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Internet Engineering Task Force                                W. George
Internet-Draft                                         Time Warner Cable
Intended status: Informational                                 R. Shakir
Expires: June 24, 2013                                                BT
                                                       December 21, 2012

                     IP VPN Scaling Considerations


   This document discusses scaling considerations unique to
   implementation of Layer 3 (IP) Virtual Private Networks, discusses a
   few best practices, and identifies gaps in the current tools and
   techniques which are making it more difficult for operators to cost-
   effectively scale and manage their L3VPN deployments.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 24, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Intention of this Document . . . . . . . . . . . . . . . .  4
     1.2.  Horizontal vs. Vertical Scaling  . . . . . . . . . . . . .  5
     1.3.  Developing Requirements for Scaled L3VPN Environments  . .  6
   2.  PE-CE routing protocols  . . . . . . . . . . . . . . . . . . .  6
     2.1.  Best Common Practice . . . . . . . . . . . . . . . . . . .  7
     2.2.  Common Problems at Scale Limits  . . . . . . . . . . . . .  9
   3.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Best Common Practices  . . . . . . . . . . . . . . . . . . 11
     3.2.  Common Problems at Scale Limits  . . . . . . . . . . . . . 11
   4.  Network Events . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Best Common Practices  . . . . . . . . . . . . . . . . . . 11
     4.2.  Common Problems at Scale Limits  . . . . . . . . . . . . . 12
   5.  General Route Scale  . . . . . . . . . . . . . . . . . . . . . 13
     5.1.  Route-reflection and scaling . . . . . . . . . . . . . . . 16
     5.2.  Best Common Practices  . . . . . . . . . . . . . . . . . . 18
       5.2.1.  Topology-related optimizations . . . . . . . . . . . . 19
     5.3.  Common problems at scale limits  . . . . . . . . . . . . . 20
   6.  Known issues and gaps  . . . . . . . . . . . . . . . . . . . . 21
     6.1.  PE-CE routing protocols  . . . . . . . . . . . . . . . . . 21
     6.2.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.3.  Network Events . . . . . . . . . . . . . . . . . . . . . . 22
     6.4.  General Route Scale  . . . . . . . . . . . . . . . . . . . 22
     6.5.  Modeling and Capacity planning . . . . . . . . . . . . . . 22
     6.6.  Performance issues . . . . . . . . . . . . . . . . . . . . 23
     6.7.  High Availability and Network Resiliency . . . . . . . . . 24
     6.8.  New methods of horizontal scaling  . . . . . . . . . . . . 25
   7.  To-Do list . . . . . . . . . . . . . . . . . . . . . . . . . . 25
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 26
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 26
     11.2. Informative References . . . . . . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28

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1.  Introduction

   As IP networking has become more ubiquitous and mature, many
   enterprises have begun migration away from legacy point to point or
   layer 2 virtual private network (VPN) implementations toward layer 3
   VPNs.  The VPN implementation as defined by RFC 4364 [RFC4364]
   enables flexible and robust implementations of IP VPNs.  However, in
   practice, it has become clear that it suffers from significant
   scaling considerations beyond those discussed in RFC4364.  In many
   cases, the limits of scale for a given platform are not in sync with
   the maximum physical and logical interface density supported by the
   platform, such that a platform may be considered "full" long before
   the physical slots and ports have all been filled with equipment and
   connections.  This represents an inefficient use of space and power,
   as well as stranded capital assets, which increase the operator's
   cost to provide the service as well as the complexity of managing the
   platform to ensure proper service levels in a wide variety of
   circumstances.  While these scaling considerations are somewhat
   similar to the scaling concerns experienced in the Global Internet,
   those are at best a subset of the overall problem, and may not have a
   great deal of overlap between solutions and best practices.  The
   added complexity and feature set required to support today's
   enterprise IP networks drives additional scaling considerations for
   large deployments.  A common response to concerns about control plane
   scale is simply to "throw hardware at the problem" in the form of
   ever-increasing amounts of memory and CPU resources.  In some cases,
   this may be the only solution, but similarly to the concerns
   identified in RFC 4984 [RFC4984], there are limits to the growth
   curve that can be supported and cost-effectively deployed by a VPN
   provider such that their service remains profitable, and therefore it
   is necessary to explore the potential for optimization to make the
   existing resources stretch further.

   Generally, router scale can be considered in one of three areas:
   forwarding capacity, interface density, and control plane capacity.
   This draft will focus almost exclusively on control plane capacity,
   because while the others are important considerations for most
   operators, they are less affected by the details of how L3VPN is
   implemented either by the router vendor or the operator.  Interface
   density is usually a factor of the forwarding capacity of a given
   module or slot as well as physical packaging.  In this application,
   interface density is interesting from the perspective of its impact
   to the control plane - more interfaces means more of all of the
   different factors that contribute to control plane load, and the
   operator wants to be able to strike a balance between interface
   density and control plane capacity such that neither grows out of
   pace with the other.

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1.1.  Intention of this Document

   This document is intended to provide a discussion of the challenges
   that network operators face in deploying large-scale L3VPN
   environments at the time of writing, with two key sets of
   recommendations.  As such, these outcomes can be divided into those
   that apply to network operators regarding the deployment of
   particular technologies, and those that apply to network protocol and
   operating system implementors relating to allowing better
   understanding of scaling characteristics in deployments of such

   The best practices defined in this document are intended to allow
   more optimal scaling of L3VPN networks, whilst minimising the impact
   on end-customer network behaviour.  It is intended that such guidance
   can be directly utilised by Service Providers to improve the
   scaleability of network elements.  However, the guidance in this
   document should not be viewed as a panacea to the problems of scaling
   network elements.  It is the intention of the authors to document a
   number of key problems experienced in such environments and provide
   information to the SP that may result in more optimal deployment of
   existing technologies to this audience.  It is appreciated that there
   is a point at which the limits of hardware will be reached, and hence
   new network elements are required.  The key intention of the
   recommendations provided to Service Providers within this document
   are intended to allow the resources that exist within existing
   elements to be utilised in the most efficient manner.  Clearly, the
   optimal point in this balance is that the data-plane and control-
   plane scale to support similar levels of service termination, so as
   to result in minimal "over provisioning" of one element.

