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Versions: (draft-ietf-idr-optional-transitive) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18

Internet Engineering Task Force                             E. Chen, Ed.
Internet-Draft                                       Cisco Systems, Inc.
Updates: 1997, 4271, 4360, 4456, 4760, 5701 (if approved)J. Scudder, Ed.
Intended status: Standards Track                        Juniper Networks
Expires: August 18, 2014                                    P. Mohapatra
                                                  Cumulus Networks, Inc.
                                                                K. Patel
                                                     Cisco Systems, Inc.
                                                       February 14, 2014


             Revised Error Handling for BGP UPDATE Messages
                    draft-ietf-idr-error-handling-06

Abstract

   According to the base BGP specification, a BGP speaker that receives
   an UPDATE message containing a malformed attribute is required to
   reset the session over which the offending attribute was received.
   This behavior is undesirable as a session reset would impact not only
   routes with the offending attribute, but also other valid routes
   exchanged over the session.  This document partially revises the
   error handling for UPDATE messages, and provides guidelines for the
   authors of documents defining new attributes.  Finally, it revises
   the error handling procedures for a number of existing attributes.

   This document updates error handling for RFCs 1997, 4271, 4360, 4456,
   4760, and 5701.

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 August 18, 2014.






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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
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Revision to Base Specification  . . . . . . . . . . . . . . .   4
   3.  Parsing of NLRI Fields  . . . . . . . . . . . . . . . . . . .   6
     3.1.  Inconsistency of Attribute Length Fields  . . . . . . . .   6
     3.2.  Syntactic Correctness of NLRI Fields  . . . . . . . . . .   7
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
   5.  Error Handling Procedures for Existing Attributes . . . . . .   8
     5.1.  ORIGIN  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  AS_PATH . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  NEXT_HOP  . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.4.  MULTI_EXIT_DESC . . . . . . . . . . . . . . . . . . . . .   9
     5.5.  LOCAL_PREF  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.6.  ATOMIC_AGGREGATE  . . . . . . . . . . . . . . . . . . . .   9
     5.7.  AGGREGATOR  . . . . . . . . . . . . . . . . . . . . . . .   9
     5.8.  Community . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.9.  Extended Community  . . . . . . . . . . . . . . . . . . .  10
     5.10. IPv6 Address Specific BGP Extended Community Attribute  .  10
     5.11. ORIGINATOR_ID . . . . . . . . . . . . . . . . . . . . . .  11



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     5.12. CLUSTER_LIST  . . . . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Why not discard UPDATE messages? . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   According to the base BGP specification [RFC4271], a BGP speaker that
   receives an UPDATE message containing a malformed attribute is
   required to reset the session over which the offending attribute was
   received.  This behavior is undesirable as a session reset would
   impact not only routes with the offending attribute, but also other
   valid routes exchanged over the session.  In the case of optional
   transitive attributes, the behavior is especially troublesome and may
   present a potential security vulnerability.  The reason is that such
   attributes may have been propagated without being checked by
   intermediate routers that do not recognize the attributes -- in
   effect the attribute may have been tunneled, and when they do reach a
   router that recognizes and checks them, the session that is reset may
   not be associated with the router that is at fault.

   The goal for revising the error handling for UPDATE messages is to
   minimize the impact on routing by a malformed UPDATE message, while
   maintaining protocol correctness to the extent possible.  This can be
   achieved largely by maintaining the established session and keeping
   the valid routes exchanged, but removing the routes carried in the
   malformed UPDATE from the routing system.

   This document partially revises the error handling for UPDATE
   messages, and provides guidelines for the authors of documents
   defining new attributes.  Finally, it revises the error handling
   procedures for a number of existing attributes.  Specifically, the
   error handling procedures of, [RFC1997], [RFC4271], [RFC4360],
   [RFC4456], [RFC4760] and [RFC5701] are revised.

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].








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2.  Revision to Base Specification

   The first paragraph of Section 6.3 of [RFC4271] is revised as
   follows:

   Old Text:

      All errors detected while processing the UPDATE message MUST be
      indicated by sending the NOTIFICATION message with the Error Code
      UPDATE Message Error.  The error subcode elaborates on the
      specific nature of the error.

