path: root/doc/rfc/rfc7705.txt

Internet Engineering Task Force (IETF)                         W. George
Request for Comments: 7705                             Time Warner Cable
Updates: 4271                                                  S. Amante
Category: Standards Track                                    Apple, Inc.
ISSN: 2070-1721                                            November 2015


                 Autonomous System Migration Mechanisms
             and Their Effects on the BGP AS_PATH Attribute

Abstract

   This document discusses some existing commonly used BGP mechanisms
   for Autonomous System Number (ASN) migration that are not formally
   part of the BGP4 protocol specification.  It is necessary to document
   these de facto standards to ensure that they are properly supported
   in future BGP protocol work.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7705.

Copyright Notice

   Copyright (c) 2015 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
   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Documentation Note  . . . . . . . . . . . . . . . . . . .   3
   2.  ASN Migration Scenario Overview . . . . . . . . . . . . . . .   3
   3.  External BGP Autonomous System Migration Mechanisms . . . . .   5
     3.1.  Modify Inbound BGP AS_PATH Attribute  . . . . . . . . . .   5
     3.2.  Modify Outbound BGP AS_PATH Attribute . . . . . . . . . .   7
     3.3.  Implementation  . . . . . . . . . . . . . . . . . . . . .   8
   4.  Internal BGP Autonomous System Migration Mechanisms . . . . .   9
     4.1.  Internal BGP AS Migration . . . . . . . . . . . . . . . .  10
     4.2.  Implementation  . . . . . . . . . . . . . . . . . . . . .  12
   5.  Additional Operational Considerations . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Implementation Report  . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   This document discusses some existing commonly used BGP mechanisms
   for Autonomous System Number (ASN) migration that are not formally
   part of the BGP4 [RFC4271] protocol specification.  These mechanisms
   are local to a given BGP speaker and do not require negotiation with
   or cooperation of BGP neighbors.  The deployment of these mechanisms
   do not need to interwork with one another to accomplish the desired
   results, so slight variations between existing vendor implementations
   exist and will not necessarily be harmonized due to this document.
   However, it is necessary to document these de facto standards to
   ensure that new implementations can be successful, and 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 mechanisms.

   The migration mechanisms discussed here are useful to ISPs and
   organizations of all sizes, but it is important to understand the
   business need for these mechanisms and illustrate why they are so
   critical for ISPs' operations.  During a merger, acquisition, or
   divestiture involving two organizations it is necessary to seamlessly
   migrate both internal and external BGP speakers from one ASN to a
   second ASN.  The overall goal in doing so is to simplify operations
   through consistent configurations across all BGP speakers in the
   combined network.  In addition, given that the BGP Path Selection
   algorithm selects routes with the shortest AS_PATH attribute, it is



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   critical that the ISP does not increase AS_PATH length during or
   after ASN migration, because an increased AS_PATH length would likely
   result in sudden, undesirable changes in traffic-patterns in the
   network.

   By default, the BGP protocol requires an operator to configure a
   router to use a single remote ASN for the BGP neighbor, and the ASN
   must match on both ends of the peering in order to successfully
   negotiate and establish a BGP session.  Prior to the existence of
   these migration mechanisms, it would have required an ISP to
   coordinate an ASN change with, in some cases, tens of thousands of
   customers.  In particular, as each router is migrated to the new ASN,
   to avoid an outage due to ASN mismatch, the ISP would have to force
   all customers on that router to change their router configurations to
   use the new ASN immediately after the ASN change.  Thus, it was
   critical to define a more asymmetric migration method so that the ISP
   could seamlessly change the ASN within its network(s) but allow the
   customers to gradually migrate to the ISP's new ASN when convenient.
   These customer migrations could occur either by coordinating
   individual session reconfigurations or, to allow for truly asymmetric
   migration, by accepting sessions using either the old or new ASN.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Documentation Note

   This document 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 use ASNs as documented in Section 5 of RFC
   6996 [RFC6996].

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 prior to the ASN migration use AS 64500 and 64510, respectively.
   AS 64500 will be the permanently retained ASN used 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 BGP speakers configured to use AS
   64500.  To all external BGP speakers, the AS_PATH length will not be
   increased.



