draft-ietf-idr-bgp-optimal-route-reflection-08.txt		draft-ietf-idr-bgp-optimal-route-reflection-09.txt
	IDR Working Group R. Raszuk		IDR Working Group R. Raszuk
	Internet-Draft Mirantis Inc.		Internet-Draft Mirantis Inc.
	Intended status: Standards Track C. Cassar		Intended status: Standards Track C. Cassar
	Expires: April 25, 2015 Cisco Systems		Expires: October 28, 2015 Cisco Systems
	E. Aman		E. Aman
	TeliaSonera		TeliaSonera
	B. Decraene		B. Decraene
	S. Litkowski		S. Litkowski
	Orange		Orange
	October 22, 2014		April 26, 2015
	BGP Optimal Route Reflection (BGP-ORR)		BGP Optimal Route Reflection (BGP-ORR)
	draft-ietf-idr-bgp-optimal-route-reflection-08		draft-ietf-idr-bgp-optimal-route-reflection-09
	Abstract		Abstract
	[RFC4456] asserts that, because the Interior Gateway Protocol (IGP)		[RFC4456] asserts that, because the Interior Gateway Protocol (IGP)
	cost to a given point in the network will vary across routers, "the		cost to a given point in the network will vary across routers, "the
	route reflection approach may not yield the same route selection		route reflection approach may not yield the same route selection
	result as that of the full IBGP mesh approach." One practical		result as that of the full IBGP mesh approach." One practical
	implication of this assertion is that the deployment of route		implication of this assertion is that the deployment of route
	reflection may thwart the ability to achieve hot potato routing. Hot		reflection may thwart the ability to achieve hot potato routing. Hot
	potato routing attempts to direct traffic to the closest AS egress		potato routing attempts to direct traffic to the closest AS egress
	skipping to change at page 1, line 45		skipping to change at page 1, line 45
	approach. Such a deployment approach would make route reflection		approach. Such a deployment approach would make route reflection
	compatible with the application of hot potato routing policy.		compatible with the application of hot potato routing policy.
	As networks evolved to accommodate architectural requirements of new		As networks evolved to accommodate architectural requirements of new
	services, tunneled (LSP/IP tunneling) networks with centralized route		services, tunneled (LSP/IP tunneling) networks with centralized route
	reflectors became commonplace. This is one type of common deployment		reflectors became commonplace. This is one type of common deployment
	where it would be impractical to satisfy the constraints described in		where it would be impractical to satisfy the constraints described in
	Section 11 of [RFC4456]. Yet, in such an environment, hot potato		Section 11 of [RFC4456]. Yet, in such an environment, hot potato
	routing policy remains desirable.		routing policy remains desirable.
	This document proposes two new solutions which can be deployed to		This document proposes a new solution which can be deployed to
	facilitate the application of closest exit point policy centralized		facilitate the application of closest exit point policy in
	route reflection deployments.		centralized route reflection deployments.
	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, 2015.		This Internet-Draft will expire on October 28, 2015.
	Copyright Notice		Copyright Notice
	Copyright (c) 2014 IETF Trust and the persons identified as the		Copyright (c) 2015 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
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	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
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	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
	2. Proposed solutions . . . . . . . . . . . . . . . . . . . . . 5		2. Proposed solution . . . . . . . . . . . . . . . . . . . . . . 4
	3. Best path selection for BGP hot potato routing from		3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5
	customized IGP network position . . . . . . . . . . . . . . . 6		4. Advantages and deployment considerations . . . . . . . . . . 6
	3.1. Client's perspective best path selection algorithm . . . 7		5. Security considerations . . . . . . . . . . . . . . . . . . . 7
	3.1.1. Flat IGP network . . . . . . . . . . . . . . . . . . 7		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	3.1.2. Hierarchical IGP network . . . . . . . . . . . . . . 8		7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
	3.2. Aside: Configuration-based flexible route reflector		8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
	placement . . . . . . . . . . . . . . . . . . . . . . . . 9		8.1. Normative References . . . . . . . . . . . . . . . . . . 7
	3.3. Route reflector client grouping . . . . . . . . . . . . . 10		8.2. Informative References . . . . . . . . . . . . . . . . . 8
	3.3.1. Route Reflector Client Group ID . . . . . . . . . . . 10		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
	3.4. Discussion . . . . . . . . . . . . . . . . . . . . . . . 11
	3.5. Advantages . . . . . . . . . . . . . . . . . . . . . . . 12
	4. Angular distance approximation for BGP warm potato routing . 13
	4.1. Problem statement . . . . . . . . . . . . . . . . . . . . 13
	4.2. Proposed solution . . . . . . . . . . . . . . . . . . . . 14
	4.3. Centralized vs distributed route reflectors . . . . . . . 15
	5. Client's perspective policy based best path selection . . . . 16
	5.1. Proposal . . . . . . . . . . . . . . . . . . . . . . . . 17
	5.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 17
	5.3. Avoiding routing loops . . . . . . . . . . . . . . . . . 18
	6. Deployment considerations . . . . . . . . . . . . . . . . . . 19
	7. Security considerations . . . . . . . . . . . . . . . . . . . 20
	8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
	9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
	10.1. Normative References . . . . . . . . . . . . . . . . . . 20
	10.2. Informative References . . . . . . . . . . . . . . . . . 21
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
	1. 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.		today: full mesh, confederations and route reflection.