   New text:

      An error detected while processing the UPDATE message for which a
      session reset is specified MUST be indicated by sending the
      NOTIFICATION message with the Error Code UPDATE Message Error.
      The error subcode elaborates on the specific nature of the error.

   The error handling of the following case described in Section 6.3 of
   [RFC4271] remains unchanged:

      If the Withdrawn Routes Length or Total Attribute Length is too
      large (i.e., if Withdrawn Routes Length + Total Attribute Length +
      23 exceeds the message Length), then the Error Subcode MUST be set
      to Malformed Attribute List.

   The error handling of the following case described in Section 6.3 of
   [RFC4271] is revised

      If any recognized attribute has Attribute Flags that conflict with
      the Attribute Type Code, then the Error Subcode MUST be set to
      Attribute Flags Error.  The Data field MUST contain the erroneous
      attribute (type, length, and value).

   as follows:

      If any recognized attribute has Attribute Flags that conflict with
      the Attribute Type Code, then the attribute MUST be treated as
      malformed and the treat-as-withdraw approach (see below) used,
      unless the specification for the attribute mandates different
      handling for incorrect Attribute Flags.

   The error handling of all other cases involving path attributes as
   described in Section 6.3 of [RFC4271] that specify a session reset is
   revised as follows.





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   When a path attribute (other than the MP_REACH_NLRI attribute
   [RFC4760] or the MP_UNREACH_NLRI attribute [RFC4760]) in an UPDATE
   message is determined to be malformed, the UPDATE message containing
   that attribute MUST be treated as though all contained routes had
   been withdrawn just as if they had been listed in the WITHDRAWN
   ROUTES field (or in the MP_UNREACH_NLRI attribute if appropriate) of
   the UPDATE message, thus causing them to be removed from the Adj-RIB-
   In according to the procedures of [RFC4271].  In the case of an
   attribute which has no effect on route selection or installation, the
   malformed attribute MAY instead be discarded and the UPDATE message
   continue to be processed.  For the sake of brevity, the former
   approach is termed "treat-as-withdraw", and the latter as "attribute
   discard".

   If any of the well-known mandatory attributes are not present in an
   UPDATE message, then the approach of "treat-as-withdraw" MUST be used
   for the error handling.

   The approach of "treat-as-withdraw" MUST be used for the error
   handling of the cases described in Section 6.3 of [RFC4271] that
   specify a session reset and involve any of the following attributes:
   ORIGIN, AS_PATH, NEXT_HOP, MULTI_EXIT_DISC, and LOCAL_PREF.

   The approach of "attribute discard" MUST be used for the error
   handling of the cases described in Section 6.3 of [RFC4271] that
   specify a session reset and involve any of the following attributes:
   ATOMIC_AGGREGATE and AGGREGATOR.

   If the MP_REACH_NLRI attribute or the MP_UNREACH_NLRI attribute
   appears more than once in the UPDATE message, then a NOTIFICATION
   message MUST be sent with the Error Subcode "Malformed Attribute
   List".  If any other attribute appears more than once in an UPDATE
   message, then all the occurrences of the attribute other than the
   first one SHALL be discarded and the UPDATE message continue to be
   processed.

   When multiple attribute errors exist in an UPDATE message, if the
   same approach (either "session reset", or "treat-as-withdraw" or
   "attribute discard") is specified for the handling of these malformed
   attributes, then the specified approach MUST be used.  Otherwise the
   approach with the strongest action MUST be used following the order
   of "session reset", "treat-as-withdraw" and "attribute discard" from
   the strongest to the weakest.

   A document which specifies a new attribute MUST provide specifics
   regarding what constitutes an error for that attribute and how that
   error is to be handled.




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   Finally, we observe that in order to use the approach of "treat-as-
   withdraw", the entire NLRI field and/or the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes need to be successfully parsed.  If this
   is not possible, the procedures of [RFC4271] continue to apply.
   Alternatively the error handling procedures specified in [RFC4760]
   for disabling a particular AFI/SAFI MAY be followed.  One notable
   case where it would be not possible to successfully parse the NLRI is
   if the NLRI field is found to be "syntactically incorrect" (see
   Section 3.2).  It can be seen that therefore, this part of [RFC4271]
   Section 6.3 necessarily continues to apply:

      The NLRI field in the UPDATE message is checked for syntactic
      validity.  If the field is syntactically incorrect, then the Error
      Subcode MUST be set to Invalid Network Field.