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   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 64499) is attached to
   ISP A, which does not undergo ASN migration since the ASN for ISP A
   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),
   is as follows.  First, ISP B will change the global BGP ASN used by a
   Provider Edge (PE) router, from ASN 64510 to 64500.  At this point,
   the router will no longer be able to establish External BGP (eBGP)
   sessions toward the existing Customer Edge (CE) devices that are
   attached to it and still using AS 64510.  Second, since ISP B needs
   to do this without coordinating the simultaneous change of its ASN



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   with all of its eBGP peers, ISP B will configure two separate, but
   related ASN migration mechanisms discussed in this document on all
   eBGP sessions toward all CE devices.  These mechanisms enable the
   router to establish BGP neighbors using the legacy ASN, modify the
   AS_PATH attribute received from a CE device when advertising it
   further, and modify AS_PATH when transmitted toward CE devices to
   achieve 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 the fact that their upstream router is
   now in a new ASN 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
   artificially lengthened while these AS migration parameters are used.

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

3.  External BGP Autonomous System Migration Mechanisms

   The following section addresses optional capabilities 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 mechanisms
   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.

3.1.  Modify Inbound BGP AS_PATH Attribute

   The first instrument used in the process described above is called
   "Local AS".  This allows the router to supersede the globally
   configured ASN in the "My Autonomous System" field of the BGP OPEN
   [RFC4271] with a locally defined AS value for a specific BGP neighbor
   or group of neighbors.  This mechanism allows the PE router that was
   formerly in ISP B to establish an eBGP session toward the existing CE
   devices using the legacy AS -- AS 64510.  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 the former ISP B PE router, the second effect this
   specific mechanism has on AS_PATH is that, by default, it prepends
   all received BGP UPDATEs with the legacy AS of ISP B -- AS 64510 --



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   while advertising it (Adj-RIB-Out) to other BGP speakers (A').
   Within the Loc-RIB on ISP B prior to the migration, the AS_PATH of
   route announcements received from customer C would appear as 64496,
   whereas the same RIB on ISP A' (ISP B routers post-migration) would
   contain AS_PATH 64510 64496.

   The second instrument, referred to as "No Prepend Inbound", is
   enabled on PE routers migrating from ISP B.  The "No Prepend Inbound"
   capability causes ISP B's routers to not prepend the legacy AS (AS
   64510), when advertising UPDATES received from customer C.  This
   restores the AS_PATH within ISP A' for route announcements received
   from customer C so that it is just one ASN in length: 64496.

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

   1.  "Local AS": Allows the local BGP router to generate a BGP OPEN to
       an eBGP neighbor with the old, legacy ASN value in the "My
       Autonomous System" field.  When this capability is activated, it
       also causes the local router to prepend the <old_ASN> value to
       the AS_PATH when installing or advertising routes received from a
       CE to Internal BGP (iBGP) neighbors inside the Autonomous System.

   2.  "No Prepend Inbound (of Local AS)": The local BGP router does not
       prepend the <old_ASN> value to the AS_PATH when installing or
       advertising routes received from the CE to iBGP neighbors inside
       the Autonomous System

   PE-B is a PE that was originally in ISP B, and has a customer eBGP
   session to CE-B.  PE-B 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-B a member of ISP A'.  PE-A is a PE that was
   originally in ISP A, and has a customer peer CE-A.  Although its
   global configuration ASN remains AS 64500, throughout this exercise
   we also consider PE-A a member of ISP A'.

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

             Note: Direction of BGP UPDATE as per the arrows.

           Figure 3: Local AS and No Prepend BGP UPDATE Diagram

   As a result, using both the "Local AS" and "No Prepend Inbound"
   capabilities on PE-B, CE-A will see an AS_PATH of 64500 64496.  CE-A
   will not receive a BGP UPDATE containing AS 64510 in the AS_PATH.
   (If only the "Local AS" mechanism was configured without "No Prepend



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   Inbound" on PE-B, then CE-A would have seen an AS_PATH of 64500 64510
   64496, which results in an unacceptable lengthening of the AS_PATH).
   Note: If there are still routers in the old ASN (64510), it is
   possible for them to accept these manipulated routes (i.e., those
   with 64510 removed from the AS_PATH by this command) as if they have
   not already passed through their ASN, potentially causing a loop,
   since BGP's normal loop-prevention behavior of rejecting routes that
   include its ASN in the path will not catch these.  Careful filtering
   between routers remaining in the old ASN and routers migrated to the
   new ASN is necessary to minimize the risk of routing loops.

3.2.  Modify Outbound BGP AS_PATH Attribute

   The two aforementioned mechanisms, "Local AS" and "No Prepend
   Inbound", only modify the AS_PATH attribute received by the ISP's PEs
   in the course of processing BGP UPDATEs from CE devices when CE
   devices still have an eBGP session established with the ISPs legacy
   AS (AS64510).

