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Versions: 00 01 02 03 draft-ietf-idr-as-migration

Internet Engineering Task Force                                W. George
Internet-Draft                                         Time Warner Cable
Intended status: Informational                                 S. Amante
Expires: July 10, 2014                                       Apple, Inc.
                                                         January 6, 2014


 Autonomous System (AS) Migration Features and Their Effects on the BGP
                           AS_PATH Attribute
                      draft-ga-idr-as-migration-03

Abstract

   This draft discusses common methods of managing an ASN migration
   using some BGP feaures that while commonly-used are not formally part
   of the BGP4 protocol specification and may be vendor-specific in
   exact implementation.  It is necessary to document these de facto
   standards to ensure that they are properly supported in future BGP
   protocol work such as BGPSec.

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 July 10, 2014.

Copyright Notice

   Copyright (c) 2014 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



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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Documentation note  . . . . . . . . . . . . . . . . . . .   3
   2.  ASN Migration Scenario Overview . . . . . . . . . . . . . . .   3
   3.  External BGP Autonomous System Migration Features . . . . . .   5
     3.1.  Local AS: Modify Inbound BGP AS_PATH Attribute  . . . . .   6
     3.2.  Replace AS: Modify Outbound BGP AS_PATH Attribute . . . .   7
   4.  Internal BGP Autonomous System Migration Features . . . . . .   8
     4.1.  Internal BGP Alias  . . . . . . . . . . . . . . . . . . .   9
   5.  Additional Operational Considerations . . . . . . . . . . . .  11
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   This draft discusses common methods of managing an ASN migration
   using some BGP features that while commonly-used are not formally
   part of the BGP4 [RFC4271] protocol specification and may be vendor-
   specific in exact implementation.  This draft does not attempt to
   standardize these features, because they are local to a given
   implementation and do not require negotiation with or cooperation of
   BGP neighbors.  The deployment of these features do not need to
   interwork with one another to accomplish the desired results.
   However, it is necessary to document these de facto standards to
   ensure that any future protocol enhancements to BGP that propose to
   read, copy, manipulate or compare the AS_PATH attribute can do so
   without inhibiting the use of these very widely used ASN migration
   features.

   It is important to understand the business need for these features
   and illustrate why they are critical, particularly for ISPs'
   operations.  However, these features are not limited to ISPs and
   organizations of all sizes use these features for similar reasons to
   ISPs.  During a merger, acquisition or divestiture involving two
   organizations it is necessary to seamlessly migrate BGP speakers from
   one ASN to a second ASN.  The overall goal in doing so, particularly
   in the case of a merger or acquisition, is to achieve a uniform



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   operational model through consistent configurations across all BGP
   speakers in the combined network.  In addition, and perhaps more
   imporantly, it is common practice in the industry for ISPs to bill
   customers based on utilization.  ISPs bill customers based on the
   95th percentile of the greater of the traffic sent or received, over
   the course of a 1-month period, on the customer's PE-CE access
   circuit.  Given that the BGP Path Selection algorithm selects routes
   with the shortest AS_PATH attribute, it is critical for the ISP to
   not increase AS_PATH length during or after ASN migration, toward
   both downstream transit customers as well as settlement-free peers,
   who are likely sending or receiving traffic from those transit
   customers.  This would not only result in sudden changes in traffic
   patterns in the network, but also (substantially) decrease
   utilization driven revenue at the ISP.

   Lastly, it is important to note that by default, the BGP protocol
   requires an operator to configure a single remote ASN for the eBGP
   neighbor inside a router, in order to successfully negotiate and
   establish an eBGP session.  Prior to the existence of these features,
   it would have required an ISP to work with, in some cases, tens of
   thousands of customers.  In particular, the ISP would have to
   encourage those customers to change their CE router configs to use
   the new ASN, in a very short period of time, when the customer has no
   business incentive to do so.  Thus, it because critical to allow the
   ISP to make this process a bit more asymmetric, so that it could
   seamlessly migrate the ASN within its network(s), but not disturb
   existing customers, and allow the customers to gradually migrate to
   the ISP's new ASN at their leisure.

