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Versions: (draft-li-lsr-isis-hierarchical-isis) 00 01 02

Internet Engineering Task Force                                    T. Li
Internet-Draft                                           Arista Networks
Intended status: Standards Track                             L. Ginsberg
Expires: January 6, 2021                                        P. Wells
                                                           Cisco Systems
                                                            July 5, 2020


                        IS-IS Extended Hierarchy
               draft-ietf-lsr-isis-extended-hierarchy-02

Abstract

   The IS-IS routing protocol was originally defined with a two level
   hierarchical structure.  This was adequate for the networks at the
   time.  As we continue to expand the scale of our networks, it is
   apparent that additional hierarchy would be a welcome degree of
   flexibility in network design.

   This document defines IS-IS Levels 3 through 8.

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 https://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 January 6, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://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



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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  PDU changes . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Circuit Type  . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  PDU Type  . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Additional PDUs . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Level n LAN IS to IS hello PDU (Ln-LAN-HELLO-PDU) . . . .   5
     3.2.  Level n Point-to-point IS to IS hello PDU (Ln-P2P-HELLO-
           PDU)  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Level Specific Area Identifiers . . . . . . . . . . . . . . .   5
     4.1.  IS-IS Area Hierarchy TLV  . . . . . . . . . . . . . . . .   6
     4.2.  Adjacency Formation Rules . . . . . . . . . . . . . . . .   8
       4.2.1.  Level 3-8 Adjacency Formation Rules . . . . . . . . .   8
       4.2.2.  Special Level-1 and Level-2 Adjacency Formation Rules   9
         4.2.2.1.  Actions on a Point-to-Point Circuit . . . . . . .   9
         4.2.2.2.  Actions on a LAN Circuit  . . . . . . . . . . . .   9
         4.2.2.3.  Reporting of Mismatched Area Hierarchies  . . . .   9
   5.  New Flooding Scopes . . . . . . . . . . . . . . . . . . . . .  10
   6.  MAC Addresses . . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Inheritance of TLVs . . . . . . . . . . . . . . . . . . . . .  13
   8.  Behavior of Level n . . . . . . . . . . . . . . . . . . . . .  13
   9.  Relationship between levels . . . . . . . . . . . . . . . . .  13
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     11.1.  PDU Type . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.2.  New PDUs . . . . . . . . . . . . . . . . . . . . . . . .  13
     11.3.  New TLVs . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.4.  New Flooding Scopes  . . . . . . . . . . . . . . . . . .  14
     11.5.  New MAC Addresses  . . . . . . . . . . . . . . . . . . .  15
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   13. Normative References  . . . . . . . . . . . . . . . . . . . .  16
   Appendix A.  Preventing Cross Branching in the Hierarchy  . . . .  16
   Appendix B.  Guidelines for Introducing a new level . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   The IS-IS routing protocol IS-IS [ISO10589] currently supports a two
   level hierarchy of abstraction.  The fundamental unit of abstraction
   is the 'area', which is a (hopefully) connected set of systems




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   running IS-IS at the same level.  Level 1, the lowest level, is
   abstracted by routers that participate in both Level 1 and Level 2.

   Practical considerations, such as the size of an area's link state
   database, cause network designers to restrict the number of routers
   in any given area.  Concurrently, the dominance of scale-out
   architectures based around small routers has created a situation
   where the scalability limits of the protocol are going to become
   critical in the foreseeable future.

   The goal of this document is to enable additional hierarchy within
   IS-IS.  Each additional level of hierarchy has a multiplicative
   effect on scale, so the addition of six levels should be a
   significant improvement.  While all six levels may not be needed in
   the short term, it is apparent that the original designers of IS-IS
   reserved enough space for these levels, and defining six additional
   levels is only slightly harder than adding a single level, so it
   makes sense to expand the design for the future.

   The modifications described herein are designed to be fully backward
   compatible and have no effect on existing networks.  The
   modifications are also designed to have no effect whatsoever on
   networks that only use Level 1 and/or Level 2.

   Section references in this document are references to sections of IS-
   IS [ISO10589].

   Note that [ISO10589] uses a bit encoding convention where bit numbers
   are 1 based and Bit 1 is the Least Significant Bit (LSB) of the
   datatype.  Traditionally IETF documents have used a bit encoding
   convention where bit numbers are 0 based and Bit 0 is the Most
   Significant Bit (MSB) of the datatype.  This document uses [ISO10589]
   conventions throughout.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  PDU changes

   In this section, we enumerate all of the redefinitions of protocol
   header fields necessary to add additional levels.