   In some existing implementations, "Local AS" and "No Prepend Inbound"
   do not concurrently modify the AS_PATH attribute for BGP UPDATEs that
   are transmitted by the ISP's PEs to CE devices.  In these
   implementations, with "Local AS" and "No Prepend Inbound" used on
   PE-B, there is an automatic lengthening of the AS_PATH in outbound
   BGP UPDATEs from ISP A' toward directly attached eBGP speakers
   (customer C in AS 64496).  The externally observed 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, there
   will be an unacceptable increase in the AS_PATH length within the
   customer's networks directly attached to ISP A'.

   A tertiary mechanism, referred to as "Replace Old AS", is used to
   resolve this problem.  This capability allows ISP A' to prevent
   routers from appending the globally configured ASN in outbound BGP
   UPDATEs toward directly attached eBGP neighbors that are using the
   "Local AS" mechanism.  Instead, only the old (or previously used) AS
   will be prepended in the outbound BGP UPDATE toward the customer's
   network, restoring the AS_PATH length to what it what was before AS
   Migration occurred.












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   To reuse the above diagram, but in the opposite direction, we have:

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

             Note: Direction of BGP UPDATE as per the arrows.

                  Figure 4: Replace AS BGP UPDATE Diagram

   By default, without the use of "Replace Old AS", CE-B would see an
   AS_PATH of 64510 64500 64499.  After ISP A' changes PE-B to use
   "Replace Old AS", CE-B would receive an AS_PATH of 64510 64499, which
   is the same AS_PATH length pre-AS migration.

3.3.  Implementation

   The mechanisms introduced in this section MUST be configurable on a
   per-neighbor or per-neighbor-group basis to allow for maximum
   flexibility.  (Here, "neighbor group" refers to a group of similar
   BGP neighbor statements that reuse some common configuration to
   simplify provisioning.)  When the "Local AS" capability is used, a
   local ASN will be provided in the configuration that is different
   from the globally configured ASN of the BGP router.  To implement
   this mechanism, a BGP speaker SHOULD send BGP OPEN [RFC4271] (see
   Section 4.2) messages to the configured eBGP peer(s) using the local
   ASN configured for this session as the value sent in "My Autonomous
   System".  The BGP router SHOULD NOT use the ASN configured globally
   within the BGP process as the value sent in "My Autonomous System" in
   the OPEN message.  This prevents causing the eBGP neighbor to
   unnecessarily generate a BGP OPEN Error message "Bad Peer AS".  This
   method is typically used to re-establish eBGP sessions with peers
   expecting the legacy ASN after a router has been moved to a new ASN.

   Implementations MAY support a more flexible model where the eBGP
   speaker attempts to open the BGP session using either the ASN
   configured as "Local AS" or the globally configured AS as discussed
   in BGP Alias (Section 4.2).  If the session is successfully
   established to the globally configured ASN, then the modifications to
   AS_PATH described in this document SHOULD NOT be performed, as they
   are unnecessary.  The benefit to this more flexible model is that it
   allows the remote neighbor to reconfigure to the new ASN without
   direct coordination between the ISP and the customer.

   Note that this procedure will vary slightly if the locally or
   globally configured ASN is a 4-octet ASN.  See Section 3 of
   [RFC6793].



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   When the BGP router receives UPDATEs from its eBGP neighbor
   configured with the "Local AS" mechanism, it processes the UPDATE as
   described in RFC4271 Section 5.1.2 [RFC4271].  However, the presence
   of a second ASN due to "Local AS" adds the following behavior to
   processing UPDATEs received from an eBGP neighbor configured with
   this mechanism:

   1.  Internal: The router SHOULD append the configured "Local AS" ASN
       in the AS_PATH attribute before installing the route or
       advertising the UPDATE to an iBGP neighbor.  The decision of when
       to append the ASN is an implementation detail outside the scope
       of this document.  Some considerations factoring into this
       decision include consistency in the AS_PATH throughout the AS,
       and implementation of the loop detection mechanism.

   2.  External: The BGP router SHOULD first append the globally
       configured ASN to the AS_PATH immediately followed by the "Local
       AS" value before advertising the UPDATE to an eBGP neighbor.

   Two options exist to manipulate the behavior of the basic "Local AS"
   mechanism.  They modify the behavior as described below:

   1.  "No Prepend Inbound": When the BGP router receives inbound BGP
       UPDATEs from its eBGP neighbor configured with this option, it
       MUST NOT append the "Local AS" ASN value in the AS_PATH attribute
       when installing the route or advertising that UPDATE to iBGP
       neighbors, but it MUST still append the globally configured ASN
       as normal when advertising the UPDATE to other local eBGP
       neighbors (i.e., those natively peering with the globally
       configured ASN).