1.1.  Documentation note

   This draft uses Autonomous System Numbers (ASNs) from the range
   reserved for documentation as described in RFC 5398 [RFC5398].  In
   the examples used here, they are intended to represent Globally
   Unique ASNs, not private ASNs as documented in RFC 1930 [RFC1930]
   section 10.

2.  ASN Migration Scenario Overview

   The use case being discussed here is an ISP merging two or more ASNs,
   where eventually one ASN subsumes the other(s).  In this use case, we
   will assume the most common case where there are two ISPs, A and B,
   that use AS 64500 and 64510, respectively, before the ASN migration
   is to occur.  AS 64500 will be the permanently retained ASN used
   going forward across the consolidated set of both ISPs network
   equipment and AS 64510 will be retired.  Thus, at the conclusion of
   the ASN migration, there will be a single ISP A' with all internal




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   BGP speakers configured to use AS 64500.  To all external BGP
   speakers, the AS_PATH length will not be increased.

   In this same scenario, AS 64496 and AS 64499 represent two, separate
   customer networks: C and D, respectively.  Originally, customer C (AS
   64496) is attached to ISP B, which will undergo ASN migration from AS
   64510 to AS 64500.  Furthermore, customer D (AS 64496) is attached to
   ISP A, which does not undergo ASN migration since ISP A's ASN will
   remain constant, (AS 64500).  Although this example refers to AS
   64496 and 64499 as customer networks, either or both may be
   settlement-free or other types of peers.  In this use case they are
   referred to as "customers" merely for convenience.

         ------                  ------
       / ISP  A \              / ISP  B \
      | AS 64500 |            | AS 64510 |
       \        /              \        /
        -------                 -------
           |                       |
           |                       |
     ------------             -------------
     |  Cust D  |             |  Cust C   |
     | AS 64499 |             | AS 64496  |
     ------------             -------------

                        Figure 1: Before Migration

                ---------------
              /                \
             |     ISP A'       |
             |     AS 64500     |
              \                /
                ---------------
             /                  \
           /                      \
          |                         |
     ------------             -------------
     |  Cust D  |             |  Cust C   |
     | AS 64499 |             | AS 64496  |
     ------------             -------------

                         Figure 2: After Migration

   The general order of operations, typically carried out in a single
   maintenance window by the network undergoing ASN migration, ISP B,
   are as follows.  First, ISP B, will change the global BGP ASN used by
   a PE router, from ASN 64510 to 64500.  At this point, the router will
   no longer be able to establish eBGP sessions toward the existing CE



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   devices that are attached to it and still using AS 64510.  Second,
   ISP B will configure two separate, but related ASN migration features
   discussed in this document on all eBGP sessions toward all CE
   devices.  These features modify the AS_PATH attribute received from
   and transmitted toward CE devices to acheive the desired effect of
   not increasing the length of the AS_PATH.

   At the conclusion of the ASN migration, the CE devices at the edge of
   the network are not aware of and do not observe any change in the
   length of the AS_PATH attribute.  However, after the changes
   discussed in this document are put in place by ISP A', there is a
   change to the contents of the AS_PATH attribute to ensure the AS_PATH
   is not artifically lengthened for the duration of time that these AS
   migration parameters are used.

   In this use case, neither ISP is using BGP Confederations RFC 5065
   [RFC5065] internally.

   Additional information about this scenario, including vendor-specific
   implementation details can be found as follows:

   o  Cisco [CISCO]

   o  Juniper [JUNIPER]

   o  Alcatel-Lucent [ALU]

   Equivalent features do exist in other implementations, however the
   authors were unable to find publicly available documentation of the
   vendor-specific implementation to reference.  Finally, the examples
   cited below use Cisco IOS CLI for ease of illustration purposes only.