   The scaling considerations presented in this document are intended to
   provide both network operators and network equipment implementors
   further guidance around the toolset, and information required to
   provide accurate means of capacity planning in L3VPN environments.
   Again, the authors consider that the scaling characteristics, and
   toolsets required of L3VPN PE equipment diverge somewhat from those
   required by Internet network equipment.  In Internet deployments,
   relatively standardised interconnects exist across all deployments -
   typically utilising either static routing, or BGP-4.  As such, each
   connected port comes with a relatively standard overhead in terms of
   the protocols required.  Whilst there is some variance in how
   "chatty" each customer connection may be, this is balanced by the
   fact that the whole Internet routing table is typically held on such
   edge equipment (and hence individual customer's instability tends to
   be relatively small when compared to the instability of the Internet
   DFZ).  In addition, since such instability is limited to relatively
   few impacts to a node (interface or BGP session flapping, and BGP

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   UPDATE messages) routers can be optimised to cope with such
   instability.  Counter to this, the L3VPN environment does not have a
   standardised connectivity model, and typically connects to much less
   controlled environments.  Further details of this are provided within
   later sections of this document.  The result of this difference is
   that 'headline' scaling figures presented for particular equipment
   tends to be of limited utility to a network operator.  The
   recommendations within this document outline some of the
   considerations that must be made in considering the scaling of such
   elements, and provide guidance as to the missing inputs and tools
   that are required to provide information around the capacity of such

1.2.  Horizontal vs. Vertical Scaling

   Within this document, two forms of 'scaling' are referred to - the
   "throw hardware at the problem" approach outlined previously involves
   deploying additional network elements in order to provide further
   network capacity.  Throughout this document, this approach is
   referred to a horizontal scaling - insofar as it requires parallel
   deployment of numerous similar elements and balancing the load across
   the combined capacity of all of the elements.  The approach of
   increasing the capacity of an individual node through allowing the
   control plane capacity to support the maximum forwarding plane
   capacity (be it data forwarded, or available ports) is referred to as
   vertical scaling.  It is obvious that at some point the approach of
   horizontal scaling of elements is required - due to either exhausting
   available port capacities, or available forwarding plane - however,
   it should be noted that there are a number of motivations for
   delaying such provisioning, some of which relate directly to the
   characteristics of L3VPN environments.

   Since a significant proportion of the customers who purchase L3VPN
   services are Enterprise customers, typically the service is utilised
   as a WAN for their inter-location connectivity.  Clearly, as such
   customer base tends to be distributed based on differing factors,
   this implies that such customers connect in numerous geographical
   locations.  The requirement to support service in these locations
   therefore results in a requirement for the service provider network
   architecture to support geographically distributed access into such
   services.  A balance must be struck between the extent to which
   access networks are utilised to backhaul traffic to the service
   layer, and the geographical distribution of the service layer itself.
   Both scale and performance characteristics of such networks tend to
   result in more geographical distribution of service layer elements
   than in Internet deployments.  This distribution results in two
   particular changes - primarily that the idea of a "point-of-presence"
   must be reconsidered - where an assumption in Internet environments

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   may be that there are separated core and access elements within a
   single location, within a distributed L3VPN environment, a point of
   presence may be a single PE device.  The result of these small scale
   points of presence is that numerous core and edge functions must be
   collapsed onto a single device.  For this reason, the approach of
   adding additional devices to the network may have an impact on a
   further subset of devices within the network (particularly due to any
   mesh-based protocols that are deployed), and hence result in a change
   in the scaling characteristics of these devices.  In this case, there
   is further motivation to avoid large numbers of devices in the
   network where possible.  Further to this, the smaller PoP profile may
   result in physical constraints around the deployment of additional
   network elements, particularly due to the availability of power and
   physical space to deploy such elements.

1.3.  Developing Requirements for Scaled L3VPN Environments

   Whilst the collected scaling considerations outlined in this document
   are based on the author's collective experience within various
   Service Provider networks, and discussions with operators of similar
   networks, it should be noted that the problems outlined in this
   document are not static.  With the growth in the use of IP as the
   underlying transport of many services, the demand for L3VPN
   environments has grown.  As such, this has meant that various
   technologies are being considered to allow growth of these networks
   at a lower cost point to a wider footprint than was previously
   required.  A network operator must therefore consider the extent to
   which the service layer must be built - both to meet economic and
   technical requirements.  With newer aggregation methods, the service
   layer edge (and hence the L3VPN PE) acquires responsibility for
   inter-working between newer dynamic aggregation technologies, and the
   existing IP network.  As such, these edge functionalities result in
   further requirements for loading onto these network elements.

   *** Author's note: Do we want to put anything about NNI for footprint
   extension here?  Datacenter edge - perhaps Ning's problem around the
   L3VPN edge in his datacentres? ***

2.  PE-CE routing protocols

   One of the things that makes IP VPNs so flexible and robust is their
   ability to participate in the encapsulated network's routing
   protocols, where the customer edge (CE) router has a direct neighbor
   relationship with its upstream provider edge (PE) router in order to
   exchange routing information about the Virtual Route Forwarding (VRF)
   instance that represents the VPN.  In many cases, this is managed
   through a combination of static routes and BGP neighbors, but IGPs

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   such as OSPF RFC 4577 [RFC4577] are often supported, because it
   enables a more complete integration into an existing enterprise
   network design and topology.  In some single-vendor implementations,
   carriers sometimes support proprietary routing protocols such as
   EIGRP [EIGRP].  IGPs may also be chosen due to a belief that they
   will respond more rapidly during a failure than BGP will.  In
   reality, this may not be true due to the fact that VRF routing
   information is still carried in MP-BGP from PE to PE, and the PE-CE
   routing protocol's characteristics are only locally significant.  In
   fact, the increased overhead may lead to slower convergence times
   than a more standard BGP implementation.

   IGPs often translate to a significant increase in overhead due to
   their inherent characteristics as link-state routing protocols
   requiring full topology databases and flooding of updates to all
   participants, and the fact that they invoke additional processes on
   the router when compared to simply using BGP (which is already going
   to be running on a router using MP-BGP for VPNs).  While a router may
   be able to scale almost effortlessly with a few thousand routes in a
   single IGP plus hundreds of thousands of routes and many neighbors in
   BGP, it may be quickly challenged if it is also required to run
   multiple instances of an IGP each with a certain number of routes
   that must be moved into MP-BGP to be passed to the rest of the VPN
   infrastructure.  The advent of support for IPv6 within a VPN (6VPE)
   [RFC4659] has the potential to make this problem worse, especially in
   the case of OSPF, where it now requires both OSPFv2 [RFC4577] and v3
   [RFC6565] to run as separate instances for the two address families.

   Another consideration in PE-CE routing protocols is the timers used
   for each session.  These will be discussed in greater detail in the
   best practices section.

2.1.  Best Common Practice

   Ultimately, the decision as to which PE-CE routing protocols to
   support is a business decision much more often than it is a technical
   one, because there are few use cases where something other than BGP
   and static routing as PE-CE routing protocols is a technical
   requirement.  If a provider chooses to support additional protocols,
   especially IGPs, they should consider the effects that these have on
   the overall scaling profile of the PE routers and the network as a
   whole when determining if and to what extent they will support other

   Often, those designing VPN solutions attempt to use extremely
   aggressive routing protocol timer and keepalive values as a means of
   rapid failure detection and reconvergence.  This tends to make PE-CE
   routing protocols more fragile and increase the load on the PE router

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   with questionable benefit.  This is especially common in scenarios
   where the network designer is attempting to replicate native IGP-like
   failure detection and reroute capabilities using BGP.  In order to
   avoid this, the preferred values should be set to something that is
   appropriate for large-scale implementations (*** do we want to make a
   specific recommendation? ***).  Further, because timer and keepalive
   values are often negotiated based on the more aggressive neighbor, it
   is a good idea to set a minimum acceptable value, so that instead of
   being forced to support negotiated timer values that are too
   aggressive for the scale that a given PE router is expected to
   support, the neighbor session will simply stay down until the remote
   end timers are reconfigured to a more acceptable value.  This acts as
   a safety valve against abuse that can destabilize a router used by
   multiple customers.  Because aggressive timers may be unavoidable in
   certain situations, it may be advisable to track the number of
   sessions which are provisioned with aggressive timers vs how many are
   using more conservative timers on a per-router basis, so that effort
   can be made to balance aggressive and conservative timers on each
   router.  This will help to prevent "hot-spots" where given a similar
   port and VRF density, some routers have significantly higher CPU
   usage in steady-state than others.