   2.  "Replace Old AS" (outbound): When the BGP router generates
       outbound BGP UPDATEs toward an eBGP neighbor configured with this
       option, the BGP speaker MUST NOT append the globally configured
       ASN from the AS_PATH attribute.  The BGP router MUST append only
       the configured "Local AS" ASN value to the AS_PATH attribute
       before sending the BGP UPDATEs outbound to the eBGP neighbor.

4.  Internal BGP Autonomous System Migration Mechanisms

   The following section describes mechanisms that assist with a gradual
   and least service-impacting migration of Internal BGP sessions from a
   legacy ASN to the permanently retained ASN.  The following mechanism
   is very valuable to networks undergoing AS migration, but its use
   does not cause changes to the AS_PATH attribute.






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4.1.  Internal BGP AS Migration

   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, 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 all iBGP neighbors' 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 more than a few dozen internal BGP routers.  There are
   also the concomitant service interruptions as these changes are made
   to routers within the network, resulting in a reset of iBGP sessions
   and subsequent route reconvergence to reestablish optimal routing
   paths.  Operators often cannot make such sweeping changes given the
   associated risks of a 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 AS Migration" mechanism described herein, 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 mechanism 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.

   The iBGP router with the "Internal BGP AS Migration" capability
   enabled allows the receipt of a BGP OPEN message with either the
   legacy ASN value or the new, globally configured ASN value in the "My
   Autonomous System" field of the BGP OPEN message from iBGP neighbors.
   It is important to recognize that enablement of the "Internal BGP AS
   Migration" mechanism preserves the semantics of a regular iBGP
   session (i.e., 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
   AS Migration" capability are no different than those exchanged across
   an iBGP session without "Internal BGP AS Migration" configured, as
   defined by [RFC4271] and [RFC4456].






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   Typically, in medium to large networks, BGP Route Reflectors (RRs)
   [RFC4456] 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 routers.  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 AS
   Migration" mechanism.  Note that Route Reflectors are not a
   prerequisite to enable "Internal BGP AS Migration" and this mechanism
   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), one
       member of a redundant pair of RRs has its global configuration
       ASN changed to the permanently retained ASN.  Concurrently, the
       "Internal BGP AS Migration" capability is enabled on all iBGP
       sessions on that device.  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.  The above step is repeated for the other side of the redundant
       pair of RRs.  The one alteration to the above procedure is that
       the "Internal BGP AS Migration" mechanism is now removed from the
       Non-Client iBGP sessions toward the other (previously
       reconfigured) RRs, since it is no longer needed.  The "Internal
       BGP AS Migration" mechanism 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 will have
       their globally configured ASN set to the permanently retained ASN
       and "Internal BGP AS Migration" 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
       a single, consolidated network using the permanently retained
       network.




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   4.  To complete the AS migration, each RR Client (PE) in the legacy
       network still utilizing the legacy ASN is now modified.
       Specifically, each legacy PE would have its globally configured
       ASN changed to use the permanently retained ASN.  The ASN
       configured within the PE for the iBGP sessions toward each RR
       would be changed to use the permanently retained ASN.  It is
       unnecessary to enable the "Internal BGP AS Migration" mechanism
       on these migrated iBGP sessions.  During the same maintenance
       window, External BGP sessions would be modified to include the
       above "Local AS", "No Prepend", and "Replace Old AS" mechanisms
       described in Section 3 above, since all of the changes are
       service interrupting to the eBGP sessions of the PE.  At this
       point, all PEs will have been migrated to the permanently
       retained ASN.

   5.  The final step is to excise the "Internal BGP AS Migration"
       configuration from the Router Reflectors in an orderly fashion.
       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 the aforementioned "Internal BGP AS Migration"
   capability is that it is a more gradual and less externally service
   impacting change to accomplish an AS migration.  Previously, without
   "Internal BGP AS Migration", such an AS migration change would carry
   a high risk and need to be successfully accomplished in a very short
   time frame (e.g., at most several hours).  In addition, it would
   likely cause substantial routing churn and rapid fluctuations in
   traffic carried -- potentially causing periods of congestion and
   resultant packet loss -- during the period when the configuration
   changes are underway to complete the AS Migration.  On the other
   hand, with "Internal BGP AS Migration", the migration from the legacy
   ASN to the permanently retained ASN can occur over a period of days
   or weeks with reduced customer disruption.  (The only observable
   service disruption should be when each PE undergoes the changes
   discussed in step 4 above.)