3.  External BGP Autonomous System Migration Features

   The following section addresses features that are specific to
   modifying the AS_PATH attribute at the Autonomous System Border
   Routers (ASBRs) of an organization, (typically a single Service
   Provider).  This ensures that external BGP customers/peers are not
   forced to make any configuration changes on their CE routers before
   or during the exact time the Service Provider wishes to migrate to a
   new, permanently retained ASN.  Furthermore, these features eliminate
   the artificial lengthening of the AS_PATH both transmitted from and
   received by the Service Provider that is undergoing AS Migration,
   which would have negative implications on path selection by external
   networks.






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3.1.  Local AS: Modify Inbound BGP AS_PATH Attribute

   ISP B needs to reconfigure its router(s) to participate as an
   internal BGP speaker in AS 64500, to realize the business goal of
   becoming a single Service Provider: ISP A'.  ISP B needs to do this
   without coordinating the change of its ASN with all of its eBGP
   peers, simultaneously.  The first step is for ISP B to change the
   global AS in its router configuration, used by the local BGP process
   as the system-wide Autonomous System ID, from AS 64510 to AS 64500.
   The next step is for ISP B to establish iBGP sessions with ISP A's
   existing routers, thus consolidating ISP B into ISP A resulting in
   operating under a single AS: ISP A', (AS 64500).

   The next step is for ISP B to reconfigure its PE router(s) so that
   each of its eBGP sessions toward all eBGP speakers with a feature
   called "Local AS".  This feature allows ISP B's PE router to re-
   establish a eBGP session toward the existing CE devices using the
   legacy AS, AS 64510, in the eBGP session establishment.  Ultimately,
   the CE devices, (i.e.: customer C), are completely unaware that ISP B
   has reconfigured its router to participate as a member of a new AS.
   Within the context of ISP B's PE router, the second effect this
   feature has is that, by default, it prepends all received BGP
   UPDATE's with the legacy AS of ISP B: AS 64510.  Thus, within ISP A'
   the AS_PATH toward customer C would appear as: 64510 64496, which is
   an increase in AS_PATH length from previously.  Therefore, a
   secondary feature "No Prepend" is required to be added to the "Local
   AS" configuration toward every eBGP neighbor on ISP B's PE router.
   The "No Prepend" feature causes ISP B's PE router to not prepend the
   legacy AS, AS 64510, on all received eBGP UPDATE's from customer C.
   This restores the AS_PATH within ISP A' toward customer C so that it
   is just one ASN in length: 64496.

   In the direction of CE -> PE (inbound):

   1.  'local-as <old_ASN>': appends the <old_ASN> value to the AS_PATH
       of routes received from the CE

   2.  'local-as <old_ASN> no-prepend': does not prepend <old_ASN> value
       to the AS_PATH of routes received from the CE

   As stated previously, local-as <old_ASN> no-prepend, (configuration
   #2), is critical because it does not increase the AS_PATH length.
   Ultimately, this ensures that routes learned from ISP B's legacy
   customers will be transmitted through legacy eBGP sessions of ISP A,
   toward both customers and peers, will contain only two AS'es in the
   AS_PATH: 64500 64496.  Thus, the legacy customers and peers of ISP A
   will not see an increase in the AS_PATH length to reach ISP B's
   legacy customers.  Ultimately, it is considered mandatory by



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   operators that both the "Local AS" and "No Prepend" configuration
   parameters always be used in conjunction with each other in order to
   ensure the AS_PATH length is not increased.

   PE-1 is a PE that was originally in ISP B. PE-1 has had its global
   configuration ASN changed from AS 64510 to AS 64500 to make it part
   of the permanently retained ASN.  This now makes PE-1 a member of ISP
   A'.  PE-2 is a PE that was originally in ISP A. Although its global
   configuration ASN remains AS 64500, throughout this exercise we also
   consider PE-2 a member of ISP A'.

