| 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
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.
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The IS-IS routing protocol IS-IS 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 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.
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.
In this section, we enumerate all of the redefinitions of protocol header fields necessary to add additional levels.
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:
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.
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.
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:
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.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
For levels 3-8, all link state information, PSNPs, and CSNPs are relayed in conformance with RFC 7356. 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:
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
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.
The relationship between Level n and Level n-1 is analogous to the relationship between Level 2 and Level 1.
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.
This document makes many requests to IANA, as follows:
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.
IANA is requested to allocate values from the IS-IS PDU registry for the following:
To allow for PDU types to be defined independent of this document, the above values should be allocated from the range 32-254.
IANA is requested to allocate values from the IS-IS TLV registry for the following:
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
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.
| [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. |
| [RFC7356] | Ginsberg, L., Previdi, S. and Y. Yang, "IS-IS Flooding Scope Link State PDUs (LSPs)", RFC 7356, DOI 10.17487/RFC7356, September 2014. |