4.2.  Implementation

   The mechanism introduced in this section MUST be configurable on a
   per-neighbor or per-neighbor-group basis to allow for maximum
   flexibility.  When configured with this mechanism, a BGP speaker MUST
   accept BGP OPEN and establish an iBGP session from configured iBGP
   peers if the ASN value in "My Autonomous System" is either the
   globally configured ASN or a locally configured ASN provided when
   this capability is utilized.  Additionally, a BGP router configured
   with this mechanism MUST send its own BGP OPEN [RFC4271] (see
   Section 4.2) using either the globally configured or the locally



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   configured ASN in "My Autonomous System" as follows.  To avoid
   potential deadlocks when two BGP speakers are attempting to establish
   a BGP peering session and are both configured with this mechanism,
   the speaker SHOULD send BGP OPEN using the globally configured ASN
   first, and only send a BGP OPEN using the locally configured ASN as a
   fallback if the remote neighbor responds with the BGP error "Bad Peer
   AS".  In each case, the BGP speaker MUST treat UPDATEs sent and
   received to this peer as if this was a natively configured iBGP
   session, as defined by [RFC4271] and [RFC4456].

   Note that this procedure will vary slightly if the locally or
   globally configured ASN is a 4-octet ASN.  See Section 3 of
   [RFC6793].

5.  Additional Operational Considerations

   This document describes several mechanisms to support ISPs and other
   organizations that need to perform ASN migrations.  Other variations
   of these mechanisms may exist, for example, in legacy router software
   that has not been upgraded or 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 customers' 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 mechanisms
   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 mechanisms will persist in an
   operational network indefinitely.

   With respect to the Internal BGP AS Migration mechanism, 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 AS Migration" capability over a long period of
   time for reasons of not only operational simplicity of the network,



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   but also reduced reliance on that mechanism during the ongoing life
   cycle management of software, features, and configurations that are
   maintained on the network.

6.  Security Considerations

   This document 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 others' networks.  This could result in
   sudden and unexpected shifts in traffic patterns in the network,
   potentially resulting in congestion.

   Given that these mechanisms can only be enabled through configuration
   of routers within a single network, standard security measures should
   be taken to restrict access to the management interface(s) of routers
   that implement these mechanisms.  Additionally, BGP sessions SHOULD
   be protected using TCP Authentication Option [RFC5925] and the
   Generalized TTL Security Mechanism [RFC5082]

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <http://www.rfc-editor.org/info/rfc4271>.

   [RFC4456]  Bates, T., Chen, E., and R. Chandra, "BGP Route
              Reflection: An Alternative to Full Mesh Internal BGP
              (IBGP)", RFC 4456, DOI 10.17487/RFC4456, April 2006,
              <http://www.rfc-editor.org/info/rfc4456>.

7.2.  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>.





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   [CISCO]    Cisco Systems, Inc., "BGP Support for Dual AS
              Configuration for Network AS Migrations", 2013,
              <http://www.cisco.com/c/en/us/td/docs/ios-xml/
              ios/iproute_bgp/configuration/xe-3s/asr1000/
              irg-xe-3s-asr1000-book/irg-dual-as.html>.

   [JUNIPER]  Juniper Networks, Inc., "Understanding the BGP Local AS
              Attribute", December 2013,
              <http://www.juniper.net/techpubs/en_US/junos13.3/topics/
              concept/bgp-local-as-introduction.html>.

   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <http://www.rfc-editor.org/info/rfc5065>.

   [RFC5082]  Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
              Pignataro, "The Generalized TTL Security Mechanism
              (GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
              <http://www.rfc-editor.org/info/rfc5082>.

   [RFC5398]  Huston, G., "Autonomous System (AS) Number Reservation for
              Documentation Use", RFC 5398, DOI 10.17487/RFC5398,
              December 2008, <http://www.rfc-editor.org/info/rfc5398>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <http://www.rfc-editor.org/info/rfc5925>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <http://www.rfc-editor.org/info/rfc6793>.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
              2013, <http://www.rfc-editor.org/info/rfc6996>.














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Appendix A.  Implementation Report

   As noted elsewhere in this document, this set of migration mechanisms
   has multiple existing implementations in wide use.

   o  Cisco [CISCO]

   o  Juniper [JUNIPER]

   o  Alcatel-Lucent [ALU]

   This is not intended to be an exhaustive list, as equivalent features
   do exist in other implementations; however, the authors were unable
   to find publicly available documentation of the vendor-specific
   implementation to reference.

Acknowledgements

   Thanks to Kotikalapudi Sriram, Stephane Litkowski, Terry Manderson,
   David Farmer, Jaroslaw Adam Gralak, Gunter Van de Velde, Juan
   Alcaide, Jon Mitchell, Thomas Morin, Alia Atlas, Alvaro Retana, and
   John Scudder for their comments.

Authors' Addresses

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

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


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

   Email: amante@apple.com









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