   Furthermore, this document extends RFC 4271 by mandating that the
   Withdrawn Routes field SHALL be checked for syntactic correctness in
   the same manner as the NLRI field.

3.  Parsing of NLRI Fields

   To facilitate the determination of the NLRI field in an UPDATE with a
   malformed attribute, the MP_REACH_NLRI or MP_UNREACH_NLRI attribute
   (if present) SHALL be encoded as the very first path attribute in an
   UPDATE.  An implementation, however, MUST still be prepared to
   receive these fields in any position.

   If the encoding of [RFC4271] is used, the NLRI field for the IPv4
   unicast address family is carried immediately following all the
   attributes in an UPDATE.  When such an UPDATE is received, we observe
   that the NLRI field can be determined using the "Message Length",
   "Withdrawn Route Length" and "Total Attribute Length" (when they are
   consistent) carried in the message instead of relying on the length
   of individual attributes in the message.

3.1.  Inconsistency of Attribute Length Fields

   There are two error cases in which the Total Attribute Length value
   can be in conflict with the enclosed path attributes, which
   themselves carry length values.  In the "overrun" case, as the
   enclosed path attributes are parsed, the length of the last
   encountered path attribute would cause the Total Attribute Length to
   be exceeded.  In the "underrun" case, as the enclosed path attributes
   are parsed, after the last successfully-parsed attribute, fewer than
   three bytes remain, or fewer than four bytes, if the Attribute Flags
   field has the Extended Length bit set -- that is, there remains
   unconsumed data in the path attributes but yet insufficient data to
   encode a single minimum-sized path attribute.  In either of these



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   cases an error condition exists and the treat-as-withdraw approach
   MUST be used (unless some other, more severe error is encountered
   dictating a stronger approach), and the Total Attribute Length MUST
   be relied upon to enable the beginning of the NLRI field to be
   located.

3.2.  Syntactic Correctness of NLRI Fields

   The NLRI field or Withdrawn Routes field SHALL be considered
   "syntactically incorrect" if either of the following are true:

   o  The length of any of the included NLRI is greater than 32,

   o  When parsing NLRI contained in the field, the length of the last
      NLRI found exceeds the amount of unconsumed data remaining in the
      field.

   Similarly, the MP_REACH or MP_UNREACH attribute of an update SHALL be
   considered to be incorrect if any of the following are true:

   o  The length of any of the included NLRI is inconsistent with the
      given AFI/SAFI (for example, if an IPv4 NLRI has a length greater
      than 32 or an IPv6 NLRI has a length greater than 128),

   o  When parsing NLRI contained in the attribute, the length of the
      last NLRI found exceeds the amount of unconsumed data remaining in
      the attribute.

4.  Operational Considerations

   Although the "treat-as-withdraw" error-handling behavior defined in
   Section 2 makes every effort to preserve BGP's correctness, we note
   that if an UPDATE received on an IBGP session is subjected to this
   treatment, inconsistent routing within the affected Autonomous System
   may result.  The consequences of inconsistent routing can include
   long-lived forwarding loops and black holes.  While lamentable, this
   issue is expected to be rare in practice, and more importantly is
   seen as less problematic than the session-reset behavior it replaces.

   When a malformed attribute is indeed detected over an IBGP session,
   we RECOMMEND that routes with the malformed attribute be identified
   and traced back to the ingress router in the network where the routes
   were sourced or received externally, and then a filter be applied on
   the ingress router to prevent the routes from being sourced or
   received.  This will help maintain routing consistency in the
   network.





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   Even if inconsistent routing does not arise, the "treat-as-withdraw"
   behavior can cause either complete unreachability or sub-optimal
   routing for the destinations whose routes are carried in the affected
   UPDATE message.

   Note that "treat-as-withdraw" is different from discarding an UPDATE
   message.  The latter violates the basic BGP principle of incremental
   update, and could cause invalid routes to be kept.  (See also
   Appendix A.)

   For any malformed attribute which is handled by the "attribute
   discard" instead of the "treat-as-withdraw" approach, it is critical
   to consider the potential impact of doing so.  In particular, if the
   attribute in question has or may have an effect on route selection or
   installation, the presumption is that discarding it is unsafe, unless
   careful analysis proves otherwise.  The analysis should take into
   account the tradeoff between preserving connectivity and potential
   side effects.

