Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Standards Track L. Ginsberg
Expires: May 2, 2020 P. Wells
Cisco Systems
October 30, 2019
IS-IS Extended Hierarchy
draft-ietf-lsr-isis-extended-hierarchy-00
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
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This Internet-Draft will expire on May 2, 2020.
Copyright Notice
Copyright (c) 2019 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
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. PDU changes . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Circuit Type . . . . . . . . . . . . . . . . . . . . . . 3
2.2. PDU Type . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Additional PDUs . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Level n LAN IS to IS hello PDU (Ln-LAN-HELLO-PDU) . . . . 4
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 Identifier TLV . . . . . . . . . . . . . . . . 5
4.2. Adjacency Formation Rules . . . . . . . . . . . . . . . . 6
4.2.1. Level 3-8 Adjacency Formation Rules . . . . . . . . . 7
4.2.2. Special Level-2 Adjacency Formation Rules . . . . . . 7
5. New Flooding Scopes . . . . . . . . . . . . . . . . . . . . . 7
6. MAC Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
7. Inheritance of TLVs . . . . . . . . . . . . . . . . . . . . . 9
8. Behavior of Level n . . . . . . . . . . . . . . . . . . . . . 9
9. Relationship between levels . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11.1. PDU Type . . . . . . . . . . . . . . . . . . . . . . . . 9
11.2. New PDUs . . . . . . . . . . . . . . . . . . . . . . . . 10
11.3. New TLVs . . . . . . . . . . . . . . . . . . . . . . . . 10
11.4. New Flooding Scopes . . . . . . . . . . . . . . . . . . 10
11.5. New MAC Addresses . . . . . . . . . . . . . . . . . . . 11
12. Security Considerations . . . . . . . . . . . . . . . . . . . 12
13. Normative References . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
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
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
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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].
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].
2. PDU changes
In this section, we enumerate all of the redefinitions of protocol
header fields necessary to add additional levels.
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
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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
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)
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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.
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.
With the extensions defined in this document, there is a need to
define an equivalent identifier for Levels 2-8. This identifier is a
32 bit value and is advertised using the new Area Identifier TLV
defined in the following section. There is no relationship between
the Level-1 Area Addresses and the new Level Specific Area
Identifier.
Just as with Area Addresses, multiple synonomous Area Identifiers 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.
4.1. IS-IS Area Identifier TLV
The Area Identifier TLV is added to IS-IS to allow nodes to indicate
which areas they participate in for Levels 2-8. Area Identifiers are
locally administered 32 bit numbers. Each level may have multiple
Area Identifiers. The format of the TLV is:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV Type | TLV Length | Level | Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Area Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: ZZZ
TLV Length: ( 2 * number of Levels) + ( 4 * each Count field )
Level: The level number of the area. (1 octet) Legal values are
2-8
Count: The number of Area Identifier fields (1 octet)
Area Identifier: One or more identifiers associated with the area.
(4 * count octets)
The Level/Count/Area Identifier tuple MAY be repeated as necessary.
The Area Identifier TLV MAY appear in all types of IIHs except for
Level 1 LAN IS to IS hellos.
The Area Identifier TLV MAY appear in LSP #0 of non-pseudo-node Level
3-8 Flooding Scoped LSPs defined in Section 5. 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.
A system may participate in more than one level. At a given level,
an area may have a number of synonymous identifiers. A system MUST
advertise all of the levels it supports and the associated Area
Identifiers.
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.
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4.2.1. Level 3-8 Adjacency Formation Rules
When the Area Identifier TLV appears in a Level N Point-to-point IS
to IS hello PDU or a Level N LAN IS to IS Hello PDU, the Circuit Type
field is inspected. For all levels with their corresponding bit set
in the Circuit Type in the range 3-8 the following checks are
performed:
o Check for a matching Area Identifier at the same level
o Check for a matching Area Identifier at all supported levels
greater than the level being checked
If both checks pass, then an adjacency can be formed supporting the
level. If any of the checks fail, then that level MUST NOT be
supported by an adjacency formed on that circuit.
On a Point-to-Point circuit, a single adjacency is formed which
supports all of the levels which pass the above checks.
On a LAN circuit, an adjacency is formed only for the level specified
by the PDU type. Nevertheless, the checks for all levels with the
corresponding bit set in the Circuit Type MUST be performed.
Note that (as previously specified) the set of levels supported MUST
be contiguous.
4.2.2. Special Level-2 Adjacency Formation Rules
The Area Identifier TLV MAY appear in a Point-to-point IS to IS hello
PDU or Level 2 LAN IS to IS Hello PDU (both specified in [ISO10589]).
In such a case, the neighbor may or may not support the Area
Identifier TLV. If the Area Identifier TLV is present and Level 2 is
indicated as being supported in the Circuit Type field, then in
addition to the checks specified in [ISO10589] the checks specified
in the previous section SHOULD be performed for Level 2.
5. New Flooding Scopes
For levels 3-8, all link state information, PSNPs, and CSNPs are
relayed in conformance with RFC 7356 [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
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
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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.
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.
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.
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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)
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 Identifier: 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>.
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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