   It is important to realize that while use of aggressive routing
   protocol timers is not a scalable way to do fast failure detection,
   fast failure detection is still a requirement for many customers.
   Because this is becoming such a table-stakes requirement, the
   provider must consider other alternatives such as Bidirectional
   Forwarding Detection ([RFC5880]), Ethernet OAM 802.1ag [IEEE802.1],
   ITU-T &.1731 [Y.1731] LACP 802.3ad [IEEE802.3] and the like.  These
   extensions often come with their own scaling considerations, but more
   and more they are implemented in a distributed fashion so that
   instead of affecting the main router CPU like a routing protocol
   might, they offload that processing to the linecard CPU, and
   therefore can support more aggressive scale.  The general philosophy
   is that these lower-layer detection mechanisms should serve as the
   primary detection and failure point, with the upper layer routing
   protocols only serving as a backstop if the failure is not detected
   by the lower level protocols for some period of time.

   Another important consideration is that there is not likely to be a
   "one-size-fits-all" solution when it comes to setting timers and
   policies around PE-CE routing protocols.  At a minuimum, a
   distinction should be made between sites that have only a single
   upstream connection and those that have two or more diverse
   connections to the network.  Further distinction can be made based on
   the importance of the site, whether it is a hub site or an end site.
   These can all be used to determine the aggressiveness that is
   appropriate for the timers and perhaps even which routing protocols

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   are appropriate.  For example, an end site with a single upstream
   connection likely does not need very aggressive timers and may be
   able to get by using only static routing, while a hub site with
   multiple connections and a need for rapid restoration and reaction to
   any routing changes may need BGP along with aggressive lower-layer
   timers for fault detection.

2.2.  Common Problems at Scale Limits

   Two common problems when working on a heavily-loaded system:

   CPU cycle constraints, even before the system reaches the point of
   scheduler thrashing often lead to one or more routing protocol
   neighbor hello drops.  If several consecutive drops occur, the remote
   neighbor may declare the session dead, which triggers a restart of
   the connection and a resync of the routing data.  Because this
   connection initialization requires dedicated CPU cycles to generate,
   receive, acknowledge, and process the updates, it increases the CPU
   utilization further, which may trigger additional hello failures and
   neighbor resets, resulting in a snowball effect where a relatively
   minor event rapidly becomes a major one due to interactions between
   multiple scaling limitations.  This problem is made worse by
   extremely aggressive timer values, because they raise the baseline
   CPU load with more frequent hellos and responses, and are more
   sensitive to drops caused by increased CPU load.  Further, because
   failures brought on by loss of hello packets are unlikely to invoke
   any graceful restart [RFC4781] machinery that the system may support,
   it is unlikely that the session reset will be able to take advantage
   of optimizations like only synching the changes that occured while
   the session was dead, thus increasing the outage time and the CPU
   cycles to get things back into sync.

   Another potential issue during times of high-CPU operation is related
   to process prioritization.  This is applicable in different ways for
   both multithreaded and interrupt-driven OS architectures.  In each
   case, the scheduling algorithm that the router uses to prioritize
   different CPU cycle work items and manage the timeslices individual
   tasks are given to complete may require significant tuning and
   prioritization in order to ensure the desired behavior during high
   CPU usage.  Improperly tuned or prioritized processes may
   significantly delay completion of routing table/update processing
   such that it may take an excessive amount of time for the routing
   table to converge properly.  This issue is further exacerbated if the
   VRF instance has a large amount of routes, or is prone to frequent
   event-driven route churn.  In some cases, the routing table in a
   given VRF may never fully converge, leading to routing loops, traffic
   loss, inconsistent latency, and a generally adverse customer

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   It worth noting that these items also have a cascade effect on other
   routers in the system that participate in a given VRF that is being
   affected by this type of scaling issue.  Not only is the local PE
   router affected, but any upstream Route reflectors, as well as other
   PEs, and even CEs participating in this VRF will see increased CPU
   cycles in order to receive and process the increased flow of updates
   driven by the local churn.

   ***specific items related to different PE-CE protocols?***

3.  Multicast

   Multicast support within a VPN [RFC6513] has become an increasingly
   popular feature, but comes with its own scaling considerations.
   Depending on the application, the frequency at which multicast state
   changes within a given VPN (e.g.  PIM joins and prunes) will
   contribute to the CPU load on the router, and any instability in the
   network can potentially increase these as remote sites flap.  In
   extreme cases, PIM neighborships can be lost during events,
   disrupting the flow of multicast traffic.

   It should be noted that, in some cases, dynamic action is required by
   a PE device to support the transition of flooding of multicast data
   from a non-optimal distribution tree (the default MDT in [RFC6037],
   or the I-PMSI) onto a more optimal one (a data MDT or S-PMSI).  Where
   such a transition is required, consideration is required of the
   nature of the traffic sourced by an end user of the L3VPN service.
   The net result of this consideration is that it becomes increasingly
   difficult to reliably gauge the scaling impact of specific end-site
   deployments.  Additional scaling considerations around Multicast in a
   VPN are related to the size and number of multicast streams.  While
   this is a consideration whenever Multicast is used even outside of a
   VPN because of the bandwidth utilization it may generate in the core,
   the additional overhead of implementing multicast within a VPN makes
   this a more siginificant consideration in this case.  Related to the
   previous consideration is the stream fanout - the amount of P and PE
   router paths in the network that could potentially carry a given
   multicast stream based on the number of PEs that are configured with
   a given Multicast-enabled VRF, and the number that actually do carry
   the stream based on actual receivers joining the stream behind that

   *** This section is quite weak.  We're looking for contributors who
   can assist in fleshing this out ***

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3.1.  Best Common Practices

   Multicast BCPs???

3.2.  Common Problems at Scale Limits

   Multicast tree interruptions

   PIM neighbor adjacency drops

4.  Network Events

   Network events are an important scaling consideration because they
   can have wide-ranging impacts far beyond the individual VRF or even
   PE router that experiences the event.  At high scale, a seemingly
   innocuous event on one router or VRF can trigger secondary impacts
   and outages on remote routers elsewhere in the network.  Correlating
   these events for root cause analysis can be challenging by itself,
   and trying to characterize the impacts as they relate to scale in a
   way that informs the provider's decisions is even more difficult.
   Different types of Network Events that can contribute are: Interface
   flaps, hardware and software outages (both planned and unplanned),
   externally driven route-churn events (such as those that originate on
   an NNI partner's network) and configuration changes.

4.1.  Best Common Practices

   While this document suggests that lower layer failure detection
   protocols like BFD and Ethernet OAM be more aggressive so that
   routing protocol timers can be more conservative, it is still
   important to remember that this can generate false positives or
   excessive churn that will cascade into a scaling problem at other
   parts of the system, so the timers should not automatically be
   configured to their minimum supported values.  Rather, each
   application may be slightly different, and the timers should only be
   set as aggressively as necessary to ensure acceptable performance of
   the applications in question.  It may be appropriate to set limits
   (e.g. in provisioning logic/rules) as to the number of interfaces per
   router and per VRF that can use aggressive, moderate, and
   conservative interface timers.