   Because of these potential issues, a BGP speaker MUST provide
   debugging facilities to permit issues caused by a malformed attribute
   to be diagnosed.  At a minimum, such facilities MUST include logging
   an error listing the NLRI involved, and containing the entire
   malformed UPDATE message when such an attribute is detected.  The
   malformed UPDATE message SHOULD be analyzed, and the root cause
   SHOULD be investigated.

5.  Error Handling Procedures for Existing Attributes

5.1.  ORIGIN

   The attribute is considered malformed if its length is not 1, or it
   has an undefined value [RFC4271].

   An UPDATE message with a malformed ORIGIN attribute SHALL be handled
   using the approach of "treat-as-withdraw".

5.2.  AS_PATH

   The error conditions for the attribute have been defined in
   [RFC4271].

   An UPDATE message with a malformed AS_PATH attribute SHALL be handled
   using the approach of "treat-as-withdraw".







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5.3.  NEXT_HOP

   The error conditions for the NEXT_HOP attribute have been defined in
   [RFC4271].

   An UPDATE message with a malformed NEXT_HOP attribute SHALL be
   handled using the approach of "treat-as-withdraw".

5.4.  MULTI_EXIT_DESC

   The attribute is considered malformed if its length is not 4
   [RFC4271].

   An UPDATE message with a malformed MULTI_EXIT_DESC attribute SHALL be
   handled using the approach of "treat-as-withdraw".

5.5.  LOCAL_PREF

   The attribute is considered malformed if its length is not 4
   [RFC4271].

   An UPDATE message with a malformed LOCAL_PREF attribute SHALL be
   handled as follows:

   o  using the approach of "attribute discard" if the UPDATE message is
      received from an external neighbor, or

   o  using the approach of "treat-as-withdraw" if the UPDATE message is
      received from an internal neighbor.

   In addition, if the attribute is present in an UPDATE message from an
   external neighbor, the approach of "attribute discard" SHALL be used
   to handle the unexpected attribute in the message.

5.6.  ATOMIC_AGGREGATE

   The attribute SHALL be considered malformed if its length is not 0
   [RFC4271].

   An UPDATE message with a malformed ATOMIC_AGGREGATE attribute SHALL
   be handled using the approach of "attribute discard".

5.7.  AGGREGATOR

   The error conditions specified in [RFC4271] for the attribute are
   revised as follows:





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   The AGGREGATOR attribute SHALL be considered malformed if any of the
   following applies:

   o  Its length is not 6 (when the "4-octet AS number capability" is
      not advertised to, or not received from the peer [RFC6793]).

   o  Its length is not 8 (when the "4-octet AS number capability" is
      both advertised to, and received from the peer).

   An UPDATE message with a malformed AGGREGATOR attribute SHALL be
   handled using the approach of "attribute discard".

5.8.  Community

   The error handling of [RFC1997] is revised as follows:

   The Community attribute SHALL be considered malformed if its length
   is nonzero and is not a multiple of 4.

   An UPDATE message with a malformed Community attribute SHALL be
   handled using the approach of "treat-as-withdraw".

5.9.  Extended Community

   The error handling of [RFC4360] is revised as follows:

   The Extended Community attribute SHALL be considered malformed if its
   length is nonzero and is not a multiple of 8.

   An UPDATE message with a malformed Extended Community attribute SHALL
   be handled using the approach of "treat-as-withdraw".

   Note that a BGP speaker MUST NOT treat an unrecognized Extended
   Community Type or Sub-Type as an error.

5.10.  IPv6 Address Specific BGP Extended Community Attribute

   The error handling of [RFC5701] is revised as follows:

   The IPv6 Address Specific Extended Community attribute SHALL be
   considered malformed if its length is nonzero and is not a multiple
   of 20.

   An UPDATE message with a malformed IPv6 Address Specific Extended
   Community attribute SHALL be handled using the approach of "treat-as-
   withdraw".





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   Note that a BGP speaker MUST NOT treat an unrecognized IPv6 Address
   Specific Extended Community Type or Sub-Type as an error.