                  ISP A'                    ISP A'
        CE-1 ---> PE-1 -------------------> PE-2 ---> CE-2
        64496     Old_ASN: 64510   New_ASN: 64500     64499
                  New_ASN: 64500

   Note: Direction of BGP UPDATE as per the arrows.

                   Figure 3: Local AS BGP UPDATE Diagram

   The final configuration on PE-1 after completing the "Local AS"
   portion of the AS migration is as follows:

               router bgp 64500
                neighbor <CE-1_IP> remote-as 64496
                neighbor <CE-1_IP> local-as 64510 no-prepend

   As a result of the "Local AS No Prepend" configuration, on PE-1, CE-2
   will see an AS_PATH of: 64500 64496.  CE-2 will not receive a BGP
   UPDATE containing AS 64510 in the AS_PATH.  (If only the "local-as
   64510" feature was configured without the keyword "no-prepend" on
   PE-1, then CE-2 would see an AS_PATH of: 64496 64510 64500, which is
   unacceptable).

3.2.  Replace AS: Modify Outbound BGP AS_PATH Attribute

   The previous feature, "Local AS No Prepend", was only designed to
   modify the AS_PATH Attribute received from CE devices by the ISP,
   when CE devices still have an eBGP session established with the ISPs
   legacy AS, (AS64510).  Use of "Local AS No Prepend" has an
   unfortunate side effect where its use does not concurrently modify
   the AS_PATH Attribute for BGP UPDATEs that are transmitted by the ISP
   to CE devices.  Specifically, with "Local AS No Prepend" enabled on
   ISP A's PE-1, it automatically causes a lengthening of the AS_PATH in
   outbound BGP UPDATEs from ISP A' toward directly attached eBGP
   speakers, (Customer C in AS 64496).  This is the result of the "Local
   AS No Prepend" feature automatically appending the new global
   configuration ASN, AS64500, after the legacy ASN, AS64510, on ISP A'



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   PE-1 in BGP UPDATEs that are transmitted by PE-1 to CE-1.  The end
   result is that customer C, in AS 64496, will receive the following
   AS_PATH: 64510 64500 64499.  Therefore, if ISP A' takes no further
   action, it will cause an increase in AS_PATH length within customer's
   networks directly attached to ISP A', which is unacceptable.

   A second feature, called "Replace AS", was designed to resolve this
   problem.  This feature allows ISP A' to not append the global
   configured AS in outbound BGP UPDATEs toward its customer's networks
   configured with the "Local AS" feature.  Instead, only the historical
   (or, legacy) AS will be prepended in the outbound BGP UPDATE toward
   customer's network, restoring the AS_PATH length to what it what was
   before AS Migration occurred.

   To re-use the above diagram, but in the opposite direction, we have:

                  ISP A'                    ISP A'
        CE-1 <--- PE-1 <------------------- PE-2 <--- CE-2
        64496     Old_ASN: 64510   New_ASN: 64500     64499
                  New_ASN: 64500

   Note: Direction of BGP UPDATE as per the arrows.

                  Figure 4: Replace AS BGP UPDATE Diagram

   The final configuration on PE-1 after completing the "Replace AS"
   portion of the AS migration is as follows:

                router bgp 64500
                 neighbor <CE-1_IP> remote-as 64496
                 neighbor <CE-1_IP> local-as 64510 no-prepend replace-as

   By default, without "Replace AS" enabled, CE-1 would see an AS_PATH
   of: 64510 64500 64499, which is artificially lengthened by the ASN
   Migration.  After ISP A' changes PE-1 to include the "Replace AS"
   feature, CE-1 would receive an AS_PATH of: 64510 64499, which is the
   same AS_PATH length pre-AS migration.