	BGP route reflection is the most popular way to distribute BGP routes		BGP route reflection is the most popular way to distribute BGP routes
	between BGP speakers belonging to the same administrative domain.		between BGP speakers belonging to the same administrative domain.
	Traditionally route reflectors have been deployed in the forwarding		Traditionally route reflectors have been deployed in the forwarding
	path and carefully placed on the POP to core boundaries. That model		path and carefully placed on the POP to core boundaries. That model
	skipping to change at page 3, line 50		skipping to change at page 3, line 35
	address families including IPv4 and IPv6 Internet routing. This is		address families including IPv4 and IPv6 Internet routing. This is
	also applicable to new services achieved with BGP as control plane		also applicable to new services achieved with BGP as control plane
	for example 6PE.		for example 6PE.
	Such centralized route reflectors can be placed on the POP to core		Such centralized route reflectors can be placed on the POP to core
	boundaries, but they are often placed in arbitrary locations in the		boundaries, but they are often placed in arbitrary locations in the
	core of large networks.		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		best path selection. A route reflector with knowledge of multiple
	paths for a given prefix will pick the best path and only advertise		paths for a given prefix will typically (unless other techniques like
	that best path to the the route reflector clients. If the best path		add paths are in use) pick the best path and only advertise that best
	for a prefix is selected on the basis of an IGP tie break, the best		path to the the route reflector clients. If the best path for a
	path advertised from the route reflector to its clients will be the		prefix is selected on the basis of an IGP tie break, the best path
	exit point closest to the route reflector. But route reflector		advertised from the route reflector to its clients will be the exit
	clients will be in a place in the network toplogy which is different		point closest to the route reflector. But route reflector clients
	from the route reflector. In networks with centralized route		will be in a place in the network topology which is different from
	reflectors, this difference will be even more acute. It follows that		the route reflector. In networks with centralized route reflectors,
	the best path chosen by the route reflector is not necessarily the		this difference will be even more acute. It follows that the best
	same as the path which would have been chosen by the client if the		path chosen by the route reflector is not necessarily the same as the
	client considered the same set of candidate paths as the route		path which would have been chosen by the client if the client
	reflector. Furthermore, the path chosen by the client might have		considered the same set of candidate paths as the route reflector.
	been a better path from that chosen by the route reflector for		Furthermore, the path chosen by the client might have been a better
	traffic entering the network at the client. The path chosen by the		path from that chosen by the route reflector for traffic entering the
	client would have guaranteed the lowest cost and delay trajectory		network at the client. The path chosen by the client would have
	through the network.		guaranteed the lowest cost and delay trajectory through the network.
	Route reflector clients switch packets using routing information		Route reflector clients switch packets using routing information
	learnt from route reflectors which are not on the forwarding path of		learnt from route reflectors which are not on the forwarding path of
	the packet through the network even in the absence of end-to-end		the packet through the network even in the absence of end-to-end
	encapsulation. In those cases the path chosen as best and propagated		encapsulation. In those cases the path chosen as best and propagated
	to the clients will often not be the optimal path chosen by the		to the clients will often not be the optimal path chosen by the
	client given all available paths.		client given all available paths.
	Eliminating the IGP distance to the BGP nexthop as a tie breaker on		Eliminating the IGP distance to the BGP nexthop as a tie breaker on
	centralized route reflectors does not address the issue. Ignoring		centralized route reflectors does not address the issue. Ignoring
	skipping to change at page 5, line 11		skipping to change at page 4, line 45
	result in the distribution of 2, 3 or n (where n is a small number)		result in the distribution of 2, 3 or n (where n is a small number)
	'good' paths rather than all domain external paths. While the route		'good' paths rather than all domain external paths. While the route
	reflector chooses one set of n paths and distributes those same n		reflector chooses one set of n paths and distributes those same n
	paths to all its route reflector clients, those n paths may not be		paths 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		the right n paths for all clients. In the context of the problem
	described above, those n paths will not necessarily include the		described above, those n paths will not necessarily include the
	closest egress point out of the network for each route reflector		closest egress point out of the network for each route reflector
	client. The mechanisms proposed in this document are likely to be		client. The mechanisms proposed in this document are likely to be
	complementary to mechanisms aimed at improving path diversity.		complementary to mechanisms aimed at improving path diversity.
	2. Proposed solutions		2. Proposed solution
	This document proposes two simple solutions to the problem described
	above. Both of these solutions make it possible for route reflector
	clients to direct traffic to their closest exit point in hot potato
	routing deployments, without requiring further state to be pushed out
	to the edge. These solutions are primarily applicable in deployments
	using centralized route reflectors, which are typically implemented
	in devices without a capable forwarding plane.
	The two alternatives are:		This document proposes a simple solution to the problem described
			above - overwrite of the default IGP location placement of the route
			reflector - which is used for determining cost to the next hop
			contained in BGP paths.
	"Best path selection for BGP hot potato routing from client's IGP		The presented solution makes it possible for route reflector clients
	network position"		to direct traffic to their closest exit point in hot potato routing
			deployments, without requiring further state to be pushed out to the
			edge. This solution is primarily applicable in deployments using
			centralized route reflectors, which are typically implemented in
			devices without a capable forwarding plane or which are being moved
			to the NFV enabled cloud.
	"Angular distance approximation for BGP warm potato routing"		The solution rely upon all route reflectors learning all paths which
			are eligible for consideration for hot potato routing. In order to
			satisfy this requirement, path diversity enhancing mechanisms such as
			add paths/diverse paths may need to be deployed between route
			reflectors.