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2.1.  Circuit Type

   In the fixed header of some IS-IS PDUs, a field is named 'Reserved/
   Circuit Type' (Section 9.5).  The high order six bits are reserved,
   with the low order two bits indicating Level 1 (bit 1) and Level 2
   (bit 2).

   This field is renamed to be 'Circuit Type'.  The bits are redefined
   as follows:

   1.  Level 1

   2.  Level 2

   3.  Level 3

   4.  Level 4

   5.  Level 5

   6.  Level 6

   7.  Level 7

   8.  Level 8

   The value of zero (no bits set) is reserved.  PDUs with a Circuit
   Type of zero SHALL be ignored.

   The set bits of the Circuit Type MUST be contiguous.  If bit n and
   bit m are set in the Circuit Type, then all bits in the interval
   [n:m] must be set.

2.2.  PDU Type

   The fixed header of IS-IS PDUs contains an octet with three reserved
   bits and the 'PDU Type' field.  The three reserved bits are
   transmitted as zero and ignored on receipt.  (Section 9.5)

   To allow for additional PDU space, this entire octet is renamed the
   'PDU Type' field.

3.  Additional PDUs








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3.1.  Level n LAN IS to IS hello PDU (Ln-LAN-HELLO-PDU)

   The 'Level n LAN IS to IS hello PDU' (Ln-LAN-HELLO-PDU) is identical
   in format to the 'Level 2 LAN IS to IS hello PDU' (Section 9.6),
   except that the PDU Types are defined as follows:

      Level 3 (L3-LAN-HELLO-PDU): 33 (Suggested - to be assigned by
      IANA)

      Level 4 (L4-LAN-HELLO-PDU): 34 (Suggested - to be assigned by
      IANA)

      Level 5 (L5-LAN-HELLO-PDU): 35 (Suggested - to be assigned by
      IANA)

      Level 6 (L6-LAN-HELLO-PDU): 36 (Suggested - to be assigned by
      IANA)

      Level 7 (L7-LAN-HELLO-PDU): 37 (Suggested - to be assigned by
      IANA)

      Level 8 (L8-LAN-HELLO-PDU): 38 (Suggested - to be assigned by
      IANA)

   The Circuit Type field MUST be set to indicate all levels supported
   on that circuit - not just the level associated with the containing
   PDU type.

3.2.  Level n Point-to-point IS to IS hello PDU (Ln-P2P-HELLO-PDU)

   The 'Point-to-point IS to IS hello PDU' (Section 9.7) is used on
   Level 1 and Level 2 circuits.  Legacy systems will not expect the
   circuit type field to indicate other levels, so a new PDU is used if
   the circuit supports other levels.  The additional PDU is the 'Level
   n Point-to-point IS to IS hello PDU' (Ln-P2P-HELLO-PDU) and has PDU
   Type 39 (Suggested - to be assigned by IANA).  The format of this PDU
   is identical to the existing Point-to-Point IS to IS hello PDU.  Both
   PDUs may be used on the same circuit.

4.  Level Specific Area Identifiers

   [ISO10589] defines an Area Address to uniquely identify a Level-1
   area.  A given area may have multiple synonymous area addresses -
   which is useful in support of hitless merging or splitting of areas.
   Area address matching is part of the adjacency formation rules
   defined in Section 8 which determine whether a given adjacency
   supports Level-1, Level-2, or both.  Area addresses are advertised in
   IIHs and LSPs using the Area Address TLV.



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   With the extensions defined in this document, there is a need to
   define an equivalent identifier for Levels 2-8.  The Level Specific
   Area Identifier (LSAI) is a 16 bit value and is advertised using the
   new Area Hierarchy TLV defined in Section 4.1.  There is no
   relationship between a Level-1 Area Address and an LSAI.

   Just as with Area Addresses, multiple synonomous LSAIs may be
   assigned to a given level.  This supports hitless merging or
   splitting of the level specific area.  Although it is legal to do so,
   it is generally not useful to define more than two Area Identifiers
   for a given level.

   A node MAY support any set of contiguous levels.  Support for non-
   contiguous levels is undefined.