4.  Internal BGP Autonomous System Migration Features

   The following section describes features that are specific to
   performing an ASN migration within medium to large networks in order
   to realize the business and operational benefits of a single network
   using one, globally unique Autonomous System.  These features assist
   with a gradual and least service impacting migration of Internal BGP
   sessions from a legacy ASN to the permanently retained ASN.  It
   should be noted that the following feature is very valuable to




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   networks undergoing AS migration, but its use does not cause changes
   to the AS_PATH attribute.

4.1.  Internal BGP Alias

   In this case, all of the routers to be consolidated into a single,
   permanently retained ASN are under the administrative control of a
   single entity.  Unfortunately, though, the traditional method of
   migrating all Internal BGP speakers, particularly within larger
   networks, is both time consuming and widely service impacting.

   The traditional method to migrate Internal BGP sessions was strictly
   limited to reconfiguration of the global configuration ASN and,
   concurrently, changing of iBGP neighbor's remote ASN from the legacy
   ASN to the new, permanently retained ASN on each router within the
   legacy AS.  These changes can be challenging to swiftly execute in
   networks with with more than a few dozen internal BGP speakers.
   There is also the concomitant service interruptions as these changes
   are made to routers within the network, resulting in a reset of iBGP
   sessions and subsequent reconvergence times to reestablish optimal
   routing paths.  Operators do not and, in some cases, cannot make such
   changes given the associated risks and highly visible service
   interruption; rather, they require a more gradual method to migrate
   Internal BGP sessions, from one ASN to a second, permanently retained
   ASN, that is not visibly service-impacting to its customers.

   With the "Internal BGP Alias" [JUNIPER] feature, it allows an
   Internal BGP speaker to form a single iBGP session using either the
   old, legacy ASN or the new, permanently retained ASN.  The benefits
   of using this feature are several fold.  First, it allows for a more
   gradual and less service-impacting migration away from the legacy ASN
   to the permanently retained ASN.  Second, it (temporarily) permits
   the coexistence of the legacy and permanently retained ASN within a
   single network, allowing for uniform BGP path selection among all
   routers within the consolidated network.

   When the "Internal BGP Alias" feature is enabled, typically just on
   one side of a iBGP session, it allows that iBGP speaker to establish
   a single iBGP session with either the legacy ASN or the new,
   permanently retained ASN, depending on which one it receives in the
   "My Autonomous System" field of the BGP OPEN message from its iBGP
   session neighbor.  It is important to recognize that enablement of
   the "Internal BGP Alias" feature preserves the semantics of a regular
   iBGP session, (using identical ASNs).  Thus, the BGP attributes
   transmitted by and the acceptable methods of operation on BGP
   attributes received from iBGP sessions configured with "Internal BGP
   Alias" are no different than those exchanged across an iBGP session




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   without "Internal BGP Alias" configured, as defined by [RFC4271] and
   [RFC4456].

   Typically, in medium to large networks, BGP Route Reflectors
   [RFC4456] (RRs) are used to aid in reduction of configuration of iBGP
   sessions and scalability with respect to overall TCP (and, BGP)
   session maintenance between adjacent iBGP speakers.  Furthermore, BGP
   Route Reflectors are typically deployed in pairs within a single
   Route Reflection cluster to ensure high reliability of the BGP
   Control Plane.  As such, the following example will use Route
   Reflectors to aid in understanding the use of the "Internal BGP
   Alias" feature.  It should be noted that Route Reflectors are not a
   prerequisite to enable "Internal BGP Alias" and this feature can be
   enabled independent of the use of Route Reflectors.

   The general order of operations is as follows:

   1.  Within the legacy network, (the routers comprising the set of
       devices that still have a globally configured legacy ASN), take
       one member of a redundant pair of RRs and change its global
       configuration ASN to the permanently retained ASN.  Concurrently,
       enable use of "Internal BGP Alias" on all iBGP sessions.  This
       will comprise Non-Client iBGP sessions to other RRs as well as
       Client iBGP sessions, typically to PE devices, both still
       utilizing the legacy ASN.  Note that during this step there will
       be a reset and reconvergence event on all iBGP sessions on the
       RRs whose configuration was modified; however, this should not be
       service impacting due to the use of redundant RRs in each RR
       Cluster.