	Both solutions rely upon all route reflectors learning all paths		The core of the proposed solution is the ability for operator to
	which are eligible for consideration for hot potato routing. In		specify on a per route reflector basis or per peer/update group basis
	order to satisfy this requirement, path diversity enhancing		or per neighbour basis the virtual IGP location placement allowing to
	mechanisms such as add paths/diverse paths may need to be deployed		have given group of clients to consider optimal distance to the next
	between route reflectors.		hops from the position of the configured virtual IGP location. The
			choice of specific granularity is left to the implementation
			decision. Implementation is considered as compliant with the
			document if it supports at least one listed grouping of virtual IGP
			placement.
	In both of these solutions the route reflector selects and		The computation of the virtual IGP location with any of the above
	distributes a route to each client based on what would be optimal		described granularity is outside of the scope of this document.
	from the client's perspective. By optimal we refer in this document		Operator may configure it manually, implementation may automate it
	to the decision made during best path selection at the IGP metric to		based on specified heuristic or it can be computed centrally and
	BGP next hop comparison step. Clearly the overall path selection		configured by external system.
	preference may be chosen based other policy step and provisions as
	defined in this document would not apply.
	In the respective solutions the choice is made either factoring in		By optimal we refer in this document to the decision made during best
	IGP costs or the configured angular distance to the next hop. The		path selection at the IGP metric to BGP next hop comparison step.
	route reflector makes different decisions for different clients only		Clearly the overall path selection preference may be chosen based
	in the case where the tie breaker for path selection would have been		other policy step and provisions as defined in this document would
	the IGP distance to the BGP nexthop (as in hot potato routing).		not apply.
	A significant advantage of this approach is that the RR clients do		A significant advantage of this approach is that the RR clients do
	not need to run new software or hardware.		not need to run new software or hardware.
	Besides these solutions to manage hot potato routing, there are		3. Discussion
	deployment scenarios where service providers want to have more
	control of traffic exiting the AS by assigning per client preference
	to gateways.
	This document proposes to introduce a solution to perform a policy
	based route-reflection to address those scenarios. This solution has
	the same requirements (regarding path diversity) and advantages than
	the two IGP metric based solutions.
	3. Best path selection for BGP hot potato routing from customized IGP
	network position
	This section describes a method for calculating the order of
	preference of BGP paths from the point of view of each separate route
	reflector client. More specifically, the route reflector will
	compute the IGP metric to the BGP nexthop from the position of the
	client to which the resulting path will be distributed, if the IGP
	metric is the tie breaker applied to a set of possible paths. In the
	subsequent model authors will propose virtual reflector placement at
	operator's selected IGP location.
	In the case of a hierarchical IGP deployment where the client is in a
	different level in the hierarchy to the route reflector, the route
	reflector will compute IGP distance to the BGP nexthop from the Area
	Border Routers (ABR) leading to the client in lieu of the route
	reflector client itself, and use the shortest distance from these
	ABRs to the nexthop. This provides an approximation to the desired
	functionality. Rather than a client picking the closest path, the
	client would be picking the exit point closest to the client region
	as defined by area or level. In cases where one or more nexthops are
	in the same region as the client, one of those nexthops would be
	preferred, with tie breaking within those nexthops performed from the
	route reflector's position in the network.
	It is assumed that reachability through a set of ABRs is always
	advertised through identical prefixes from those ABRs. If a nexthop
	is reachable through multiple ABRs but the ABRs advertise
	reachability through prefixes of different length, then only the ABR
	advertising the longest prefix will be considered as a viable path to
	the nexthop.
	BGP best path selection and its distribution has a natural
	consequence of limiting the amount of state in the network. That is
	not in itself a drawback. BGP speakers will rarely need to receive
	all available BGP paths. In network deployments with multiple
	upstream peerings or with very dense peering schemes, the number of
	available BGP paths for a given BGP prefix can be high. Real network
	deployments with the number of paths for a prefix ranging from 10s to
	100s have been observed. It would be wasteful to propagate all of
	those paths to all clients, such that each client can select paths
	according to the position of the nexthop relative to the client.
	Whenever a BGP route reflector would need to decide what path or
	paths need to be selected for advertisement to one of its clients,
	the route reflector would need to virtually position itself in its
	client IGP network location in order to choose the right set of paths
	based on the IGP metric to the next hops from the client's
	perspective.
	This technique applies in deployments with or without diverse paths
	or the various path selection modes contemplated in add-paths.
	In the network architectures consisting of more then single pair of
	route reflectors it is required that all reflectors are fully meshed
	and have ability to learn and maintain all external BGP paths. In
	the event of constructing a hierarchy of reflectors to relax the full
	RR mesh requirements ORR should not be run between such route
	reflectors.
	3.1. Client's perspective best path selection algorithm
	For each centralized route reflector the proposal assumes that the
	route reflector participates in a common IGP with its clients. There
	are two scenarios to consider - flat versus hierarchical IGP network.
	3.1.1. Flat IGP network
	Reflectors run SPF from the client IGP node point of view such
	that the cost of BGP nexthops from the client can be determined if
	necessary. For the purpose of BGP path selection the interesting
	product of this calculation is the ability to determine the IGP
	distance from a client to a BGP next hop. This distance to a
	nexthop would be interesting in cases where that next hop is for a
	path which is contending with otherwise equally preferred paths.
	This approach works in tunneled as well as conventional hop-by-hop
	IP forwarding cores.
