| 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
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 Circuit Type field MUST be set to indicate all levels supported on that circuit.
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.
[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.
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:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
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.
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:
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.
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.
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
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. [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.
When operating in Point-to-Point mode on a broadcast network
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 behavior of Level n is analogous to the behavior of Level 2.
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.
The authors would like to thank Dinesh Dutt for inspiring this document and Huaimo Chen for his 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:
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
IANA is requested to allocate values from the IANA Multicast 48-bit MAC Addresses block for the following:
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. |
| [RFC5309] | Shen, N. and A. Zinin, "Point-to-Point Operation over LAN in Link State Routing Protocols", RFC 5309, DOI 10.17487/RFC5309, October 2008. |
| [RFC7356] | Ginsberg, L., Previdi, S. and Y. Yang, "IS-IS Flooding Scope Link State PDUs (LSPs)", RFC 7356, DOI 10.17487/RFC7356, September 2014. |