   2.  Repeat the above step for the other side of the redundant pair of
       RRs.  The one alteration to the above procedure is to disable use
       of "Internal BGP Alias" on the Non-Client iBGP sessions toward
       the other (previously reconfigured) RRs, since it is no longer
       needed.  "Internal BGP Alias" is still required on all RRs for
       all RR Client iBGP sessions.  Also during this step, there will
       be a reset and reconvergence event on all iBGP sessions whose
       configuration was modified, but this should not be service
       impacting.  At the conclusion of this step, all RRs should now
       have their globally configured ASN set to the permanently
       retained ASN and "Internal BGP Alias" enabled and in use toward
       RR Clients.

   3.  At this point, the network administrators would then be able to
       establish iBGP sessions between all Route Reflectors in both the
       legacy and permanently retained networks.  This would allow the
       network to appear to function, both internally and externally, as




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       a single, consolidated network using the permanently retained
       network.

   4.  The next steps to complete the AS migration are to gradually
       modify each RR Client, (PE), in the legacy network still
       utilizing the legacy ASN.  Specifically, each legacy PE would
       have its globally configured ASN changed to use the permanently
       retained ASN.  The ASN used by the PE for the iBGP sessions,
       toward each RR, would be changed to use the permanently retained
       ASN.  (It is unnecessary to enable "Internal BGP Alias" on the
       migrated iBGP sessions).  During the same maintenance window,
       External BGP sessions would be modified to include the above
       "Local AS No Prepend" and "Replace-AS" features, since all of the
       changes are service interrupting to the eBGP sessions of the PE.
       At this point, all PE's will have been migrated to the
       permanently retained ASN.

   5.  The final step is to excise the "Internal BGP Alias"
       configuration from the first half of the legacy RR Client pair --
       this will expunge "Internal BGP Alias" configuration from all
       devices in the network.  After this is complete, all routers in
       the network will be using the new, permanently retained ASN for
       all iBGP sessions with no vestiges of the legacy ASN on any iBGP
       sessions.

   The benefit of using "Internal BGP Alias" is a more gradual and less
   externally visible, service-impacting change to accomplish an AS
   migration.  Previously, without "Internal BGP Alias", such an AS
   migration change would carry a high risk and need to be successfully
   accomplished in a very short timeframe, (e.g.: at most several
   hours).  In addition, it would cause substantial routing churn and,
   likely, rapid fluctuations in traffic carried -- potentially causing
   periods of congestion and resultant packet loss -- during the period
   the configuration changes are underway to complete the AS Migration.
   On the other hand, with "Internal BGP Alias", the migration from the
   legacy ASN to the permanently retained ASN can occur over a period of
   days or weeks with little disruption experienced by customers of the
   network undergoing AS migration.  (The only observable service
   disruption should be when each PE undergoes the changes discussed in
   step 4 above.)

5.  Additional Operational Considerations

   This document describes several implementation-specific features to
   support ISPs and other organizations that need to perform ASN
   migrations.  Other variations of these features may exist, for
   example, in legacy router software that has not been upgraded or




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   reached End of Life, but continues to operate in the network.  Such
   variations are beyond the scope of this document.

   Companies routinely go through periods of mergers, acquisitions and
   divestitures, which in the case of the former cause them to
   accumulate several legacy ASNs over time.  ISPs often do not have
   control over the configuration of customer's devices, (i.e.: the ISPs
   are often not providing a managed CE router service, particularly to
   medium and large customers that require eBGP).  Furthermore, ISPs are
   using methods to perform ASN migration that do not require
   coordination with customers.  Ultimately, this means there is not a
   finite period of time after which legacy ASNs will be completely
   expunged from the ISP's network.  In fact, it is common that legacy
   ASNs and the associated External BGP AS Migration features discussed
   in this document can and do persist for several years, if not longer.
   Thus, it is prudent to plan that legacy ASNs and associated External
   BGP AS Migration features will persist in a operational network
   indefinitely.