   Even with timers set as conservatively as the application will allow,
   churn is unavoidable.  For this reason, it is also a good idea to use
   interface-level dampening such as hold-down timers or event dampening
   in order to ensure that interfaces that flap too rapidly will not
   telegraph that churn into the upper-layer routing protocols any more
   than necessary.  BGP Peer Oscillation Dampening

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   (DampPeerOscillations, RFC4271 [RFC4271] ) may also help to reduce
   intermittent outage-based churn while leaving the interface itself
   unaffected.  All of these dampening measures help to ensure that
   problems are localized to a single PE or even a single interface,
   rather than causing instability and routing churn throughout the VRF
   and the provider network.

   In addition to interface dampening, it may be advisable to consider
   implementing some manner of route flap dampening to assist in
   reducing the impact that route churn may have on the SP's network
   infrastructure.  This is currently fairly uncommon within VPN
   environments, and is not without controversy.  While it may help with
   scaling, it also requires each PE to maintain more state to store and
   compute the per-prefix penalty values, which may reduce the benefits
   gained by implementing RFD.  Further, customers typically expect a
   fair amount of transparency in the provider's participation in their
   routing instances.  Many providers and customers view a VPN or VRF as
   a part of the customer's internal network and therefore
   compartmentalized so that the customer can only affect their own
   routing if they have a problem with excessive route flaps.  Further,
   if routes are dampened it requires intervention from the SP to clear
   the dampening, which can potentially add to the outage time that a
   customer experiences once the issue that triggered the dampening is
   resolved.  Implementing RFD may even drive the need for a customer-
   accessible looking glass, which is far more complex in the VPN space
   owing to the requirement to prevent one customer from looking at
   another's VRF routes on a common platform.

4.2.  Common Problems at Scale Limits

   Network events are both a cause and a symptom of a system running at
   or near its scaling limits.  As noted above, event-driven routing
   table churn or routing protocol interactions can significantly drive
   up CPU usage on the locally connected PE as well as on other PEs and
   CEs participating in the VRF.  If routes are constantly changing due
   to a preferred path repeatedly being added and removed, latency and
   jitter numbers can be affected in a way that adversely effects
   applications sensitive to this sort of change.  Network events can
   also be triggered by routers with high CPU, because similarly to
   systems which may have aggressive routing protocol timers for
   enhanced failure detection, systems with centralized CPU-based
   implementations for lower-layer protocols (such as HDLC [ISO13239]
   PPP [RFC1661], LACP, BFD/EOAM) may start losing keepalives and
   declaring outages that result in physical interfaces being torn down
   and restored.  Again, implementations that choose timer and
   multiplier values or numbers of sessions at or near the maximum rated
   scaling for the device put the operator in a position where there is
   very little headroom to deal with an event that momentarily spikes

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   CPU usage, meaning that the likelihood of a cascade failure
   dramatically increases.

   As above, these network events may be something that occurs elsewhere
   in the network, and may trigger a failure on a completely different
   PE or CE router.  The danger with this is that it is extremely
   difficult to troubleshoot and correlate root causes when the outage
   observed isn't caused by an event on the same router.  Failures
   become increasingly non-deterministic and difficult for operators to
   manage and address.

5.  General Route Scale

   PE routers in a carrier network can have many different
   implementation scenarios.  Some carriers implement a dedicated PE
   router that is only responsible for carrying VPN routes and therefore
   may only carry IGP routes in its global routing table, rather than a
   full internet routing table.  Others use combined edge routers that
   carry full routes plus a complement of customer VPN routes, and some
   even place the full internet routing table into one or more VRF
   instances.  The issue here is that the weight of all of these routes
   and paths must be combined when considering the maximum scale of the
   router, both in terms of memory footprint and in terms of convergence
   times.  The addition of an 8-byte RD appended to the IP address to
   ensure uniqueness means that each VPN prefix takes up incrementally
   more physical space in memory than an equivalent non-VPN route.
   Further, the greater number of Address-families running
   simultaneously on the same router, the more sensitive it will be to
   event-induced churn since each address-family (and VRF) often has its
   own independent computation/SPF run.  The addition of IPv6 support
   within both the global routing table and within a VPN adds yet
   another source for routing table bloat.  A PE router can be running a
   combination of any of the following address-families:

   o  Global IPv4 unicast

   o  Global IPv4 multicast

   o  VPN IPv4 unicast

   o  VPN IPv4 multicast

   o  Global IPv6 unicast

   o  Global IPv6 multicast

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   o  VPN IPv6 unicast

   o  VPN IPv6 multicast

   Even PE routers that do not carry the full internet routing table are
   still required to carry a minimal number of IGP routes, LDP
   information, and some amount of TE tunnel state, adding to the items
   competing for scale.  On high-scale PE routers, the VPN routing
   tables are often as large as or larger than the equivalent global
   routing table in both number of routes and number of paths.  This is
   at least partially due to the fact that there are no constraints on
   the customer addressing plan within a VPN other than they cannot
   conflict within a given VRF, or with any extranet with which the VRF
   interconnects.  As such, they may not necessarily adhere to any best
   practices to control the deaggregation of the routing table such as
   hierarchical addressing, aggregation and summarization of
   announcements, and minimum prefix lengths.  It's also quite likely
   that connected interfaces will be redistributed, and little or no
   route filtering may take place.  Most PE routers use the absence of a
   given VRF instance (or RD/RT filtering) to limit the number of routes
   that they must actually carry, but this is sometimes of limited
   utility for a couple of reasons.  First, it leads to an inconsistent
   routing table footprint from one PE router to the next, and it can
   change with every new customer turned up on the router.  These lead
   to non-deterministic performance and scale from PE to PE and from
   customer to customer.  In other words, PE1 may be fine from a scale
   perspective, while PE2, which has the same number of occupied ports
   has significant scaling problems on account of which VRFs are
   present/absent.  Then, PE1 may find itself suddenly having the same
   scaling concerns because a new customer was provisioned with a large
   or high-churn VRF that was previously not present on the router.
   Second, many customer VPNs are so large and have such stringent
   diversity requirements that they have a presence on nearly every PE
   router in a provider's network, meaning that one cannot rely heavily
   on statistical distribution to reduce the percentage of VRFs that
   must be installed on a specific PE router.  In addition, customers
   may request the use of BGP multipath for faster failover or better
   load balancing, which has the net effect of installing more active
   routes into the table, rather than simply selecting the single best
   path.  The scaling considerations for enabling BGP Multipath are not
   unique to L3VPN, but they are more pertinent here because SPs are
   less likely to be willing to enable MP for standard internet traffic,
   while they will do it for L3VPN.  The application as an enterprise
   network instead of internet connectivity drives a different set of
   expectations about the performance of the network, design tradeoffs
   that must be made to meet the SP's requirements, etc.  In many cases,
   L3VPNs are replacing old point-to-point networks or L2VPNs using
   legacy Frame Relay, ATM, or L2TPv3.  Customers often don't want to

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   make major architectural changes to their routing, and therefore
   expect the SP to do the same things that they were doing between
   their routers before, including things like multipath.