	When the path selection tie breaker for a prefix is the IGP metric
	to the BGP nexthops of the contending paths, then the route
	reflector will determine the order of preference of the contending
	paths by considering the distance from the client to the path
	nexthops in order to decide what path/s to advertise to a client
	(or group of clients where feasible). It should be noted that an
	operator may wish to provide a distance tolerance value, such that
	beyond a certain granularity, differences between IGP metric are
	invisible to the path selection algorithm. This will allow a
	route reflector some leeway in selecting between paths such that
	rather than pick one path over another on the basis of a
	difference in distance which is operationally irrelevant, the
	route reflector can choose to optimize for update generation
	grouping. Furthermore, this tolerance will reduce the likelihood
	of generation of BGP updates when the IGP topology changes in a
	way which is not operationally relevant. In the case that a path
	is selected from a set for a given prefix while ignoring
	differences in distance within the tolerance figure, then that
	same path must always be preferred for all clients where the paths
	are within the tolerance figure
	3.1.2. Hierarchical IGP network
	Hierarchy introduces two challenges:
	The first challenge is that the RR IGP view may differ from a
	client IGP view by virtue of one or the other having a summarized
	view versus the other. Summarization, by its nature, loses
	information. Consider the example where a client within a PoP
	sees two prefixes with two metrics for two egress points within
	the PoP, but where the RR only sees a single summary covering
	reachability to both nexthops as injected by the ABR. For
	clarification purposes in the case of ISIS by ABR we refer to L1/
	L2 node. However it needs to be observed that inter area networks
	running LDP are required to disable summarisation of all FEC
	advertised in LDP (typically all loopbacks) unless [RFC5283] is
	deployed. Such deployments are not likely to suffer summarization
	difficulties.
	The second challenge is that in cases where the client is in a
	different level of hierarchy from the RR, the RR can not build a
	Shortest Path First (SPF) tree with the client node as root,
	simply because the topology derived by the IGP will not include
	the client node. It will instead only include reachability to the
	client from one or more ABRs. In order to overcome this problem,
	the RR could compute an SPF tree from the ABRs in the area. The
	RR would then determine the shortest distance from a client which
	lives behind the ABRs, to a nexthop, by adding the advertised
	distances from an ABR to the client and the distance from the ABR
	to a nexthop, for each ABR, and picking the minimum. This assumes
	that IGP metrics on links are symmetric; i.e. that the distance
	from the ABR to the client or nexthop is equal to the distance
	from the client or nexthop to the ABR.
	There are cases where the above approach does not help. If RR is
	trying to arbitrate amongst a set of paths for a client which is
	in the same hierarchy as some of those paths, and in a different
	hierarchy to the RR, the opaqueness of the region containing the
	client at the RR defeats the selection process. It is impossible
	to determine the relative position of the RR client and the paths
	within the client region.
	The solution for hierarchical IGP networks also assumes that if
	RRs are present and are responsible for calculation of BGP best
	path to clients they are either placed in each local area
	coinciding with area containing clients or they are placed in the
	core (area 0/level 2) of the network.
	3.2. Aside: Configuration-based flexible route reflector placement
	The ability to exploit topology information available in the IGP in
	ways described above can also be used to virtually place the RR at
	different points in the network for purposes other than hot potato
	routing.
	A route reflector can be globally configured to "pretend" its logical
	location is one of any of the other nodes within a given IGP area/
	level flooding scope regardless of its physical connectivity.
	Such flexibility provides a useful tool for reflector virtualization,
	and supports moving or replacing physical route reflectors without
	any effect on routing. Such a change can be permanent or it could be
	performed during network maintenance in order to minimize network
	impact.
	A possible variation would allow the virtual placement of RR to be
	effected on a per-AF or AF plus update/peer group granularity. It
	should be noted that this approach provides for splitting one
	centralized route reflector such that it is virtually positioned at
	various network locations, with the network location depending upon
	of address family or address family plus update/peer group.
	Virtual slicing of a centralized route reflector relaxes the need to
	propagate all BGP paths between RRs in a alternative conventional
	distributed RR deployment. It is expected that such RRs would be
	deployed in redundant sets, and that those RRs would not need to be
	physically collocated, while still benefiting from the possibility of
	being logically collocated, and therefore not compromising any of the
	best path selection symmetry.
	3.3. Route reflector client grouping
	It may be appropriate to allow the operator, or the route reflector
	itself, to group clients together using IGP distance between clients
	to determine grouping. All the operation discussed above which
	relied upon computing best path for each client, and measuring
	distances from each client to different nexthops, would instead be
	performed for each group of clients. Configurable thresholds can be
	used to determine which IGP metric changes should be visible to BGP,
	and trigger best paths recomputation. The latter would be beneficial
	in existing BGP RR code too.
	Alternatively route reflector client grouping could be accomplished
	statically by the operator by coloring clients belonging to a common
	group (for example being part of the same POP). In order to
	accomplish such marking it is proposed that BGP OPEN message be
	augmented with an optional parameter indicating the Group ID given
	peer belongs to.
	3.3.1. Route Reflector Client Group ID
	This is an Optional Parameter in BGP OPEN message that is used by a
	BGP speaker to convey to its route reflectors the Group ID value.
	Such value will allow automatic and predictable peer grouping on the
	route reflectors as deemed necessary from operator's network
	architecture.