   With respect to the Internal BGP AS Migration Features, all of the
   routers to be consolidated into a single, permanently retained ASN
   are under the administrative control of a single entity.  Thus,
   completing the migration from iBGP sessions using the legacy ASN to
   the permanently retained ASN is more straightforward and could be
   accomplished in a matter of days to months.  Finally, good
   operational hygiene would dictate that it is good practice to avoid
   using "Internal BGP Alias" over a long period of time for reasons of
   not only operational simplicity of the network, but also reduced
   reliance on that feature during the ongoing lifecycle management of
   software, features and configurations that are maintained on the
   network.

6.  Conclusion

   Although the features discussed in this document are not formally
   recognized as part of the BGP4 specification, they have been in
   existence in commercial implementations for well over a decade.
   These features are widely known by the operational community and will
   continue to be a critical necessity in the support of network
   integration activities going forward.  Therefore, these features are
   extremely unlikely to be deprecated by vendors.  As a result, these
   features must be acknowledged by protocol designers, particularly
   when there are proposals to modify BGP's behavior with respect to
   handling or manipulation of the AS_PATH Attribute.  More
   specifically, assumptions should not be made with respect to the
   preservation or consistency of the AS_PATH Attribute as it is
   transmitted along a sequence of ASN's. In addition, proposals to
   manipulate the AS_PATH that would gratuitously increase AS_PATH



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   length or remove the capability to use these features described in
   this document will not be accepted by the operational community.

7.  Acknowledgements

   Thanks to Kotikalapudi Sriram, Stephane Litkowski, and Terry
   Manderson for their comments.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Security Considerations

   This draft discusses a process by which one ASN is migrated into and
   subsumed by another.  This involves manipulating the AS_PATH
   Attribute with the intent of not increasing the AS_PATH length, which
   would typically cause the BGP route to no longer be selected by BGP's
   Path Selection Algorithm in other's networks.  This could result in a
   loss of revenue if the ISP is billing based on measured utilization
   of traffic sent to/from entities attached to its network.  This could
   also result in sudden, and unexpected shifts in traffic patterns in
   the network, potentially resulting in congestion, in the most extreme
   cases.

   Given that these features can only be enabled through configuration
   of router's within a single network, standard security measures
   should be taken to restrict access to the management interface(s) of
   routers that implement these features.

10.  References

10.1.  Normative References

   [RFC5398]  Huston, G., "Autonomous System (AS) Number Reservation for
              Documentation Use", RFC 5398, December 2008.

10.2.  Informative References

   [ALU]      Alcatel-Lucent, "BGP Local AS attribute", 2006-2012,
              <https://infoproducts.alcatel-lucent.com/html/0_add-h-f/
              93-0074-10-01/7750_SR_OS_Routing_Protocols_Guide/BGP-
              CLI.html#709567>.

   [CISCO]    Cisco Systems, Inc., "BGP Support for Dual AS
              Configuration for Network AS Migrations", 2003,
              <http://www.cisco.com/en/US/docs/ios/12_3t/12_3t11/feature
              /guide/gtbgpdas.html>.



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   [JUNIPER]  Juniper Networks, Inc., "Configuring the BGP Local
              Autonomous System Attribute", 2012, <https://
              www.juniper.net/techpubs/en_US/junos12.3/topics/reference/
              configuration-statement/local-as-edit-protocols-bgp.html>.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

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

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, April 2006.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065, August 2007.

Authors' Addresses

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

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


   Shane Amante
   Apple, Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   US

   Email: samante@apple.com













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