   In addition to such intended behaviour, within many L3VPN networks, a
   balance must be struck between complexity in OSS such as provisioning
   and inventory systems, and complexity in network deployments.  One
   such example of this is the assignment of route distinguisher (RD)
   attributes.  Where it may be possible to assign a single RD per L3VPN
   instance, and hence achieve some level of route aggregation for
   multi-homed CE routes on BGP speakers within the solution, this has
   some consequences for both convergence in the VPN (due to BGP
   convergence being relied upon) and in its potential to exacerbate
   geographic distance between PE and Route-reflector and is therefore
   undesirable in some circumstances.  In order to avoid this, multiple
   RDs are then required, which requires OSS and inventory support to
   control the namespace.  As such, due to this requirement, often each
   VRF instance is deployed with a specific RD - which, whilst achieving
   the desired convergence effect, places load on all BGP control-plane
   elements of the provider network.

   Total supportable route scale on a given PE router will be driven by
   multiple different variables, which have a roughly inverse
   relationship to one another: Number of VRFs per router, number of
   routes per VRF, number of neighbors per VRF.  For example, a router
   can support a low number of VRFs per router if each VRF has a large
   number of routes per VRF and/or a large number of neighbors per VRF.
   Conversely, a router can support a relatively high number of VRFs if
   each VRF is kept to a much lower number of routes per VRF, and/or
   lower numbers of neighbors per VRF.  This provides a baseline that
   then must be reduced based on the expected level of event-driven
   churn, the type of protocol chosen, etc.  In short, this is a
   difficult problem from a modeling and capacity planning perspective.

   It is fairly common for the contract or Service Level Agreement
   between SP and customer to include a maximum limit as to how many
   routes can be carried in a VRF.  At its most basic, this maximum can
   be used as a method to estimate the number of VRFs that can be
   present on a PE given its scaling limitations.  However, there is a
   wide gulf between a contractual limitation of no more than N routes
   per VRF with a corresponding configured limit and the fact that many
   customers will not carry nearly that many routes.  This leads to the
   potential for significant stranded capacity.  Therefore the provider
   needs a way to have different tiers of "maximum routes allowed" so
   that the capacity management can be done in such a way as to enable
   better loading of PE routers to take this relationship into account
   (e.g. populating a PE with a combination of high-scale and low-scale
   VRFs).  The alternative to this method would be to assume a standard

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   maximum routes per VRF, and then similarly to the way that carriers
   use statistical multiplexing and oversubscription to assume that not
   all customers will have their pipes full of bandwidth at the same
   time, make some assumptions about control plane capacity.  This may
   come in the form of an average that is calculated based on the actual
   size of the routes in each VRF.  This has many challenges.  Among
   them- Should it be calculated per-PE?  Network-wide?  What happens
   when there are too many VRFs that exceed the average on a given PE?
   How does one add control plane capacity to a "full" router?  This may
   be a manageable model in a network with a robust and flexible
   provisioning system, such that high-scale VRFs can be moved between
   PE routers to balance the load, but each of these moves likely
   represents an outage for the customer and the potential for other
   errors to creep in, and is not likely to be attractive due to the
   operational costs of managing the network.  In other words, it
   doesn't scale, but for a completely different reason.  Further, this
   VRF route limit may or may not be a physically enforced value.  Some
   PEs have an additional configuraiton knob per VRF that places a hard
   limit on the number of routes the VRF will accept.  This works well
   as a last-chance safety valve to protect the PE and the network in
   the case where there are misconfigurations in the VRF that cause a
   sudden and significant increase in the number of routes, but can
   create inconsistencies in the VRF's routing table if there is a
   periodic or intermittent increase in the routes that causes the
   maximum to be periodically exceeded.  Unlike something like a BGP
   maximum prefix limit, which shuts down the BGP neighbor when a
   threshold is exceeded, there is no direct feedback to the peers that
   the VRF route limit threshold is exceeded, and different
   implementations handle this in different ways in terms of how they
   drop or buffer routes, and how they resynch once the routes are below
   the threshold again.  It may be appropriate to identify a common way
   for implementations to handle this limit, perhaps triggering one or
   more PE-CE peering sessions to drop, etc. so that this is a more
   useful tool to protect the PE from increases that would cause it to
   have scaling problems.

5.1.  Route-reflection and scaling

   Most of this document focuses on scaling at the PE router, but a
   discussion of route scaling would not be complete without at least a
   cursory mention of route-reflection [RFC4456].  While using route-
   reflectors to eliminate the need for a full mesh of your PE routers
   is a common optimization, there are many different deployment models
   as far as whether dedicated route-reflectors are deployed vs. running
   an existing PE or P router as a route-reflector, how many are
   deployed and where, the method for ensuring diversity and redundancy,
   and even whether a router is used vs. a commodity PC running some
   sort of routing daemon.  From a scaling perspective, there are

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   several considerations that are unique to the route-reflector design
   that will be discussed here.

   Starting with the route-reflector itself, these devices are often
   experiencing a worst-case scenario when it comes to storing entries
   in the RIB, exposure to route-churn, etc.  This is because they are
   not capable of filtering the routes from VRFs not locally configured
   on themselves, and they must carry all of the routes for all of the
   VRFs in the ASN.  This requires significant amounts of CPU and memory
   to store and manage these updates, and an underpowered route-
   reflector can quickly cause widespread convergence problems if it is
   unable to keep up with the load of receiving, processing, and
   propagating these updates.  Beyond CPU and memory, it may also be
   necessary to know how the router manages its FIB when running as a
   route-reflector.  A route-reflector is almost 100% control-plane, but
   if it tries to install all of the routes that it has in its RIB into
   the FIB, it may require very high-scale (and therefore costly)
   forwarding hardware to manage the large FIB.  It may be useful to
   select a device that is capable of optimizing for this control-plane
   only mode and suppressing unnecessary routes from its FIB to reduce
   the overhead.  This is why some providers choose to use commodity
   PCs, which are well-suited for high-scale, processor and memory-
   intensive control plane work, and can easily and cost-effectively be
   horizontally scaled.  The main consideration with using a PC instead
   of a router for route-reflection is that there may be implementation
   differences that lead to incompatibililties in terms of supported
   features, and there may be a different model in terms of how high-
   scale applications are managed, or even what bugs are exposed at
   maximum scale, all of which will require significant additional

   Route-reflector placement is another important consideration.
   Because route-reflectors are control-plane devices, and the scale
   requirements for them are high enough that they can be expensive, the
   tendency might be to deploy two large geographically-diverse and
   horizontally scaled sets of them in order to provide an acceptable
   amount of diversity while deploying the fewest possible devices.
   However, this leads to potential problems with the geographic
   distance between the PE and the route-reflector leading to geographic
   "routing artifacts".  (Geographic routing artifacts in this case is
   referring to the phenomenon where the PE and the route-reflector are
   significantly distant from one another in the network, and the route-
   reflector chooses one or more best paths based on its view of the
   IGP, and then reflects those to its neighbors, even though there may
   be a better path at a given PE based on its location in the network
   and its view of IGP.  Also, propagation delay and the latency it
   induces for updates and convergence may be a factor.)  Use of a small
   number of route-reflectors network-wide also can result in scaling

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   problems based on the number of BGP sessions a given route-reflector
   must maintain.  Both of these items point to a larger deployment of
   smaller, more geographically diverse route-reflectors throughout the
   network, so that a given route-reflector is maintaining fewer BGP
   sessions with PE routers, has an IGP view of the network that is
   closer to that of the PEs it peers with, and can rapidly propagate
   local updates to the surrounding PEs.