4.1.  IS-IS Area Hierarchy TLV

   The Area Hierarchy TLV specifies the set of LSAIs which comprise the
   branch of the network hierarchy to which the advertising node is
   connected.  The TLV MUST include at least one LSAI for Levels 2-N,
   where N is >= 2 and N represents the highest level supported in the
   IS-IS domain.  It is RECOMMENDED that N == 8 even when not all 8
   levels are currently in use, but in cases where a network does not
   support higher levels a number less than 8 MAY be used.

   Note that the levels advertised MAY include levels which are not
   supported by the advertising node.

   The Area Hierarchy TLV has the following format:






















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       8 7 6 5 4 3 2 1
      +-+-+-+-+-+-+-+-+
      |   TLV Type    |
      +-+-+-+-+-+-+-+-+
      | TLV Length    |
      +-+-+-+-+-+-+-+-+
      | Supp-Levels   |
      +-+-+-+-+-+-+-+-+

   Followed by one or more Level Specific Area ID Sets:

       1             0
       6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Level     | # of LSAIs    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Level Specific Area Id(s)      |
      ...
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      TLV Type: ZZZ (1 octet)

      TLV Length: Variable (1 octet)

      Supp-Levels: A contiguous bitmask representing the set of levels
        supported by the advertising node (1 octet)
        Bit #8 of this field is set if Level 8 is supported.
        Bit #7 of this field is set if Level 7 is supported.
        Bit #6 of this field is set if Level 6 is supported.
        Bit #5 of this field is set if Level 5 is supported.
        Bit #4 of this field is set if Level 4 is supported.
        Bit #3 of this field is set if Level 3 is supported.
        Bit #2 of this field is set if Level 2 is supported.
        Bit #1 of this field is set if Level 1 is supported

       If the Supp-level bit mask is non-contiguous all advertised LSAIs
       are ignored.

   Each Level Specific Area ID Set consists of:

       Level: 2-8 (1 octet)
       # of LSAI: >=1 (1 octet)
       LSAIs: The set of synonomous LSAIs associated with this level
         (2 * # of LSAIs octets)







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   The Area Hierarchy TLV MUST appear in all new IIH PDUs defined in
   Section 3.  It MAY appear in P2P-HELLO-PDUs, L1-LAN-HELLO-PDUs, or
   L2-LAN-HELLO-PDUs.

   The Area Hierarchy TLV MUST appear in LSP #0 of non-pseudo-node Level
   3-8 Flooding Scoped LSPs defined in Section 5.  It MAY appear in L1
   or L2 LSP #0.  It MUST NOT be present in any LSP with non-zero LSP
   number.  If present in an LSP with non-zero LSP number it MUST be
   ignored on receipt.

   Multiple Area Hierarchy TLVs MUST NOT be sent.  In the event multiple
   Area Hierarchy TLVs are received, the first such TLV in the PDU is
   used.  Subsequent TLVs in the same PDU MUST be ignored.

4.2.  Adjacency Formation Rules

   Adjacency formation rules for Levels 1 and 2 are defined in
   [ISO10589] and are not altered by these extensions except where noted
   below.

   Adjacency Formation rules for Levels 3 and above are defined to
   insure that adjacency support for a given level is only enabled when
   there is a matching Area Identifier.  Adjacency formation rules also
   are defined so as to prevent interconnection of neighbors which will
   connect to different areas at levels above any supported level.

   The checks discussed below need to be performed on receipt of an IIH.

4.2.1.  Level 3-8 Adjacency Formation Rules

   The Area Hierarchy TLV MUST be present in a Level N Point-to-point IS
   to IS hello PDU or a Level N LAN IS to IS Hello PDU and the TLV
   content MUST adhere to the definition in Section 4.1.  Beginning with
   the lowest level supported by the receiving node on this circuit and
   including all higher levels for which the receiver has an assigned
   LSAI regardless as to whether the higher levels are supported on this
   circuit, the set of LSAIs defined on the receiving node is compared
   against the set of LSAIs advertised in the received TLV.  A matching
   LSAI MUST be found for each level.

   If all of the checks pass then a new adjacency is formed or an
   existing adjacency is maintained.

   NOTE: The absence of the advertisement of an LSAI for a given level
   is considered as a failure to find a matching LSAI.

   On a Point-to-Point circuit, a single adjacency is formed which
   supports all of the levels supported by both nodes on this circuit.



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   On a LAN circuit, an adjacency is formed supporting only the level
   specified by the PDU type.