	The parameter contains precisely one set of [Group_ID Code, Group_ID
	Length, Group_ID Value] encoded as shown below:
	+----------------------------+
	| Group ID Code (1 octet) |
	+----------------------------+
	| Group ID Length (1 octet) |
	+----------------------------+
	| Group ID Value (4 octets) |
	+----------------------------+
	The use and meaning of these fields are as follows:
	Group ID Code:
	Group ID Code is a one octet field that identifies Group ID
	optional parameter of BGP OPEN message. Value TBD by IANA
	Recommended value: 3.
	Group ID Length:
	Group ID Length is a one octet field that contains the length
	of the Group ID Value field in octets. It is fixed and equals
	to 4.
	Group ID Value:
	Group ID Value is a fixed length field of size equal to
	four octets that contains the numerical value of group given
	BGP speaker should be part of on the route reflector.
	Two special values are reserved:
	0x00000000 - No grouping preference
	0xFFFFFFFF - Do not group this BGP speaker
	An implementation may allow automatic population of
	GROUP_ID value using IGP area identifier.
	Route reflectors or EBGP speakers receiving such Group IDs from their
	respective BGP peers as part of the BGP OPEN procedure MAY use them
	when constructing update or peer groups in addition to any of the
	existing grouping mechanism already available. An implementation may
	allow operator to explicitly allow or disallow honoring such grouping
	or provide means for manual overwrite via explicit configuration.
	3.4. Discussion
	This is not the first instance where a router participating in an IGP
	is required to build the SPF tree using a root other than itself.
	Determination of loop free alternate paths as described in [RFC5714]
	is one such example.
	Determining the shortest path and associated cost between any two		Determining the shortest path and associated cost between any two
	arbitrary points in a network based on the IGP topology learned by a		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 resource.		router is expected to add some extra cost in terms of CPU resource.
	However SPF tree generation code is now implemented efficiently in a		However SPF tree generation code is now implemented efficiently in a
	number of implementations, and therefor this is not expected to be a		number of implementations, and therefor this is not expected to be a
	major drawback. The number of SPTs computed in the general non-		major drawback. The number of SPTs computed in the general non-
	hierarchical case is expected to be of the order of the number of		hierarchical case is expected to be of the order of the number of
	clients of an RR whenever a topology change is detected. Advanced		clients of an RR whenever a topology change is detected. Advanced
	optimizations like partial and incremental SPF may also be exploited.		optimizations like partial and incremental SPF may also be exploited.
	skipping to change at page 12, line 12		skipping to change at page 6, line 18
	number of clients. While this is not expected to be necessary in		number of clients. While this is not expected to be necessary in
	existing networks with best in class route reflectors available		existing networks with best in class route reflectors available
	today, this avenue to scaling up the route reflection infrastructure		today, this avenue to scaling up the route reflection infrastructure
	would be available. If we consider the overall network wide cost/		would be available. If we consider the overall network wide cost/
	benefit factor, the only alternative to achieve the same level of		benefit factor, the only alternative to achieve the same level of
	optimality would require significantly increasing state on the edges		optimality would require significantly increasing state on the edges
	of the network, which, in turn, will consume CPU and memory resources		of the network, which, in turn, will consume CPU and memory resources
	on all BGP speakers in the network. Building this client perspective		on all BGP speakers in the network. Building this client perspective
	into the route reflectors seems appropriate.		into the route reflectors seems appropriate.
	3.5. Advantages		4. Advantages and deployment considerations
	The solution described provides a model for integrating the client		The solution described provides a model for integrating the client
	perspective into the best path computation for RRs. More		perspective into the best path computation for RRs. More
	specifically, the choice or BGP path factors in the IGP metric		specifically, the choice of BGP path factors in the IGP metric
	between the client and the nexthop, rather than the distance from the		between the client and the nexthop, rather than the distance from the
	RR to the nexthop. The documented method does not require any BGP or		RR to the nexthop. The documented method does not require any BGP or
	IGP protocol changes as required changes are contained within the RR		IGP protocol changes as required changes are contained within the RR
	implementation.		implementation.
	This solution 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 the		networks as well as in end-to-end tunneled environments. In the
	networks where there are multiple route reflectors and hop-by-hop		networks where there are multiple route reflectors and hop-by-hop
	forwarding without encapsulation, such optimizations should be		forwarding without encapsulation, such optimizations should be
	enabled on all route reflectors. Otherwise clients may receive an		enabled in a consistent way on all route reflectors. Otherwise
	inconsistent view of the network and in turn lead to intra-domain		clients may receive an inconsistent view of the network and in turn
	forwarding loops.		lead to intra-domain forwarding loops.
	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 from the forwarding plane to		even if route reflection has been moved from the forwarding plane
	the core and hop-by-hop switching has been replaced by end to end		(example ABR) tothe core and hop-by-hop switching has been replaced
	MPLS or IP encapsulation.		by end to end MPLS or IP encapsulation.
	As per above, the approach reduces the amount of state which needs to		As per above, the approach reduces the amount of state which needs to
	be pushed to the edge in order to perform hot potato routing. The		be pushed to the edge in order to perform hot potato routing. The
	memory and CPU resource required at the edge to provide hot potato		memory and CPU resource required at the edge to provide hot potato
	routing using this approach is lower than what would be required in		routing using this approach is lower than what would be required in
	order to achieve the same level of optimality by pushing and		order to achieve the same level of optimality by pushing and
	retaining all available paths (potentially 10s) per each prefix at		retaining all available paths (potentially 10s) per each prefix at
	the edge.		the edge.