   The number of route-reflectors peering with each PE is a scaling
   consideration as well.  While a minimum of two discrete route-
   reflector BGP sessions is the minimum to ensure proper redundancy,
   adding additional route-reflectors requires each PE to carrry the
   additional state of those sessions, adding significant overhead with
   questionable value.

   Related to route-reflector placement and the number of PE to route-
   reflector peering sessions is the use of cluster-IDs within a set of
   route-reflectors.  Cluster-IDs can be effectively used to reduce the
   amount of duplicate route updates propagated between route-
   reflectors, thus reducing some of the same state and churn impact
   that is so critical in high-scale implementations.  However, it can
   have unintend side effects.  In order to prevent inconsistency in the
   routing table, a PE MUST peer with all of the route-reflectors in a
   given cluster.  As a result, depending on how route-reflectors are
   spread out throughout the network and clustered together, it may
   create the need for a PE to either peer with multiple clusters, or to
   peer with one or more route-reflectors that are not optimal in terms
   of geographic placement in relation to the PE.  For example, if each
   cluster has two route-reflectors for redundancy, and there are three
   regional clusters (East, Central, West), PEs that sit in the overlap
   area between two cluster "regions" may have to peer with one or more
   route-reflectors that are farther away, lest they have to now peer
   with four route-reflectors in order to peer with the two closest to

5.2.  Best Common Practices

   A number of things can be done to improve the general route scaling.
   Most BGP sessions can be configured with a similar set of protections
   as they would be if they were global Internet eBGP sessions, such as
   maximum prefix limits, inbound and outbound prefix filtering, etc.
   Prefix filtering is less common within VPNs because it is treated
   more like iBGP, where filtering is typically not recommended
   (***reference?***), or as noted above, it's part of the customer's
   network and therefore not the SP's business/problem to do filtering
   in an application that can only break that customer's network.  What
   is often more important in the case of individual VRFs is to
   configure an acceptable maximum number of routes that the VRF is

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   permitted to carry.  This allows the SP to control their exposure to
   sudden increases in the memory footprint of the routing table,
   especially if a misconfiguration on the CE side leads to significant
   amounts of route leakage, such as to suddenly leak a significant
   amount of the Global Internet Routing Table into their VRF.  However,
   it can also be used to enforce the assumptions on number of routes
   per VRF that the SP has used to determine what the other max scaling
   values such as number of VRFs per router, number of sessions per
   router, etc.

   As noted above, the number of VRFs per router, number of routes per
   VRF, and number of sessions per router and per VRF are all inter-
   related values in the way that they contribute to overall router
   scale.  The more of this information is known in advance based on the
   design of the customer's network, the more it can be used as input to
   the provisioning system to determine the best available PE router on
   which to terminate the connections for consistent loading.  Since
   these values are usually estimates, and considerations like diverse
   router terminations may drive a specific choice, this is not by any
   means fool-proof, but is a valuable optimization to improve the
   density of customers on a given router and maximize the return on
   investment for the capacity deployed.  It is worth noting, however,
   that many SP VPN networks have a different geographic spread than do
   their Internet service counterparts, where there will be more POPs
   with fewer routers, as it is important to provide more local handoffs
   to customers.  This may limit the SP's flexibility in terms of homing
   locations and router choices, and thus may be of limited value when
   controlling scale impacts on individual PE routers.

   *** Discuss incremental SPF, next-hop tracking, SPF timer tuning (By
   protocol and AF), prefix prioritization, etc?  All of these are
   generally thought of as convergence optimizations, and may be
   applicable here as a way to both reduce the CPU load and ensure that
   behavior is more deterministic, but I'm not sure how much depth we
   want to get into here, especially since some are vendor-specific or
   FIB-specific optimizations... ***

5.2.1.  Topology-related optimizations

   As has been discussed above, the topology of a given VPN and its
   placement on the available PE routers can be a significant
   contributing factor to the impacts of that VPN on the scaling limits
   of a given PE.  For example, a hub and spoke arrangement allows for
   some amount of aggregation and route summarization to be used, but
   there are limitations to its effectiveness at minimizing routing
   table growth since this is typically implemented by the end customer,
   and is dependent on how hierarchical their topology and IP addressing
   plan is.  While there are plenty of other good reasons to use a hub

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   and spoke design, including security (traffic separation) between
   spoke sites, etc., generally, a customer does not have much incentive
   to expend the time and effort to maintain a proper hierarchy or deal
   with the added complexity of a hub and spoke design if the only
   benefit is to improve route scaling.  A possible solution for some
   full-mesh topologies is to use Virtual Hub-and-Spoke in BGP/MPLS VPNs
   [I-D.ietf-l3vpn-virtual-hub].  From the abstract:

   "With BGP/MPLS VPNs, any-to-any connectivity among sites of a given
   Virtual Private Network would require each Provider Edge router that
   has one or more of these sites connected to it to hold all the routes
   of that Virtual Private Network.  The approach described in this
   document allows to reduce the number of Provider Edge routers that
   have to maintain all these routes by requiring only a subset of these
   routers to maintain all these routes."

   The value of this approach is that it is much less dependent on the
   individual customer to implement a hierarchy in order to conserve
   routing table entries.  The potential downside to this approach is
   that it requires additional provisioning and troubleshooting
   complexity due to the way that routes are/are not imported, the use
   of default/summary routes, etc.  This approach also potentially
   exacerbates the problem discussed above where PE's are inconsistently
   loaded (in terms of total number of routes) from one PE to the next
   and the potential provisioning difficulty that comes from a desire to
   find and use as much spare control plane capacity as possible without
   overloading a given PE.

5.3.  Common problems at scale limits

   As mentioned above, systems that are carrying a large number of VRFs
   and/or VRFs with large numbers of routes tend to be more sensitive
   during events due to the increased amount of periodic and event-
   driven processing that must be done to complete a walk of the routing
   table to process updates.  While optimization techniques may reduce
   the overhead of (re)programming the FIB after an update, there are
   less tricks to be employed in managing the RIB, and they are often
   vendor-specific, which leads to a lowest-common-denominator threshold
   in multivendor environments.

   In addition to CPU constraints, it's common for route memory
   footprint to be a consideration if there are large numbers of VRFs
   with large numbers of routes.  Similarly to the way that high scale
   reduces the cushion of available CPU resources to absorb temporary
   peaks, as memory use reaches its high threshold, allocation of the
   remaining memory becomes less efficient and more fragmented, such
   that memory allocations may begin to fail well before the available
   memory is actually exhausted.  Depending on the specific

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   implementation, the "largest free" may be more important than the
   "total free" and it may be difficult or impossible to coalesce the
   free memory to reduce fragmentation to an acceptable level.  As with
   other scaling problems, a failure of this type has the nasty habit of
   causing a cascade of problems.  Depending on how robust the system is
   at recovering from memory allocation failures, it may trigger
   restarts of critical routing processes or even the entire system.
   These may or may not be graceful and hitless, and even if they are
   locally a fairly low impact, these may trigger events on other
   routers due to the ripple effect of the network event itself.  It is
   also worth noting that there are hardware and software limits to how
   much memory a given system can use - if the router in question does
   not use a 64-bit OS, then it is unable to address more than 4GB of
   RAM, for example.  This may make an otherwise robust system incapable
   of scaling to the necessary level, and make memory usage an even more
   significant consideration.