   Note that (as previously specified) the set of levels advertised MUST
   be contiguous.

4.2.2.  Special Level-1 and Level-2 Adjacency Formation Rules

   The Area Hierarchy TLV MAY appear in a Point-to-point IS to IS hello
   PDU, Level 1 LAN IS to IS Hello PDU, or Level 2 LAN IS to IS Hello
   PDU (PDUs specified in [ISO10589]).  In such a case, the neighbor may
   or may not support the Area Hierarchy TLV.  The following sub-
   sections define modified adjacency formation rules for point-to-point
   and LAN circuits.

4.2.2.1.  Actions on a Point-to-Point Circuit

   If the Area Hierarchy TLV is present, then in addition to the checks
   specified in [ISO10589] the checks specified in Section 4.2.1 MUST be
   performed for all levels for which the receiver has an assigned LSAI
   beginning with Level 2.  If those checks fail an adjacency MUST NOT
   be formed and any existing matching adjacency MUST transition to DOWN
   state.

4.2.2.2.  Actions on a LAN Circuit

   Adjacency formation MUST follow the rules defined in [ISO10589].  If
   the Area Hierarchy TLV is present in the Level 1 or Level 2 LAN IS to
   IS Hello PDU then the checks specified in Section 4.2.1 SHOULD be
   performed for all levels for which the receiver has an assigned LSAI
   beginning with Level 2.  If those checks fail an error SHOULD be
   reported, but the level specific adjacency is still allowed.  This
   prevents violation of the assumption of transitivity on the LAN in
   the presence of systems which do not support the extensions defined
   in this document.

4.2.2.3.  Reporting of Mismatched Area Hierarchies

   When forming adjacencies at Level-1 and/or Level-2, it is possible to
   have a mixture of legacy nodes (which do NOT support the extensions
   defined in this document) and new nodes which do support the
   extensions.

   In Point-to-Point mode, legacy nodes will not advertise the new Area
   Hierarchy TLV and will not have an assigned LSAI for Level-2.  It
   then becomes possible for new nodes with mismatched Area Hierarchies
   to form adjacencies with legacy nodes and form an L1 or L2 area where
   not all new nodes have a matching Area Hierarchy.  This cannot be



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   detected when forming adjacencies if the new nodes are not directly
   connected - but it can be detected after the adjacencies have been
   formed by inspecting the set of Area Hierarchy TLVs in the level
   specific LSPs of all routers in the area.

   Similarly in LAN mode, the transitivity requirement means that new
   nodes MUST form adjacencies with all nodes connected to the LAN even
   when the Area Hierarchy TLV mismatch check fails (see
   Section 4.2.2.2).  This can occur both at Level-1 and Level-2.

   New nodes MUST report these inconsistencies.

5.  New Flooding Scopes

   For levels 3-8, all link state information, PSNPs, and CSNPs are
   relayed in conformance with [RFC7356].  Additional flooding scopes
   are defined for each new level, for both circuit flooding scope and
   level flooding scope.  Level flooding scopes are defined for both
   Standard and Extended TLV formats.  The list of additional flooding
   scopes is:































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                                          FS LSP ID Format/
     Value Description                    TLV Format
     ----- ------------------------------ -----------------
     6     Level 3 Circuit Flooding Scope Extended/Standard
     7     Level 4 Circuit Flooding Scope Extended/Standard
     8     Level 5 Circuit Flooding Scope Extended/Standard
     9     Level 6 Circuit Flooding Scope Extended/Standard
     10    Level 7 Circuit Flooding Scope Extended/Standard
     11    Level 8 Circuit Flooding Scope Extended/Standard
     12    Level 3 Flooding Scope         Extended/Standard
     13    Level 4 Flooding Scope         Extended/Standard
     14    Level 5 Flooding Scope         Extended/Standard
     15    Level 6 Flooding Scope         Extended/Standard
     16    Level 7 Flooding Scope         Extended/Standard
     17    Level 8 Flooding Scope         Extended/Standard
     18    Level 3 Flooding Scope         Standard/Standard
     19    Level 4 Flooding Scope         Standard/Standard
     20    Level 5 Flooding Scope         Standard/Standard
     21    Level 6 Flooding Scope         Standard/Standard
     22    Level 7 Flooding Scope         Standard/Standard
     23    Level 8 Flooding Scope         Standard/Standard
     70    Level 3 Circuit Flooding Scope Extended/Extended
     71    Level 4 Circuit Flooding Scope Extended/Extended
     72    Level 5 Circuit Flooding Scope Extended/Extended
     73    Level 6 Circuit Flooding Scope Extended/Extended
     74    Level 7 Circuit Flooding Scope Extended/Extended
     75    Level 8 Circuit Flooding Scope Extended/Extended
     76    Level 3 Flooding Scope         Extended/Extended
     77    Level 4 Flooding Scope         Extended/Extended
     78    Level 5 Flooding Scope         Extended/Extended
     79    Level 6 Flooding Scope         Extended/Extended
     80    Level 7 Flooding Scope         Extended/Extended
     81    Level 8 Flooding Scope         Extended/Extended