5.11.  ORIGINATOR_ID

   The error handling of [RFC4456] is revised as follows.

   o  If the ORIGINATOR_ID attribute is received from an external
      neighbor, it SHALL be discarded using the approach of "attribute
      discard", or

   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not equal to 4.  If malformed, the
      UPDATE SHALL be handled using the approach of "treat-as-withdraw".

5.12.  CLUSTER_LIST

   The error handling of [RFC4456] is revised as follows.

   o  If the CLUSTER_LIST attribute is received from an external
      neighbor, it SHALL be discarded using the approach of "attribute
      discard", or

   o  if received from an internal neighbor, it SHALL be considered
      malformed if its length is not a multiple 4.  If malformed, the
      UPDATE SHALL be handled using the approach of "treat-as-withdraw".

6.  IANA Considerations

   This document makes no request of IANA.

7.  Security Considerations

   This specification addresses the vulnerability of a BGP speaker to a
   potential attack whereby a distant attacker can generate a malformed
   optional transitive attribute that is not recognized by intervening
   routers (which thus propagate the attribute unchecked) but that
   causes session resets when it reaches routers that do recognize the
   given attribute type.

   In other respects, this specification does not change BGP's security
   characteristics.

8.  Acknowledgements

   The authors wish to thank Juan Alcaide, Ron Bonica, Mach Chen, Andy
   Davidson, Bruno Decraene, Dong Jie, Rex Fernando, Joel Halpern, Akira
   Kato, Miya Kohno, Tony Li, Alton Lo, Shin Miyakawa, Tamas Mondal,



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   Jonathan Oddy, Robert Raszuk, Yakov Rekhter, Eric Rosen, Rob Shakir,
   Naiming Shen, Shyam Sethuram, Ananth Suryanarayana, Kaliraj
   Vairavakkalai and Lili Wang for their observations and discussion of
   this topic, and review of this document.

9.  Normative References

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

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

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

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

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

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760, January
              2007.

   [RFC5701]  Rekhter, Y., "IPv6 Address Specific BGP Extended Community
              Attribute", RFC 5701, November 2009.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December
              2012.

Appendix A.  Why not discard UPDATE messages?

   A commonly asked question is "why not simply discard the UPDATE
   message instead of treating it like a withdraw?  Isn't that safer and
   easier?"  The answer is that it might be easier, but it would
   compromise BGP's correctness so is unsafe.  Consider the following
   example of what might happen if UPDATE messages carrying bad
   attributes were simply discarded:









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                                AS1 ---- AS2
                                  \      /
                                   \    /
                                    \  /
                                     AS3

                                 Figure 1

   o  AS1 prefers to reach AS3 directly, and advertises its route to
      AS2.

   o  AS2 prefers to reach AS3 directly, and advertises its route to
      AS1.

   o  Connections AS3-AS1 and AS3-AS2 fail simultaneously.

   o  AS1 switches to prefer AS2's route, and sends an update message
      which includes a withdraw of its previous announcement.  The
      withdraw is bundled with some advertisements.  It includes a bad
      attribute.  As a result, AS2 ignores the message.

   o  AS2 switches to prefer AS1's route, and sends an update message
      which includes a withdraw of its previous announcement.  The
      withdraw is bundled with some advertisements.  It includes a bad
      attribute.  As a result, AS1 ignores the message.

   The end result is that AS1 forwards traffic for AS3 towards AS2, and
   AS2 forwards traffic for AS3 towards AS1.  This is a permanent (until
   corrected) forwarding loop.

   Although the example above discusses route withdraws, we observe that
   in BGP the announcement of a route also withdraws the route
   previously advertised.  The implicit withdraw can be converted into a
   real withdraw in a number of ways; for example, the previously-
   announced route might have been accepted by policy, but the new
   announcement might be rejected by policy.  For this reason, the same
   concerns apply even if explicit withdraws are removed from
   consideration.

Authors' Addresses

   Enke Chen (editor)
   Cisco Systems, Inc.

   Email: enkechen@cisco.com






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   John G. Scudder (editor)
   Juniper Networks

   Email: jgs@juniper.net


   Pradosh Mohapatra
   Cumulus Networks, Inc.

   Email: pmohapat@cumulusnetworks.com


   Keyur Patel
   Cisco Systems, Inc.

   Email: keyupate@cisco.com



































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