	The proposal allows for a fast and safe transition to BGP control		The proposal allows for a fast and safe transition to BGP control
	plane route reflection without compromising an operator's closest		plane route reflection without compromising an operator's closest
	exit operational principle. Hot potato routing is important to most		exit operational principle. Hot potato routing is important to most
	ISPs. The inability to perform hot potato routing effectively stops		ISPs. The inability to perform hot potato routing effectively stops
	migrations to centralized route reflection and edge-to-edge LSP/IP		migrations to centralized route reflection and edge-to-edge LSP/IP
	encapsulation for traffic to IPv4 and IPv6 prefixes.		encapsulation for traffic to IPv4 and IPv6 prefixes.
	4. Angular distance approximation for BGP warm potato routing		Regarding potential for intra-domain forwarding loops at ASBR level,
			this could be solved by enforcing external route preference or by
	This section describes an alternative solution to the use of IGP
	topology information to virtually position the RR at the client
	location in the network. This solution involves modeling the network
	topology as a set of elements (regions, PoPs or routers) arranged in
	a circle. Route reflector clients and inter-domain exit points would
	then be statically assigned to those elements such that one can
	compute the angular distance between route-reflector clients and the
	various exit points in order to infer the distance between any two
	elements. This measure of distance can be used as an effective
	alternative to the IGP distance as a tie breaker in the path
	selection algorithm if necessary.
	4.1. Problem statement
	This solution addresses the problem described in earlier sections,
	while attempting to minimize computational overhead. The aim of the
	proposed solution is to enable a route reflector to provide a route
	reflector client with an exit point for a prefix which is 'closest'
	to the client rather than the route-reflector, without having to
	distribute all paths to that client, or having to derive each
	client's view of the network topology. The measure of closest is
	based on a simplistic description of network topology provided by the
	operator.
	Consider the following example of an ISP network topology drawn to
	reflect the location of the nodes and POPs:
	N4 POP4
	CLIENT B
	POP4 POP1 N1
	CORE
	RR(s) POP2 N2
	N5 POP3 POP2 N3
	CLIENT A
	POP3
	N - represents the different exit points for a given prefix. POP2 is
	a geographically large PoP with two paths; N2 and N3.
	In a deployment where the centralized RRs tie break on the basis of
	their IGP-based view of the network, N1 above would be advertised to
	all clients on the basis that it is closest to the RR. Path N4 would
	be a more appropriate choice for client B. Similarly, N5 would be
	more appropriate for client A since path N5 is closer to client A
	then path N1.
	4.2. Proposed solution
	The proposed solution revolves around the operator establishing the
	angular position of the route-reflector clients and inter-domain exit
	points in the network. The route reflector then picks the path to
	advertise to a client based on the client's angular position versus
	the angular position of the inter-domain exit points originating the
	paths. The operator can choose the granularity of angular position
	appropriate to the desired goals. On one hand, the coarseness of the
	angular position will effect the operator overhead; versus the
	optimality of routing on the other. The finest granularity possible
	will be the relative position of originating clients.
	Note that this solution has nothing to do with actual IGP link
	metrics and resulting topology in the network.
	It can be shown that for each network topology, elements such as AS
	exit points can be mapped on to a circle. By putting POPs, Regions
	or individual clients onto the hypothetical circle we can identify an
	angular location for each element relative to some fixed direction;
	for example defining the angular north of the circle at 0 degrees.
	The angular position of elements in the network can be conveyed to a
	route reflector in a number of ways:
	Assignment of angular position of each RR client through
	configuration on the route reflector itself; per client
	configuration on RR
	Assignment of angular position of an RR client at each client,
	then propagating it to RRs.
	The proposed angular distance approximation is compatible with both
	flat and hierarchical IGP deployments.
	In the example illustrated above the route reflector might learn or
	be configured with the following set of paths and corresponding
	angular positions:
	Prefix X/Y N1 N2 N3 N4 N5
	Location
	in degrees 60 85 120 290 260
	If the absolute angular position of clients A and B were as follows:
	Client A: 260 degrees
	Client B: 290 degrees
	Then the corresponding angular distances for those clients versus the
	exit points can be calculated as follows:
	Prefix X/Y N1 N2 N3 N4 N5
	Client A 200 175 140 30 0
	Client B 230 205 170 0 30
	With an RR running the BGP best path algorithm modified to use the
	angular distance from the client to the nexthops, rather than its IGP
	distance to the nexthops as tie breaker, each client is provided with
	its closest path with the measure of closeness reflecting the angular
	position as configured by the operator.
	The model used by the operator in order to determine the angular
	position of a client or exit point, might involve grouping elements
	together by region or PoP, or might involve no grouping at all.
	Implementations should allow the operator to pick the appropriate
	granularity.
	4.3. Centralized vs distributed route reflectors
	In an environment where the RR clusters are distributed (yet
	centralized enough to make hot potato routing hard), and each RR
	cluster serves a subset of clients, it becomes necessary to propagate
	the angular position of the clients between route reflectors. This
	can be achieved as follows:
	Deploy add-paths between route reflectors in order to maximize
	path diversity within the cluster.
	A non AS transitive BGP community of type (TBA by IANA) can be
	used to encode and propagate angular position between 0 and 359 of
	a client. This community is only relevant to the route reflectors
	of a given BGP domain and should be stripped either at the ASBR
	boundary or when propagating updates to BGP peers which are not
	route reflectors.
	The angular position marking could also be added by clients and
	advertised to the route reflector. This would require some
	configuration effort.