   The final octet of the header of a Flooding Scoped LSP as defined in
   [RFC7356] contains Reserved/LSPDBOL/IS Type information.  This field
   is redefined for the new flooding scopes defined in this document as
   follows:













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   Reserved/ATT/LSPDBOL

     Bits 8-5 Reserved
      Transmitted as 0 and ignored on receipt
     Bit 4 ATT
       If set to 1 indicates that the sending IS is attached to
       routers in other Level N areas via Level N+1
     Bit 3 LSDBOL
       As defined in RFC7356
     Bits 2-1
       Transmitted as 0 and ignored on receipt.


   Note that the levels supported (analogous to the IS-type information
   in L1 and L2 LSPs) can be obtained from the Area Hierarchy TLV
   advertised in the associated LSP #0.

   Note that the definition of the ATT bit specified above also applies
   to L2 LSPs.  Previously this bit would have no meaning as [ISO10589]
   does not define support for Level 3.

6.  MAC Addresses

   On a broadcast network, PDUs are currently sent to the AllL1Iss or
   AllL2Iss MAC addresses.  We will need additional MAC addresses for
   Levels 3-8.

      AllL3ISs: MAC3

      AllL4ISs: MAC4

      AllL5ISs: MAC5

      AllL6ISs: MAC6

      AllL7ISs: MAC7

      AllL8ISs: MAC8

   When operating in Point-to-Point mode on a broadcast network
   [RFC5309], a Level N Point-to-Point Hello PDU will be sent.  Any of
   the above MAC addresses could be used in this case, but it is
   recommended to use the AllL3ISs MAC address.








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7.  Inheritance of TLVs

   All existing Level 2 TLVs may be used in the corresponding Level 3
   through Level 8 PDUs.  When used in a Level 3 through Level 8 PDU,
   the semantics of these TLVs will be applied to the Level of the
   containing PDU.  If the original semantics of the PDU was carrying a
   reference to Level 1 in a Level 2 TLV, then the semantics of the TLV
   at level N will be a reference to level N-1.  The intent is to retain
   the original semantics of the TLV at the higher level.

8.  Behavior of Level n

   The behavior of Level n is analogous to the behavior of Level 2.

9.  Relationship between levels

   The relationship between Level n and Level n-1 is analogous to the
   relationship between Level 2 and Level 1.

   An area at Level n has at most one parent at Level n+1.

10.  Acknowledgements

   The authors would like to thank Dinesh Dutt for inspiring this
   document and Huaimo Chen for his comments.  The authors would also
   like to thank Tony Pryzienda for his careful review and excellent
   suggestions.

11.  IANA Considerations

   This document makes many requests to IANA, as follows:

11.1.  PDU Type

   The existing IS-IS PDU registry currently supports values 0-31.  This
   should be expanded to support the values 0-255.  The existing value
   assignments should be retained.  Value 255 should be reserved.

11.2.  New PDUs

   IANA is requested to allocate values from the IS-IS PDU registry for
   the following:

      L3-LAN-HELLO-PDU: 33 (Suggested - to be assigned by IANA)

      L4-LAN-HELLO-PDU: 34 (Suggested - to be assigned by IANA)

      L5-LAN-HELLO-PDU: 35 (Suggested - to be assigned by IANA)



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      L6-LAN-HELLO-PDU: 36 (Suggested - to be assigned by IANA)

      L7-LAN-HELLO-PDU: 37 (Suggested - to be assigned by IANA)

      L8-LAN-HELLO-PDU: 38 (Suggested - to be assigned by IANA)

      Ln-P2P-HELLO-PDU: 39 (Suggested - to be assigned by IANA)

11.3.  New TLVs

   IANA is requested to allocate values from the IS-IS TLV registry for
   the following:

      Area Hierarchy: ZZZ

11.4.  New Flooding Scopes

   IANA is requested to allocate the following values from the IS-IS
   Flooding Scope Identifier Registry.
































