Internet DRAFT - draft-li-hierarchical-isis
draft-li-hierarchical-isis
Internet Engineering Task Force T. Li
Internet-Draft Arista Networks
Intended status: Informational June 28, 2018
Expires: December 30, 2018
Hierarchical IS-IS
draft-li-hierarchical-isis-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
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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. LAN IS to IS hello PDU (LAN-HELLO-PDU) . . . . . . . . . 4
3.2. Point-to-point IS to IS hello PDU (P2P-HELLO-PDU) . . . . 4
3.3. Level n Link State PDU (Ln-LSP-PDU) . . . . . . . . . . . 4
3.4. Level n complete sequence numbers PDU (Ln-CSNP-PDU) . . . 5
3.5. Level n partial sequence numbers PDU (Ln-PSNP-PDU) . . . 5
4. Inheritance of TLVs . . . . . . . . . . . . . . . . . . . . . 5
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6.1. PDU Type . . . . . . . . . . . . . . . . . . . . . . . . 6
6.2. New PDUs . . . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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 by creating additional hierarchy. Each additional level of
hierarchy has a multiplicative effect on scale, so the addtion 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
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single level, so it makes some sense to expand the design for the
future.
The modifications described herein are designed to be fully backward
compatible.
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
8. Level 8
The value of zero (no bits set) is reserved. PDUs with a Circuit
Type of zero SHALL be ignored.
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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. LAN IS to IS hello PDU (LAN-HELLO-PDU)
The 'LAN IS to IS hello PDU' (LAN-HELLO-PDU) is identical in format
to the 'Level 2 LAN IS to IS hello PDU' (Section 9.6), except that
the PDU Type has value AAA. The LAN-HELLO-PDU MUST be used instead
of the 'Level 1 LAN IS to IS hello PDU' (Section 9.5) or the 'Level 2
LAN IS to IS hello PDU' on any circuit that has one or more of Level
3 through Level 8 enabled.
3.2. Point-to-point IS to IS hello PDU (P2P-HELLO-PDU)
The 'Point-to-point IS to IS hello PDU' can be used on circuits of
any Level without modification.
3.3. Level n Link State PDU (Ln-LSP-PDU)
The 'Level n Link State PDU' (Ln-LSP-PDU) has the same format as the
'Level 2 Link State PDU' (Section 9.9), except for the PDU Type. The
PDU Types for Levels 3 through 8 are defined as follows:
Level 3 (L3-LSP-PDU): BBB
Level 4 (L4-LSP-PDU): CCC
Level 5 (L5-LSP-PDU): DDD
Level 6 (L6-LSP-PDU): EEE
Level 7 (L7-LSP-PDU): FFF
Level 8 (L8-LSP-PDU): GGG
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3.4. Level n complete sequence numbers PDU (Ln-CSNP-PDU)
The 'Level n complete sequence numbers PDU' (Ln-CSNP-PDU) has the
same format as the 'Level 2 complete sequence numbers PDU'
(Section 9.11), except for the PDU Type. The PDU Types for Levels 3
through 8 are defined as follows:
Level 3 (L3-CSNP-PDU): HHH
Level 4 (L4-CSNP-PDU): III
Level 5 (L5-CSNP-PDU): JJJ
Level 6 (L6-CSNP-PDU): KKK
Level 7 (L7-CSNP-PDU): LLL
Level 8 (L8-CSNP-PDU): MMM
3.5. Level n partial sequence numbers PDU (Ln-PSNP-PDU)
The 'Level 2 partial sequence numbers PDU' (Ln-PSNP-PDU) has the same
format as the 'Level 2 partial sequence numbers PDU' (Section 9.13),
except for the PDU Type. The PDU Types for Levels 3 through 8 are
defined as follows:
Level 3 (L3-PSNP-PDU): NNN
Level 4 (L4-PSNP-PDU): OOO
Level 5 (L5-PSNP-PDU): PPP
Level 6 (L6-PSNP-PDU): QQQ
Level 7 (L7-PSNP-PDU): RRR
Level 8 (L8-PSNP-PDU): SSS
4. 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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5. Acknowledgements
The author would like to thank Dinesh Dutt for inspiring this
document.
6. IANA Considerations
This document makes many requests to IANA, as follows:
6.1. 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.
6.2. New PDUs
IANA is requested to allocate values from the IS-IS PDU registry for
the following:
LAN-HELLO-PDU: AAA
L3-LSP-PDU: BBB
L4-LSP-PDU: CCC
L5-LSP-PDU: DDD
L6-LSP-PDU: EEE
L7-LSP-PDU: FFF
L8-LSP-PDU: GGG
L3-CSNP-PDU: HHH
L4-CSNP-PDU: III
L5-CSNP-PDU: JJJ
L6-CSNP-PDU: KKK
L7-CSNP-PDU: LLL
L8-CSNP-PDU: MMM
L3-PSNP-PDU: NNN
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L4-PSNP-PDU: OOO
L5-PSNP-PDU: PPP
L6-PSNP-PDU: QQQ
L7-PSNP-PDU: RRR
L8-PSNP-PDU: SSS
To allow for PDU types to be defined independent of this document,
the above values should be allocated from the range 32-254.
7. 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.
8. 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>.
Author's Address
Tony Li
Arista Networks
5453 Great America Parkway
Santa Clara, California 95054
USA
Email: tony.li@tony.li
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