routers, BGP ORR reduces the amount of state which needs to be pushed
	As per above, these approaches reduce the amount of state which needs		to the edge of the network in order to perform hot potato routing.
	to be pushed to the edge of the network in order to perform hot
	potato routing. The memory and CPU resources required at the edge of
	the network to provide hot potato routing using these approaches is
	lower than what would be required to achieve the same level of
	optimality by pushing and retaining all available paths (potentially
	10s) per each prefix at the edge.
	The solutions above allow for a fast and safe transition to a BGP		Modifying the IGP location of BGP ORR does not interfere with
	control plane using centralized route reflection, without		policies enforced before IGP tie-breaking (step e) in the BGP
	compromising an operator's closest exit operational principle. This		Decision Process Route.
	enables edge-to-edge LSP/IP encapsulation for traffic to IPv4 and
	IPv6 prefixes.
	Regarding Best Path Selection from a different IGP location, it		Calculating routes for different IGP locations requires multiple SPF
	should be self evident that this solution does not interfere with		calculations and multiple (subsets of) BGP Decision Processes, which
	policies enforced above IGP tie-breaking in the BGP best path		requires more computing resources. This document allows for
	algorithm.		different granularity such as one Decision Process per route
			reflector, per set of clients or per client. A more fine grained
			granularity may translate into more optimal hot potato routing at the
			cost of more computing power. The ability to run fine grained
			computations depends on the platform/hardware deployed, the number of
			clients, the number of BGP routes and the size of the IGP topology.
			In essence, sizing considerations are similar to the deployments of
			BGP Route Reflector.
	7. Security Considerations		5. Security Considerations
	Similarly to [RFC4456], this extension to BGP does not change the		Similarly to [RFC4456], this extension to BGP does not change the
	underlying security issues inherent in the existing IBGP [RFC4456].		underlying security issues inherent in the existing IBGP.
	It however enables the deployment of base BGP Route Reflection as
	described in [RFC4456] to be possible using virtual compute
	environments without any negative consequence on the BGP routing path
	optimality.
	This document does not introduce requirements for any new protection		This document does not introduce requirements for any new protection
	measures, but it also does not relax best operational practices for		measures.
	keeping the IGP network stable or to pace rate of policy based IGP
	cost to next hops such that it does not have any substantial effect
	on BGP path changes and their propagation to route reflection
	clients.
	8. IANA Considerations		6. IANA Considerations
	This document does not request any IANA allocations.		This document does not request any IANA allocations.
	9. Acknowledgments		7. Acknowledgments
	Authors would like to thank Keyur Patel, Eric Rosen, Clarence		Authors would like to thank Keyur Patel, Eric Rosen, Clarence
	Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand, Jon		Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand, Jon
	Mitchell, John Scudder, Jeff Haas, Martin Djernaes, Daniele		Mitchell, John Scudder, Jeff Haas, Martin Djernaes, Daniele
	Ceccarelli, Kieran Milne, Job Snijders and Randy Bush for their		Ceccarelli, Kieran Milne, Job Snijders and Randy Bush for their
	valuable input.		valuable input.
	10. Contributors		8. Contributors
	Following persons substantially contributed to the current format of		Following persons substantially contributed to the current format of
	the document:		the document:
	Stephane Litkowski		Stephane Litkowski
	Cisco System		Cisco System
	slitkows.ietf@gmail.com		slitkows.ietf@gmail.com
	Adam Chappell		Adam Chappell
	GTT Communications, Inc.		GTT Communications, Inc.
	Aspira Business Centre		Aspira Business Centre
	Bucharova 2928/14a		Bucharova 2928/14a
	158 00 Prague 13 Stodulky		158 00 Prague 13 Stodulky
	Czech Republic		Czech Republic
	adam.chappell@gtt.net		adam.chappell@gtt.net
	11. References		9. References
	11.1. Normative References		9.1. Normative References
	[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,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[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,		DOI 10.17487/RFC4271, January 2006,
	<https://www.rfc-editor.org/info/rfc4271>.		<https://www.rfc-editor.org/info/rfc4271>.
	[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route		[RFC4456] Bates, T., Chen, E., and R. Chandra, "BGP Route
	Reflection: An Alternative to Full Mesh Internal BGP		Reflection: An Alternative to Full Mesh Internal BGP
	(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,		(IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
	<https://www.rfc-editor.org/info/rfc4456>.		<https://www.rfc-editor.org/info/rfc4456>.
	[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>.
	11.2. Informative References		9.2. Informative References
	[ISO10589]		[ISO10589]
	International Organization for Standardization,		International Organization for Standardization,
	"Intermediate system to Intermediate system intra-domain		"Intermediate system to Intermediate system intra-domain
	routeing information exchange protocol for use in		routeing information exchange protocol for use in
	conjunction with the protocol for providing the		conjunction with the protocol for providing the
	connectionless-mode Network Service (ISO 8473)", ISO/		connectionless-mode Network Service (ISO 8473)", ISO/
	IEC 10589:2002, Second Edition, Nov 2002.		IEC 10589:2002, Second Edition, Nov 2002.
