BGP Unreachability Information SAFI
draft-tantsura-idr-unreachability-safi-06
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Jeff Tantsura , Donald Sharp , Vivek Venkatraman , Karthikeya Venkat Muppalla , Maciej Rzehak , Abderrahman Jouhari , Smit Parikh | ||
| Last updated | 2026-06-17 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-tantsura-idr-unreachability-safi-06
IDR Working Group J. Tantsura
Internet-Draft D. Sharp
Intended status: Standards Track V. Venkatraman
Expires: 17 December 2026 K. Muppalla
Nvidia
M. Rzehak
CoreWeave
A. Jouhari
S. Parikh
Oracle
15 June 2026
BGP Unreachability Information SAFI
draft-tantsura-idr-unreachability-safi-06
Abstract
This document defines a new BGP Subsequent Address Family Identifier
(SAFI) called "Unreachability Information" that allows the
propagation of prefix unreachability information through BGP without
affecting the installation or removal of routes in the Routing
Information Base (RIB) or Forwarding Information Base (FIB). This
mechanism enables network operators to share information about
unreachable prefixes for monitoring, debugging, and coordination
purposes while maintaining complete separation from the active
routing plane.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 17 December 2026.
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Copyright Notice
Copyright (c) 2026 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 5
2.1. Unreachability Information SAFI . . . . . . . . . . . . . 5
2.2. Capability Advertisement . . . . . . . . . . . . . . . . 5
2.3. BGP Path Attributes for Carrying Unreachability
Information . . . . . . . . . . . . . . . . . . . . . . . 6
3. NLRI Format . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Approach to deal with multiple Reporters . . . . . . . . 7
3.2. IPv4 Unreachability NLRI (AFI=1, SAFI=81) . . . . . . . . 8
3.3. IPv6 Unreachability NLRI (AFI=2, SAFI=81) . . . . . . . . 8
3.4. Reporter TLV Format . . . . . . . . . . . . . . . . . . . 9
3.5. Sub-TLV Format . . . . . . . . . . . . . . . . . . . . . 10
3.5.1. Unreachability Reason Code Sub-TLV . . . . . . . . . 10
3.5.2. Timestamp Sub-TLV . . . . . . . . . . . . . . . . . . 11
3.6. Encoding Examples . . . . . . . . . . . . . . . . . . . . 11
3.6.1. Single Reporter . . . . . . . . . . . . . . . . . . . 11
3.6.2. Multiple Reporters . . . . . . . . . . . . . . . . . 12
4. Operation . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Generating Unreachability Information . . . . . . . . . . 13
4.2. NLRI Processing and Aggregation . . . . . . . . . . . . . 13
4.2.1. Aggregation Procedures . . . . . . . . . . . . . . . 14
4.3. Withdrawal Procedures . . . . . . . . . . . . . . . . . . 15
4.3.1. Individual Reporter Withdrawal . . . . . . . . . . . 15
4.3.2. Complete NLRI Withdrawal . . . . . . . . . . . . . . 16
4.3.3. Stale Reporter Detection . . . . . . . . . . . . . . 16
4.4. Path Selection for Aggregation . . . . . . . . . . . . . 16
4.5. Interaction with Route Reflection . . . . . . . . . . . . 17
4.6. Communities and Attributes . . . . . . . . . . . . . . . 17
4.7. Interaction with Graceful Restart . . . . . . . . . . . . 18
4.7.1. Graceful Restart Capability . . . . . . . . . . . . . 18
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4.7.2. Restarting Speaker Behavior . . . . . . . . . . . . . 18
4.7.3. Receiving Speaker Behavior . . . . . . . . . . . . . 19
4.7.4. Route Reflector Considerations . . . . . . . . . . . 20
4.7.5. Implementation Recommendations . . . . . . . . . . . 20
4.8. Preventing State Explosion . . . . . . . . . . . . . . . 21
5. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 21
5.1. NLRI Structural Errors . . . . . . . . . . . . . . . . . 21
5.2. Non-Key Field Errors . . . . . . . . . . . . . . . . . . 22
5.3. Reporter TLV Errors . . . . . . . . . . . . . . . . . . . 22
5.4. Sub-TLV Errors . . . . . . . . . . . . . . . . . . . . . 23
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 23
6.1. Incremental Deployment . . . . . . . . . . . . . . . . . 23
6.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 23
6.3. Operational Recommendations . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
8.1. SAFI Assignment . . . . . . . . . . . . . . . . . . . . . 25
8.2. BGP Capability Code . . . . . . . . . . . . . . . . . . . 25
8.3. BGP Unreachability Information Reporter TLV Types . . . . 26
8.4. BGP Unreachability Information Sub-TLV Types . . . . . . 26
8.5. BGP Unreachability Reason Codes . . . . . . . . . . . . . 26
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
