Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Standards Track L. Ginsberg
Expires: December 7, 2019 P. Wells
Cisco Systems
June 5, 2019
Hierarchical IS-IS
draft-li-lsr-isis-hierarchical-isis-01
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
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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) . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. IS-IS Area Identifier TLV . . . . . . . . . . . . . . . . . . 5
5. New Flooding Scopes . . . . . . . . . . . . . . . . . . . . . 5
6. Inheritance of TLVs . . . . . . . . . . . . . . . . . . . . . 6
7. Relationship between levels . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9.1. PDU Type . . . . . . . . . . . . . . . . . . . . . . . . 7
9.2. New PDUs . . . . . . . . . . . . . . . . . . . . . . . . 7
9.3. New TLVs . . . . . . . . . . . . . . . . . . . . . . . . 7
9.4. New Flooding Scopes . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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
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
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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
4. Level 4
5. Level 5
6. Level 6
7. Level 7
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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): AA3
Level 4 (L4-LAN-HELLO-PDU): AA4
Level 5 (L5-LAN-HELLO-PDU): AA5
Level 6 (L6-LAN-HELLO-PDU): AA6
Level 7 (L7-LAN-HELLO-PDU): AA7
Level 8 (L8-LAN-HELLO-PDU): AA8
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 indiate 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 TTT with the same format. Both PDUs may be used on the same
circuit.
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4. IS-IS Area Identifier TLV
The Area Identifier TLV is added to IS-IS to allow nodes to indicate
which areas they participate in. Area Identifiers are locally
administered 32 bit numbers. The format of the TLV is:
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 | Area |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
TLV Type: ZZZ
TLV Length: 7
Level: The level number of the area.
Area Identifier: The identifier associated with the area.
The Area Identifier TLV may appear in IIHs or in LSPs. When the Area
Identifier TLV appears in a PDU, it indicates that the system is
participating in the specified area at the indicated level. When the
Area Identifier TLV appears in a IIH, the receiving system MUST NOT
form an adjacency unless an Area Identifier TLV corresponds to the
receiver's own Area Identifier for the given level.
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. 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.
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7. Relationship between levels
The relationship between Level n and Level n-1 is analogous to the
relationship between Level 2 and Level 1.
8. Acknowledgements
The author would like to thank Dinesh Dutt for inspiring this
document. The author would also like to thank Les Ginsberg and Paul
Wells for their helpful comments.
9. IANA Considerations
This document makes many requests to IANA, as follows:
9.1. 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.
9.2. New PDUs
IANA is requested to allocate values from the IS-IS PDU registry for
the following:
L3-LAN-HELLO-PDU: AA3
L4-LAN-HELLO-PDU: AA4
L5-LAN-HELLO-PDU: AA5
L6-LAN-HELLO-PDU: AA6
L7-LAN-HELLO-PDU: AA7
L8-LAN-HELLO-PDU: AA8
Ln-P2P-HELLO-PDU: TTT
To allow for PDU types to be defined independent of this document,
the above values should be allocated from the range 32-254.
9.3. New TLVs
IANA is requested to allocate values from the IS-IS TLV registry for
the following:
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Area Identifier: ZZZ
9.4. New Flooding Scopes
IANA is requested to allocate the following values from the IS-IS
Flooding Scope Identifier Registry.
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
10. 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.
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11. 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>.
[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
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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