	[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,		DOI 10.17487/RFC2328, April 1998,
	<https://www.rfc-editor.org/info/rfc2328>.		<https://www.rfc-editor.org/info/rfc2328>.
	[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private		[RFC4364] Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
	Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February		Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
	2006, <https://www.rfc-editor.org/info/rfc4364>.		2006, <https://www.rfc-editor.org/info/rfc4364>.
	[RFC4798] De Clercq, J., Ooms, D., Prevost, S., and F. Le Faucheur,
	"Connecting IPv6 Islands over IPv4 MPLS Using IPv6
	Provider Edge Routers (6PE)", RFC 4798,
	DOI 10.17487/RFC4798, February 2007,
	<https://www.rfc-editor.org/info/rfc4798>.
	[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF		[RFC5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
	for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,		for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
	<https://www.rfc-editor.org/info/rfc5340>.		<https://www.rfc-editor.org/info/rfc5340>.
	[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
	and K. Kumaki, "Distribution of Diverse BGP Paths",
	RFC 6774, DOI 10.17487/RFC6774, November 2012,
	<https://www.rfc-editor.org/info/rfc6774>.
	[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and		[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
	S. Ray, "North-Bound Distribution of Link-State and		S. Ray, "North-Bound Distribution of Link-State and
	Traffic Engineering (TE) Information Using BGP", RFC 7752,		Traffic Engineering (TE) Information Using BGP", RFC 7752,
	DOI 10.17487/RFC7752, March 2016,		DOI 10.17487/RFC7752, March 2016,
	<https://www.rfc-editor.org/info/rfc7752>.		<https://www.rfc-editor.org/info/rfc7752>.
	[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,		[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
	"Advertisement of Multiple Paths in BGP", RFC 7911,		"Advertisement of Multiple Paths in BGP", RFC 7911,
	DOI 10.17487/RFC7911, July 2016,		DOI 10.17487/RFC7911, July 2016,
	<https://www.rfc-editor.org/info/rfc7911>.		<https://www.rfc-editor.org/info/rfc7911>.
	Appendix A. Appendix: alternative solutions with limited applicability
	One possible valid solution or workaround to the best path selection
	problem requires sending all domain external paths from the route
	reflector to all its clients. This approach suffers the significant
	drawback of pushing a large amount of BGP state and churn to all edge
	routers. Many networks receive full Internet routing information in
	a large number of locations. This could easily result in tens of
	paths for each prefix that would need to be distributed to clients.
	Notwithstanding this drawback, there are a number of reasons for
	sending more than just the single best path to the clients. Improved
	path diversity at the edge is a requirement for fast connectivity
	restoration, and a requirement for effective BGP level load
	balancing.
	In practical terms, add/diverse path deployments [RFC7911] [RFC6774]
	are expected to result in the distribution of 2, 3, or n (where n is
	a small number) good paths rather than all domain external paths.
	When the route reflector chooses one set of n paths and distributes
	them to all its route reflector clients, those n paths may not be the
	right n paths for all clients. In the context of the problem
	described above, those n paths will not necessarily include the
	closest exit point out of the network for each route reflector
	client. The mechanisms proposed in this document are likely to be
	complementary to mechanisms aimed at improving path diversity.
	Another possibility to optimize exit point selection is the
	implementation of distributed route reflector functionality at key
	IGP locations in order to ensure that these locations see their
	viewpoints respected in exit selection. Typically, however, this
	requires the installation of physical nodes to implement the
	reflection, and if exit policy subsequently changes, the reflector
	placement and position can become inappropriate.
	To counter the burden of physical installation, it is possible to
	build a logical overlay of tunnels with appropriate IGP metrics in
	order to simulate closeness to key locations required to implement
	exit policy. There is significant complexity overhead in this
	approach, however, enough so to typically make it undesirable.
	Trends in control plane decoupling are causing a shift from
	traditional routers to compute virtualization platforms, or even
	third-party cloud platforms. As a result, without this proposal,
	operators are left with a difficult choice for the distribution and
	reflection of address families with significant exit diversity:
	o centralized path selection, and tolerate the associated suboptimal
	paths, or
	o defer selection to end clients, but lose potential route scale
	capacity
	The latter can be a viable option, but it is clearly a decision that
	needs to be made on an application and address family basis, with
	strong consideration for the number of available paths per prefix
	(which may even vary per prefix range, depending on peering policy,
	e.g. consider bilateral peerings versus onward transit arrangements)
	Authors' Addresses		Authors' Addresses
	Robert Raszuk (editor)		Robert Raszuk (editor)
	NTT Network Innovations		NTT Network Innovations
	Email: robert@raszuk.net		Email: robert@raszuk.net
	Christian Cassar		Christian Cassar
	Tesla		Tesla
	43 Avro Way		43 Avro Way
End of changes. 52 change blocks.
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