6.  Known issues and gaps

6.1.  PE-CE routing protocols

   While support for route flap dampening in BGP as a PE-CE routing
   protocol is equivalent to its support in non-VPN applications, the
   addition of IGP routing protocols such as OSPF creates a new problem,
   in that there is not really a way to manage route dampening, either
   by configuring it within the context of the IGP itself, or by
   configuring it in the translation point where the IGP's routing
   information is moved into the MP-BGP control plane infrastructure to
   be exchanged between participating PEs across the VPN network.  This
   means that in the case where IGPs are used, which is often more CPU-
   intensive and performance-conscious to start with, the route flaps
   associated with an unstable network will make a bad problem even
   worse.  It may be advisable for the IETF to document updates to
   standards managing use of IGPs as PE-CE routing protocols to
   explicitly define the use of RFD in this application.

   There are also not clear guidelines based on testing and real-world
   experience for recommended timer values or appropriate use cases for
   an IGP vs BGP as a PE-CE routing protocol.  In other words, rather
   than enterprises simply defaulting to whatever IGP is already in use
   or they are most comfortable with, there may be certain cases where
   use of an IGP is recommended, and those where it is not.  Guidance in
   this area may be very useful to both the SPs supporting these
   networks and the engineers designing the corporate networks that make
   use of them.

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6.2.  Multicast

   Issues in multicast VPN scale?

6.3.  Network Events

   Guidance on interface event dampening values (research and testing),
   correlation tools to help determine root cause in a cascade failure,

6.4.  General Route Scale

   Route flap dampening may potentially be a best practice, but it has a
   number of shortcomings.  First, there is no systematic way for end
   customers to view and clear dampening without some sort of advanced-
   functionality looking glass that allows them to view only the routes
   in their authorized VRFs.  Also, allowing customers to make
   unattended clears of dampened routes may defeat the purpose of having
   dampening enabled at all, since customers may clear the dampening
   without addressing the underlying cause of the problem.  In addition,
   as noted in [I-D.ietf-idr-rfd-usable] and
   [I-D.shishio-grow-isp-rfd-implement-survey] , Route flap Dampening is
   not widely used even within the Global Internet routing table, and
   its values probably need to be tweaked.  Due to the differences in
   the characteristics of VPN routes compared with the global routing
   table, additional study and recommendations as to appropriate RFD
   values within a VPN are likely required.  Additionally, it is not
   possible to configure RFD on IGPs, either natively within the PE-CE
   routing protocol or upstream where the learned routes are carried in
   MP-BGP.  This means that in some cases, there is no way to insulate
   the SP network from the adverse impacts of rapid route churn.

6.5.  Modeling and Capacity planning

   There is a significant lack of multidimensional scale guidance and
   modeling for capacity planning and troubleshooting large-scale VPN
   deployments.  This has a number of contributing factors.  First,
   behavior at scale becomes increasingly non-deterministic the more
   variables you're working with simultaneously, so this is classically
   a difficult problem to model.  Even worse, it's difficult to account
   in a model for latent design/implementation flaws: things that work
   well enough at moderate scale, but are not efficient enough for high
   scale, or suffer some sort of secondary impact due to dependencies,
   race conditions, etc.  These problems are often only found through
   extensive testing or even escape into production.  Second, it is
   difficult to characterize an "average" implementation in such a way
   that it can be tested to failure in mulitple permutations to provide
   a reasonably accurate multidimensional model.  Consequently, the
   guidance available normally takes the form of multiple uni-

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   dimensional scale thresholds plus some very conservative multi-
   dimensional thresholds.  These conservative recommendations avoid
   risk to both the vendor and the implementer by catering to the lowest
   common denominator, but they have the adverse effect of leaving a lot
   of capacity sitting idle.  Some vendors make an effort to
   characterize their customers' large scale implementations such that
   they can better replicate real-world conditions, but gathering this
   information and devising ways to replicate the behavior in a lab is
   problematic and time-consuming.

   This leads to a follow-on issue, which is that there is a lack of
   instrumentation on critical scaling vectors.  Some routers have very
   limited abilities to provide useful data about critical scaling
   vectors (routing updates per second, changes in multicast state,
   sources of internal bottlenecks, etc), either for use in a model or
   for use as additional capacity monitoring thresholds.  While most
   routers can provide information about CPU usage and memory
   thresholds, and even which processes are consuming large amounts of
   resources, it often takes special instrumented versions of the OS to
   provide a window into what is actually causing some sort of failure
   at scale.  Because these are not routinely monitored, it means that
   the provider may be blind to one or more early warning signs that the
   router is nearing its scaling limits and cannot take action to
   prevent exceeding those limits before it causes customer impacts.

   Additionally, even if this information is available, the provisioning
   systems used by most providers do not currently have the intelligence
   or visibility to make a decision regarding which PE to provision new
   customers on to evenly load the available PE routers.  The
   provisioning system is often aware of the available physical or
   logical port capacity on a given router or site, and uses this as a
   key input to its port choice for newly provisioned customers.
   However, these additional capacity and scale vectors are based on
   real-time statistics from the router (CPU, memory load, etc) and
   there is no interaction or feedback loop between the provisioning
   system and these types of real-time router scale stats.  As a result,
   manual intervention is often required to either remove busy routers
   from the available capacity pool, move spare port capacity from a
   busy router to a full one, or even to reprovision customers to move
   them from one device to another to rebalance the load on each router.

6.6.  Performance issues

   In many ways, it's difficult to define a hard-and-fast scale limit,
   because each provider and customer have a differing view on what is
   an acceptable performance envelope both in steady state and during
   recovery from outages, whether planned or unplanned.  In the most
   extreme sorts of network events, such as a heavily loaded PE router

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   undergoing a cold restart, the scale considerations may take
   something like boot time and convergence from what the involved
   parties consider acceptable and extend them to the point where they
   significantly prolong the pain that to which an end customer is
   exposed.  They often have the added problem of making it difficult to
   predict the duration of an outage, because individual customer VRFs
   may be affected for differing amounts of time based on all of the
   factors that contribute to scaling.  For example, if a customer has
   one critical route that happens to be among the last to converge,
   they perceive the outage to be ongoing until that last route
   converges, even if the entire rest of their network has been
   functional for a significant amount of time prior to that point.