	5. Client's perspective policy based best path selection
	There is some deployment scenarios where a service provider wants to
	achieve a stronger control on traffic exiting the AS (for capacity
	planning) rather than using hot potato routing based on IGP metric.
	| | | |
	| | | |
	GW1 GW2 GW3 GW4
	RR1 RR2
	R1 R2 R3
	Considering the figure above, all gateways have iBGP sessions to RR1
	and RR2, and R1 R2 R3 have iBGP sessions as well to RR1 and RR2.
	Gateway routers are meshed to an external network (for example, a
	transit service provider).
	We would like to achieve a strong control on the gateway used
	(primary and backup) for each router (or each set of routers) in the
	network (taking into account that routers do not support ADD PATHs).
	For example, R1 using GW1 as primary and GW2 as backup; R2 using GW2
	as primary and GW3 as backup; R3 using GW3 as primary and GW4 as
	backup.
	Basically, today a prefix P1 is received on each gateway from the
	external network. Each gateway will send the prefix to both route
	reflectors. Each route-reflector will receive four paths for P1 and
	choose the best one based on his own decision process. Note that RR1
	and RR2 may choose a different path as best. Each route-reflector
	sends his best path towards R1, R2 and R3. Each router will receive
	the same paths from the route-reflectors for P1 (at max, only two
	gateways are visible from Rx routers). So default behavior does not
	fit our requirements in term of traffic flows.
	Using current BGP mechanisms available, we could achieve our
	requirements using two solutions :
	o Modify the BGP meshing: for example, R1 meshed directly to GW1 and
	GW2 and apply inbound policies on R1; R2 meshed directly to GW2
	and GW3 and apply inbound policies on R2 ...
	o Adding more route-reflectors (one RR per gateway used as primary)
	and applying inbound policies on RRs to make each RR choosing a
	different primary gateway and apply policies on routers to select
	his own primary gateway.
	These solutions have many drawbacks: first one is not flexible (re-
	meshing needed when we want to change gateway of a router), second
	one requires a lot of CAPEX.
	We would like to introduce a solution where a single currently
	deployed route-reflector chassis may take a different best path
	decision for different set of clients based on preferences.
	It should be noted that in simple scenarios (example: two RRs and two
	gateways), RFC6774 would be able to fulfill service provider needs.
	The solution proposed here would permit to handle more complex
	scenarios and fine gateway choice per client or groups of clients.
	5.1. Proposal
	Our proposal is to reuse the concept introduced in [I.D.ietf-idr-ix-
	bgp-route-server] in an iBGP context. To perform per client best
	path selection, the router should maintain a per client BGP local-RIB
	(or Adj-RIB-Out) associated with inbound policies implemented between
	Adj-RIB-In and client LOC-RIB.
	It would not be very scalable to use a per client policy (considering
	hundreds of peers on a route-reflector), therefor our proposal is to
	group clients sharing common policies inside a client group to
	minimize computation/memory overhead. Client grouping could be done
	statically (by configuration) or dynamically using the solution
	described in section 3.3.1 of this document. Client grouping would
	be performed with a per AFI/SAFI granularity as gateway/client
	mapping may change in each AFI/SAFI context. A route-reflector
	should be able to implement multiple client groups (with associated
	inbound policies) as well as a default client group for clients that
	does not require any specific policy decision: in this case, the
	overall BGP best path computation would be used.
	5.2. Example
	GW1 GW2 GW3
	\ | /
	\ | /
	RR1
	/ | \
	R1 R2 R3
	In the above figure GW1, GW2, GW3 and R3 are standard ibgp route-
	reflector clients. R1 and R2 want to use a special gateway
	combination (primary GW3, backup GW2, last resort GW1). R1 and R2
	are configured in a specific client group CG1 on the route-reflector
	while other peers are in the default client group. CG1 is associated
	with a policy achieving the expected GW preference for R1 and R2, and
	letting other paths without any change.
	All routes received by RR1 (ebgp, ibgp, ibgp rr client, ibgp rr
	client routing context) must be evaluated using overall BGP best path
	computation as well as in client group, the client group policy will
	accept or not the route to be evaluated by the local decision
	process.
	o Paths from GW1, GW2, GW3 are compared within default client group
	leading to one GW (for example GW1) to be selected as best and
	installed in global LOC-RIB. GW1 path will be advertised to GW2,
	GW3 and R3 as they are in default CG. In CG1, preference of GW
	paths has been modified, leading to GW3 being the best path and
	installed in client group LOC-RIB. GW3 path will be advertised to
	R1 and R2, as R1 and R2 are part of CG1.
	o Paths from R3 are compared within default client group and
	advertised to GW1, GW2, GW3. Those paths are also compared within
	CG1 (as accepted by policy) and advertised to R1 and R2.
	o Paths from R1 are compared within default client group and
	advertised to GW1, GW2, GW3 and R3. Those paths are also compared
	within GG1 (as accepted by policy) and advertised to R2.
	o Paths from R2 are compared within default client group and
	advertised to GW1, GW2, GW3 and R3. Those paths are also compared
	within CG1 (as accepted by policy) and advertised to R1.
	5.3. Avoiding routing loops
	Compared to the IGP approaches described in this document, the policy
	based route-reflection should be limited to end-to-end encapsulation
	environments to avoid intra-domain forwarding loops. Using end-to-
	end encapsulation permit Edge routers to transport the traffic to the
	targeted/preferred ASBR without any loop in the core.