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                                         FS LSP ID Format/ IIH Announce
    Value Description                    TLV Format        Lx-P2P Lx-LAN
    ----- ------------------------------ ----------------- ------ ------
    6     Level 3 Circuit Flooding Scope Extended/Standard  Y      Y
    7     Level 4 Circuit Flooding Scope Extended/Standard  Y      Y
    8     Level 5 Circuit Flooding Scope Extended/Standard  Y      Y
    9     Level 6 Circuit Flooding Scope Extended/Standard  Y      Y
    10    Level 7 Circuit Flooding Scope Extended/Standard  Y      Y
    11    Level 8 Circuit Flooding Scope Extended/Standard  Y      Y
    12    Level 3 Flooding Scope         Extended/Standard  Y      Y
    13    Level 4 Flooding Scope         Extended/Standard  Y      Y
    14    Level 5 Flooding Scope         Extended/Standard  Y      Y
    15    Level 6 Flooding Scope         Extended/Standard  Y      Y
    16    Level 7 Flooding Scope         Extended/Standard  Y      Y
    17    Level 8 Flooding Scope         Extended/Standard  Y      Y
    18    Level 3 Flooding Scope         Standard/Standard  Y      Y
    19    Level 4 Flooding Scope         Standard/Standard  Y      Y
    20    Level 5 Flooding Scope         Standard/Standard  Y      Y
    21    Level 6 Flooding Scope         Standard/Standard  Y      Y
    22    Level 7 Flooding Scope         Standard/Standard  Y      Y
    23    Level 8 Flooding Scope         Standard/Standard  Y      Y
    70    Level 3 Circuit Flooding Scope Extended/Extended  Y      Y
    71    Level 4 Circuit Flooding Scope Extended/Extended  Y      Y
    72    Level 5 Circuit Flooding Scope Extended/Extended  Y      Y
    73    Level 6 Circuit Flooding Scope Extended/Extended  Y      Y
    74    Level 7 Circuit Flooding Scope Extended/Extended  Y      Y
    75    Level 8 Circuit Flooding Scope Extended/Extended  Y      Y
    76    Level 3 Flooding Scope         Extended/Extended  Y      Y
    77    Level 4 Flooding Scope         Extended/Extended  Y      Y
    78    Level 5 Flooding Scope         Extended/Extended  Y      Y
    79    Level 6 Flooding Scope         Extended/Extended  Y      Y
    80    Level 7 Flooding Scope         Extended/Extended  Y      Y
    81    Level 8 Flooding Scope         Extended/Extended  Y      Y

11.5.  New MAC Addresses

   IANA is requested to allocate values from the IANA Multicast 48-bit
   MAC Addresses block for the following:

      AllL3Iss: MAC3

      AllL4Iss: MAC4

      AllL5Iss: MAC5

      AllL6Iss: MAC6

      AllL7Iss: MAC7



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      AllL8Iss: MAC8

12.  Security Considerations

   This document introduces no new security issues.  Security of routing
   within a domain is already addressed as part of the routing protocols
   themselves.  This document proposes no changes to those security
   architectures.

13.  Normative References

   [ISO10589]
              International Organization for Standardization,
              "Intermediate System to Intermediate System Intra-Domain
              Routing Exchange Protocol for use in Conjunction with the
              Protocol for Providing the Connectionless-mode Network
              Service (ISO 8473)", ISO/IEC 10589:2002, Nov. 2002.

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

   [RFC5309]  Shen, N., Ed. and A. Zinin, Ed., "Point-to-Point Operation
              over LAN in Link State Routing Protocols", RFC 5309,
              DOI 10.17487/RFC5309, October 2008,
              <https://www.rfc-editor.org/info/rfc5309>.

   [RFC7356]  Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,
              <https://www.rfc-editor.org/info/rfc7356>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Appendix A.  Preventing Cross Branching in the Hierarchy

   The use of additional levels requires careful interconnection of
   routers which support multiple levels.  Consistent association of
   LSAIs is required not only for validating the connections between
   routers in a level specific area but also for all levels above a
   given level to which any of the routers may be connected (directly or
   indirectly).  Failure to do so can result in interconnecting
   different branches of a tree leading to interarea loops.  This leads
   to the requirement that all routers advertise an LSAI for all levels




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   regardless of whether a given router is configured to participate in
   a given level or not.