   When dealing with scheduled outages, customers obviously prefer that
   they never are impacted.  Since this is not really possible, they
   expect the provider to give them very clear and accurate guidance on
   what the impacts will be, when they will occur, and for what
   duration, so that they can set expectations for their customers.
   VPNs are often carrying mission-critical services and data, so any
   downtime is bad downtime.  While a customer may be understanding of a
   scheduled maintenance with a 15-30 minute traffic interruption while
   a router reloads, they may be less so if the outage actually
   stretches for 60-90 minutes while the router runs at 100% CPU trying
   to deal with this worst-case sort of load or suffers intermittent
   cascade problems while any remaining cushion is used up dealing with
   the results of the event.  These impacts may be largely invisible to
   the provider unless they have probes within each VRF or other means
   to verify that traffic is no longer impacted for a given customer.
   It's often difficult or impossible for a provider to tell the
   difference between a router that is fully converged but running near
   100% CPU after a reload from one that is thrashing and causing delays
   in convergence and customer traffic impacts while it runs at 100% CPU
   after a reload.  Even worse, a scheduled or known outage on one
   router may trigger unplanned outages on other high-CPU devices.  Even
   in unplanned outages, communication regarding impacts and duration is
   key, and these sorts of scale issues make it difficult to predict the

6.7.  High Availability and Network Resiliency

   In many cases, L3VPN services are carrying significant amounts of
   business-critical data.  Customers and carriers design their networks
   to be robust enough to absorb single and sometimes even dual faults
   with little or no impact to the network as a whole.  However, the
   expectations as to the frequency and duration of outages due to
   either scheduled or unscheduled events continue to go higher and
   higher.  This is leading more providers to adopt features such as
   Non-Stop Forwarding and Non-Stop Routing, as well as things like In-

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   Service Software Upgrades to improve the chances that outages will be
   transparent to the underlying customers, networks, and applications
   using the network elements.  As these become more common within the
   L3VPN space, they must be considered when evaluating PE scale.
   Often, the machinery necessary to make these reliability enhancements
   work requires duplication and sharing of state between multiple
   elements.  At its most basic level, this state sharing takes more
   resources and more time the more state there is to be shared, so
   increases in the different scaling vectors discussed in this document
   will cause proportional increases in the complexity and resource
   requirements necessary for the combined feature set.  In more complex
   scenarios and implementations, it may contribute to the complexity
   associated with capacity planning, and may make the response even
   more non-deterministic as scale increases.

6.8.  New methods of horizontal scaling

   When this document was being written, there was considerable
   discussion around the area of Software Defined Networking and
   Openflow[ONF].  These are technologies which provide a way to offload
   some of the more complex control plane elements to a more central
   controller device, which then programs the routing elements for
   correct forwarding plane operation.  This is interesting in solving a
   problem such as described in this document because it effectively
   decouples the growth of the control plane from the growth of the
   forwarding plane.  In other words, it would be possible to continue
   allocating more and more CPU resources to the high-overhead control
   plane elements discussed above, and keep it almost totally
   independent from the physical forwarding plane resources necessary.
   While in some ways this would simple move the need for horizontal
   scaling elsewhere, rather than actually reducing the scaling
   considerations, the benefit is that an SP could use commodity compute
   hardware, which would potentially be a lower cost and more easily
   scaled than your average PE router's CPU.  The application of SDN/
   Openflow or any other interface to the routing system that offloads
   some control plane elements for improved BGP VPN scale is beyond the
   scope of this document, but may be a valid use case for future
   discussion within the IETF.

7.  To-Do list

   RFC EDITOR: Please remove this section before publication.

   Still not discussed in the document:

   Inter-AS VPN NNI scaling considerations (separate discussions on 10A,
   10B/hybrid, 10C?) - include discussion on number of VRFs per NNI,

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   routes per VRF, NNIs per router

   Label Exhaustion

   BGP Fast External Fallover

   additional scaling considerations if using L2TPv3 or RSVP-TE
   tunneling for PE-PE transport

   Future scaling considerations (MPLS-TP at the edge, interworking with
   L2 technologies, significant increases in density, etc)

8.  Acknowledgements

   The idea for this draft came from a presentation made by Ning So
   during the CDNI working group meeting at IETF 81 in Quebec City where
   some of these same scaling considerations are discussed.  Thanks also
   to Yakov Rekhter, Luay Jalil, Jeff Loughridge, Stephane Litkowski,
   Rajiv Asati, and Daniel Cohn for their reviews and comments.

9.  IANA Considerations

   This draft makes no request to IANA..

10.  Security Considerations

   Security considerations for IP VPNs are covered in the protocol
   definitions.  This draft does not introduce any new security
   considerations, but it is worth noting that attack vectors that
   result in minor impacts in a low-scale environment may make the
   problems observed in a high-scale or resource-constrained environment
   worse, thereby magnifying the potential for impacts.

11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

11.2.  Informative References

   [EIGRP]    Wikipedia.org, "Enhanced Interior Gateway Routing
              Protocol", <http://en.wikipedia.org/wiki/

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              Pelsser, C., Bush, R., Patel, K., Mohapatra, P., and O.
              Maennel, "Making Route Flap Damping Usable",
              draft-ietf-idr-rfd-usable-01 (work in progress),
              December 2012.

              Jeng, H., Uttaro, J., Jalil, L., Decraene, B., Rekhter,
              Y., and R. Aggarwal, "Virtual Hub-and-Spoke in BGP/MPLS
              VPNs", draft-ietf-l3vpn-virtual-hub-04 (work in progress),
              December 2012.

              Tsuchiya, S., Kawamura, S., Bush, R., and C. Pelsser,
              "Route Flap Damping Deployment Status Survey",
              draft-shishio-grow-isp-rfd-implement-survey-05 (work in
              progress), June 2012.

              IEEE, "Connectivity Fault Management", <http://

              IEEE, "Carrier Sense Multiple Access with Collision
              Detection (CSMA/CD) Access Method and Physical Layer

              ISO, "High-level Data Link Control protocol", <http://

   [ONF]      ONF, "The Open Networking Foundation",

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route

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              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC4577]  Rosen, E., Psenak, P., and P. Pillay-Esnault, "OSPF as the
              Provider/Customer Edge Protocol for BGP/MPLS IP Virtual
              Private Networks (VPNs)", RFC 4577, June 2006.

   [RFC4659]  De Clercq, J., Ooms, D., Carugi, M., and F. Le Faucheur,
              "BGP-MPLS IP Virtual Private Network (VPN) Extension for
              IPv6 VPN", RFC 4659, September 2006.

   [RFC4781]  Rekhter, Y. and R. Aggarwal, "Graceful Restart Mechanism
              for BGP with MPLS", RFC 4781, January 2007.

   [RFC4984]  Meyer, D., Zhang, L., and K. Fall, "Report from the IAB
              Workshop on Routing and Addressing", RFC 4984,
              September 2007.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC6037]  Rosen, E., Cai, Y., and IJ. Wijnands, "Cisco Systems'
              Solution for Multicast in BGP/MPLS IP VPNs", RFC 6037,
              October 2010.

   [RFC6513]  Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
              VPNs", RFC 6513, February 2012.

   [RFC6565]  Pillay-Esnault, P., Moyer, P., Doyle, J., Ertekin, E., and
              M. Lundberg, "OSPFv3 as a Provider Edge to Customer Edge
              (PE-CE) Routing Protocol", RFC 6565, June 2012.

   [Y.1731]   ITU-T, "OAM functions and mechanisms for Ethernet based
              networks", <http://www.itu.int/rec/T-REC-Y.1731/en>.

Authors' Addresses

   Wesley George
   Time Warner Cable
   13820 Sunrise Valley Drive
   Herndon, VA  20171

   Phone: +1 703-561-2540
   Email: wesley.george@twcable.com

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   Rob Shakir

   Phone: +
   Email: rob.shakir@bt.com

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