	To avoid a potential rerouting of the ASBR into the core (and
	possible loop between Edges and ASBR), we must enforce forwarding at
	the ASBR to the eBGP peer. This could be done by :
	o implementing policies on ASBR to prefer eBGP path and install it
	in FIB.
	o implementing tunneling of traffic until the outside interface
	(ASBR action to switch to outside interface).
	The exact choice of encapsulation and techniques to prevent transport
	loops (including potential loops at gateways) is left to the operator
	choice and its specification is outside of the scope of this
	document.
	6. Deployment considerations
	The solutions are primarily intended for end-to-end tunneled
	environments, i.e. where traffic is label switched or IP tunneled
	across the core. If unencapsulated hop-by-hop forwarding is used,
	either misconfigurations or conflicts between these optimizations and
	classical BGP path selection rules could lead to intra-domain
	forwarding loops. Under certain circumstances the solutions can also
	be deployable without end-to-end tunneling. In particular the best
	path selection based on the client's IGP best-path selection is
	guaranteed not to cause any forwarding loops (other than micro loops
	associated with reconvergence) when deployed in a flat IGP area
	provided that no distance tolerance value is used so that the path
	choice is truly made on a per-client basis.
	Regarding potential intra-domain forwarding loops at ASBR level, this
	could be solved by enforcing external route preference or by
	performing tunnel to external interface switching action on ASBRs.		performing tunnel to external interface switching action on ASBRs.
	Regarding client's IGP best-path selection, it should be self evident		Regarding client's IGP best-path selection, it should be self evident
	that this solution does not interfere with policies enforced above		that this solution does not interfere with policies enforced above
	IGP tie breaking in the BGP best path algorithm.		IGP tie breaking in the BGP best path algorithm.
	The solution applies to NLRIs of all address families which can be		The solution applies to NLRIs of all address families which can be
	route reflected.		route reflected.
	It should be noted that customized per-client or group of clients		5. Security considerations
	best path selection is already in use today in the context of
	Internet Exchange Point (IXP) route servers. In an IXP route server
	the client best path is selected as a result of different policies
	rather than IGP metric distance to BGP next hop.
	A possible scalability impact of optimizing path selection to take
	account of the RR client position or operator's policy based
	preference is that different RR clients receive different paths, and
	therefore update/peer group efficiency diminishes. This cost is
	imposed by the requirement to optimize the egress path from the
	client's perspective. It is also likely that groups of clients will
	end up receiving the same best path/s, in which case, inefficiency of
	update generation will be minimized. It should be noted that in the
	cases described under flexible router placement where placement is
	determined on a per update/peer group basis or per route reflector,
	the scale benefits of peer groupings are retained.
	7. Security considerations
	No new security issues are introduced to the BGP protocol by this		No new security issues are introduced to the BGP protocol by this
	specification.		specification.
	8. IANA Considerations		6. IANA Considerations
	IANA is requested to allocate a type code for the Standard BGP		IANA is requested to allocate a type code for the Standard BGP
	Community to be used for inter cluster propagation of angular		Community to be used for inter cluster propagation of angular
	position of the clients.		position of the clients.
	IANA is requested to allocate a new type code from BGP OPEN Optional		IANA is requested to allocate a new type code from BGP OPEN Optional
	Parameter Types registry to be used for Group_ID propagation.		Parameter Types registry to be used for Group_ID propagation.
	9. Acknowledgments		7. Acknowledgments
	Authors would like to thank Eric Rosen, Clarence Filsfils, Uli		Authors would like to thank Keyur Patel, Eric Rosen, Clarence
	Bornhauser Russ White, Jakob Heitz, Mike Shand and Jon Mitchell for		Filsfils, Uli Bornhauser, Russ White, Jakob Heitz, Mike Shand and Jon
	their valuable input.		Mitchell for their valuable input.
	10. References		8. References
	10.1. Normative References		8.1. 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, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway		[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
	Protocol 4 (BGP-4)", RFC 4271, January 2006.		Protocol 4 (BGP-4)", RFC 4271, January 2006.
	[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended		[RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
	Communities Attribute", RFC 4360, February 2006.		Communities Attribute", RFC 4360, February 2006.
	[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement		[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
	with BGP-4", RFC 5492, February 2009.		with BGP-4", RFC 5492, February 2009.
	10.2. Informative References		8.2. Informative References
	[I-D.ietf-idr-add-paths]		[I-D.ietf-idr-add-paths]
	Walton, D., Retana, A., Chen, E., and J. Scudder,		Walton, D., Retana, A., Chen, E., and J. Scudder,
	"Advertisement of Multiple Paths in BGP", draft-ietf-idr-		"Advertisement of Multiple Paths in BGP", draft-ietf-idr-
	add-paths-09 (work in progress), October 2013.		add-paths-10 (work in progress), October 2014.
	[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP		[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
	Communities Attribute", RFC 1997, August 1996.		Communities Attribute", RFC 1997, August 1996.
	[RFC1998] Chen, E. and T. Bates, "An Application of the BGP		[RFC1998] Chen, E. and T. Bates, "An Application of the BGP
	Community Attribute in Multi-home Routing", RFC 1998,		Community Attribute in Multi-home Routing", RFC 1998,
	August 1996.		August 1996.
	[RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114,		[RFC4384] Meyer, D., "BGP Communities for Data Collection", BCP 114,
	RFC 4384, February 2006.		RFC 4384, February 2006.
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