9.1. Normative References . . . . . . . . . . . . . . . . . . 27
9.2. Informative References . . . . . . . . . . . . . . . . . 28
Appendix A. Implementation Considerations . . . . . . . . . . . 28
Appendix B. Detailed Examples . . . . . . . . . . . . . . . . . 29
B.1. Complete UPDATE Message Example . . . . . . . . . . . . . 29
B.2. Aggregation Example . . . . . . . . . . . . . . . . . . . 30
B.3. Withdrawal Example . . . . . . . . . . . . . . . . . . . 31
Appendix C. Comparison with ADD-PATH Approach . . . . . . . . . 32
C.1. Architectural Differences . . . . . . . . . . . . . . . . 32
C.2. Advantages of Nested TLV Approach . . . . . . . . . . . . 33
C.3. Disadvantages of Nested TLV Approach . . . . . . . . . . 33
C.4. When to Use Each Approach . . . . . . . . . . . . . . . . 33
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
BGP-4 [RFC4271] withdrawals are only propagated for prefixes that
have been previously announced. This behavior, while preventing
certain attack vectors, limits the ability of operators to share
information about prefix unreachability for prefixes that were never
announced in the first place.
There are several use cases where propagating unreachability
information without affecting routing decisions would be valuable:
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* Debugging and troubleshooting routing issues across administrative
domains
* Sharing information about DDoS targets without null-routing
traffic
* Coordinating information about potentially hijacked prefixes
* Monitoring and anomaly detection systems that need visibility into
negative routing events
* Providing telemetry about routing system health without affecting
production traffic
* Correlating unreachability reports from multiple network vantage
points
This document defines a new SAFI that creates a parallel information
plane for unreachability data, allowing BGP speakers to share this
information while maintaining complete separation from the routing
plane.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Terminology
UI-RIB: Unreachability Information RIB
NLRI: Network Layer Reachability Information
AFI: Address Family Identifier
SAFI: Subsequent Address Family Identifier
TLV: Type-Length-Value
Reporter TLV: A nested TLV structure containing information about
one reporting BGP speaker and its associated unreachability
details
Aggregation: The process of combining multiple Reporter TLVs from
different paths into a single NLRI
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Advertising Speaker: The BGP speaker that sends the UPDATE message
containing the unreachability information
Reporting Speaker: The BGP speaker that originally generated the
unreachability information (identified within a Reporter
TLV)
2. Protocol Extensions
2.1. Unreachability Information SAFI
This document defines a new SAFI:
* Value: 81
* Name: Unreachability Information (UNREACH)
* Applicable to AFI: 1 (IPv4) and 2 (IPv6)
2.2. Capability Advertisement
A BGP speaker that wishes to exchange Unreachability Information MUST
advertise the corresponding AFI/SAFI capability as defined in
[RFC5492].
The Capability Code for Multiprotocol Extensions is 1. The
Capability Value field contains:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AFI | Reserved | SAFI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where:
* AFI = 1 (IPv4) or 2 (IPv6)
* Reserved = 0 (MUST be set to 0 on transmit, ignored on receive)
* SAFI = 81 (Unreachability Information)
Additionally, this document defines a new capability:
* Capability Code: TBD2 (to be assigned by IANA)
* Capability Name: Enhanced Unreachability Information
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* Capability Value: 1 octet flags field
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|A| Reserved |
+-+-+-+-+-+-+-+-+
Where:
* A bit: If set, speaker supports Aggregation of multiple Reporter
TLVs
* Reserved: MUST be zero on transmit, ignored on receive
The A bit indicates support for the aggregation procedures in
Section 4.2.1. A speaker that advertises A=1 SHALL combine Reporter
TLVs from multiple paths into a single NLRI. A speaker that
advertises A=0 MUST NOT perform aggregation and therefore only
advertises the Reporter TLVs present on its best path. A peer
receiving A=0 MUST NOT send aggregated NLRIs on that session.
2.3. BGP Path Attributes for Carrying Unreachability Information
Unreachability Information NLRI is carried in BGP UPDATE messages
using the Multiprotocol Extensions for BGP-4 as defined in [RFC4760].