   At first glance it may seem that it would be sufficient for each
   router to advertise LSAIs only for the levels that the router is
   configured to support.  However, the following simple example
   illustrates why this is problematic.

      +------------+   +------------+   +------------+
      | Rtr  A     |   | Rtr  B     |   | Rtr  C     |
      | L3 Area 30 |---| L3 Area 30 |---| L3 Area 30 |
      | L4 Area 40 |   |            |   | L4 Area 44 |
      +------------+   +------------+   +------------+


   Since Router B does not support Level 4, it chose not to advertise
   any Area for Level 4.  This means that neither Router A nor Router C
   can tell by inspecting hellos that not all routers in Level 3 area 30
   have been configured to support the same Level 4 area.  It is
   possible for Rtr A and Rtr C to discover the LSAIs advertised by all
   routers by inspecting the Level 3 LSPs - however this requires that
   Level 3 adjacencies be formed and maintained even when routing cannot
   be safely performed via all adjacencies in a given area.  It then
   needs to be decided how routing over existing adjacencies should be
   limited.  A number of possibilities exist:

      Treat the area as if it were two partitions.  In the example
      Router A would be in one partition and Router C would be in
      another partition.  But Router B could belong to either partition.

      Select a winning Level 4 Area among the set of Level 4 areas
      advertised in L3 LSPs and only allow leaking of routes to/from
      that level

   But either of these options introduce the possibility that a
   previously fully connected hierarchy becomes partially disconnected
   as a result of a single configuration change on a single router and/
   or the bringup of a new router.

   The choice made was then to require all routers suppporting the
   extensions in this document to advertise an LSAI for all levels
   regardless of what specific levels an individual router is configured
   to support.  This guarantees that any inconsistency between the
   intended connectivity of a router at all levels - direct and indirect
   - can be detected during exchange of hellos and therefore adjacency
   bringup can always be blocked when necessary.





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Appendix B.  Guidelines for Introducing a new level

   It is desirable to be able to introduce support for a new level
   without disruption.  This section discusses ways to do this.

   Initial deployment may require only the support of one additional
   level (Level 3).  However, in the future increased network scale may
   make introduction of an additional level (Level 4) desirable.  It is
   suggested that all routers be configured to advertise a single
   candidate LSAI for Level 4 - for the purposes of the example let's
   use LSAI 44.  When ready to deploy Level 4, it is then only necessary
   to enable Level 4 on those routers who will be participating in the
   additional level.

   However, perhaps at the time of deploying Level 3 the administrator
   has no idea what LSAI will be used for Level 4 in the future.  In
   such a case a "dummy" LSAI should be configured for Level 4 on all
   routers - let's use "0" in this example.  In this case, what needs to
   be done when ready to enable Level 4 is to go to every router
   (regardless of whether it will actively participate in the new level)
   and configure the intended LSAI for Level 4.  If LSAI 45 is the
   intended Level 4 area, then LSAI 45 is configured on each router.
   Each router is then advertising two LSAIs for Level 4: (0, 45).  Once
   this is completed, go to every router and remove the "dummy" Level 4
   LSAI (0) and the network is now ready to have this Level 4 area
   enabled.

   In the event that support for a new level needs to be introduced and
   no LSAI was ever advertised for that level, the introduction of LSAI
   for the new level will cause temporary adjacency flaps as the
   advertisement of the LSAI for the new level is introduced.  To avoid
   this, implementations would need to introduce support for temporary
   disablement of the LSAI check for the new level until the
   configuration of the new LSAI is complete on all nodes.  Support for
   this transition mode is outside the scope of this document.  The need
   for a transition mode can be avoided if an LSAI is configured for
   levels 2-8 from day one.

Authors' Addresses

   Tony Li
   Arista Networks
   5453 Great America Parkway
   Santa Clara, California  95054
   United States of America

   Email: tony.li@tony.li




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   Les Ginsberg
   Cisco Systems
   United States of America

   Email: ginsberg@cisco.com


   Paul Wells
   Cisco Systems
   United States of America

   Email: pauwells@cisco.com







































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