Specifically:
* MP_REACH_NLRI (Path Attribute Type Code 14): Used to announce
unreachability information for one or more prefixes. The presence
of a prefix in MP_REACH_NLRI with AFI/SAFI indicating
Unreachability Information signifies that the prefix is being
reported as unreachable by one or more Reporting Speakers.
* MP_UNREACH_NLRI (Path Attribute Type Code 15): Used to withdraw
previously announced unreachability information. The presence of
a prefix in MP_UNREACH_NLRI with AFI/SAFI indicating
Unreachability Information signifies that the unreachability
condition for that prefix has been cleared by all previously
Reporting Speakers.
The AFI field in both attributes MUST be set to 1 (IPv4) or 2 (IPv6),
and the SAFI field MUST be set to 81 (Unreachability Information).
The NLRI field within MP_REACH_NLRI contains the Unreachability
Information NLRI as described in Section 3.
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Each Unreachability Information NLRI begins with a 2-octet NLRI
Length field (Section 3) that bounds the NLRI contents. Within
MP_REACH_NLRI the NLRI carries the Prefix followed by one or more
Reporter TLVs; within MP_UNREACH_NLRI it carries only the Prefix,
with no Reporter TLVs. In both cases the NLRI Length field covers
the octets that follow it.
Standard BGP path attributes (AS_PATH, ORIGIN, NEXT_HOP via
MP_REACH_NLRI, etc.) apply as defined in [RFC4760] and [RFC4271].
These attributes represent the path taken by the UPDATE message
itself, not the paths of individual reporters (which are preserved in
Reporter TLVs).
3. NLRI Format
The NLRI is uniquely identified by the combination of Prefix Length
and Prefix. Reporter TLVs are NOT part of the NLRI key but provide
information about each Reporting Speaker. The presence of an
Unreachability Information NLRI for a prefix signifies that one or
more speakers report the prefix as unreachable. The withdrawal of
such an NLRI indicates that all reporters have cleared their
unreachability reports for that prefix.
Each NLRI begins with an explicit 2-octet NLRI Length field. This
makes each NLRI boundary unambiguous regardless of the number of
NLRIs or Reporter TLVs present, so a receiver can locate the boundary
of each NLRI without parsing its interior.
3.1. Approach to deal with multiple Reporters
When multiple BGP speakers report unreachability for the same prefix,
implementers have several options:
1. Single Reporter: Do nothing and allow the Reporter Identifier of
the best path to be used as the only Reporter. This is the
simplest approach but loses information from other reporters.
2. Nested TLV Aggregation (Recommended): Implement the nested TLV
aggregation approach described in this specification to preserve
all reporter perspectives in a single NLRI. This provides the
most comprehensive view while maintaining a single BGP path per
prefix.
3. BGP ADD-PATH: Use BGP ADD-PATH [RFC7911] to maintain multiple
paths, each carrying its own Reporter TLV. This preserves full
BGP path attributes per reporter but requires ADD-PATH support.
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This specification focuses on the nested TLV aggregation approach as
the preferred mechanism, providing detailed procedures and encodings
for this method throughout the remainder of this document.
3.2. IPv4 Unreachability NLRI (AFI=1, SAFI=81)
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLRI Length (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | IPv4 Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reporter TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
* NLRI Length: 2 octets. The total length, in octets, of the fields
that follow: in MP_REACH_NLRI, the Prefix Length, Prefix, and
Reporter TLV(s); in MP_UNREACH_NLRI, the Prefix Length and Prefix
only. It excludes the NLRI Length field itself. When ADD-PATH
[RFC7911] is in use, the 4-octet Path Identifier precedes the NLRI
Length field and is likewise not counted.
* Prefix Length: 1 octet (0-32 bits)
* IPv4 Prefix: Variable, encoded as in [RFC4271] Section 4.3, and
carried in MP_REACH_NLRI per [RFC4760]
* Reporter TLVs: One or more Reporter TLVs (Section 3.4). Present
in MP_REACH_NLRI only; absent in MP_UNREACH_NLRI (see
Section 4.3.2).
3.3. IPv6 Unreachability NLRI (AFI=2, SAFI=81)
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLRI Length (2 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | IPv6 Prefix (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reporter TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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* NLRI Length: 2 octets. The total length, in octets, of the fields
that follow: in MP_REACH_NLRI, the Prefix Length, Prefix, and
Reporter TLV(s); in MP_UNREACH_NLRI, the Prefix Length and Prefix
only. It excludes the NLRI Length field itself. When ADD-PATH
[RFC7911] is in use, the 4-octet Path Identifier precedes the NLRI
Length field and is likewise not counted.
* Prefix Length: 1 octet (0-128 bits)
* IPv6 Prefix: Variable, encoded in MP_REACH_NLRI as defined in
[RFC4760]
* Reporter TLVs: One or more Reporter TLVs (Section 3.4). Present
in MP_REACH_NLRI only; absent in MP_UNREACH_NLRI (see
Section 4.3.2).
Example IPv6 prefix: 2001:db8::/32.
3.4. Reporter TLV Format
The Reporter TLV is a container that encapsulates information about a
single reporting BGP speaker and its associated unreachability
details. Multiple Reporter TLVs MAY appear in a single NLRI to
represent reports from different speakers.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reporter Identifier (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reporter AS Number (4 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-TLVs (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reporter TLV Fields:
Type: 1 octet. Value: 1 (Reporter)
Length: 2 octets. Total length of the Reporter Identifier, Reporter
AS Number, and Sub-TLVs fields (minimum 8 octets)
Reporter Identifier: 4 octets. BGP Identifier (Router ID) of the
Reporting Speaker in network byte order
Reporter AS Number: 4 octets. 4-octet AS number of the Reporting
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Speaker in network byte order. If the AS number is less than
65536, the upper 2 octets are set to 0
Sub-TLVs: Variable length. Contains zero or more Sub-TLVs providing
additional details about this reporter's unreachability
observation. A Reporter TLV carrying no Sub-TLVs is valid: the
presence of the Reporter TLV itself conveys the fact of
unreachability, with the Reporter Identifier and Reporter AS
Number identifying the speaker that observed it. When the
Unreachability Reason Code Sub-TLV (Section 3.5.1) is absent, the
reason is treated as Unspecified (code 0).
The combination of Reporter Identifier and Reporter AS Number
uniquely identifies the Reporting Speaker. Multiple Reporter TLVs
with the same Reporter Identifier and AS Number MUST NOT appear in
the same NLRI. If such duplication occurs, only the first occurrence
SHOULD be processed.
3.5. Sub-TLV Format
Sub-TLVs appear within Reporter TLVs and provide specific details
about the unreachability observation by that reporter.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sub-Type | Sub-Length | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Sub-Value (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Defined Sub-TLV Types:
Implementations MUST ignore unknown Sub-TLV types to allow for future
extensibility. Multiple Sub-TLVs of the same type SHOULD NOT appear
within a single Reporter TLV; if present, only the first occurrence
SHOULD be processed.
3.5.1. Unreachability Reason Code Sub-TLV
* Sub-Type: 1
* Sub-Length: 2 octets
* Sub-Value: 2-octet reason code in network byte order
Defined Reason Codes:
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* 0: Unspecified
* 1: Policy Blocked
* 2: Security Filtered
* 3: RPKI Invalid
* 4: No Export Policy
* 5: Martian Address
* 6: Bogon Prefix
* 7: Maintenance
* 8: Local Administrative Action
* 9: Local Link Down
* 10-64535: Reserved
* 64536-65535: Reserved for Private Use
Inclusion of the Unreachability Reason Code Sub-TLV is RECOMMENDED.
When this Sub-TLV is absent from a Reporter TLV, receivers MUST treat
the reason as Unspecified (code 0); this is semantically equivalent
to an explicitly encoded Reason Code of 0. A bare Reporter TLV
carrying no Sub-TLVs therefore conveys the fact of unreachability
with reason Unspecified.
3.5.2. Timestamp Sub-TLV
* Sub-Type: 2
* Sub-Length: 8 octets
* Sub-Value: Unix timestamp (seconds since epoch) in network byte
order, indicates when the unreachability event occurred or was
detected by this reporter
3.6. Encoding Examples
3.6.1. Single Reporter
Example: 192.0.2.0/24 unreachable, reported by AS 65001, Router ID
198.51.100.1, Reason: RPKI Invalid
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NLRI Length: 31 (0x001F)
Prefix Length: 24 (0x18)
Prefix: 192.0.2.0 (0xC0000200)
Reporter TLV:
Type: 1
Length: 24 (0x0018)
Reporter Identifier: 198.51.100.1 (0xC6336401)
Reporter AS: 65001 (0x0000FDE9)
Sub-TLV (Reason):
Sub-Type: 1
Sub-Length: 2 (0x0002)
Sub-Value: 3 (0x0003) [RPKI Invalid]
Sub-TLV (Timestamp):
Sub-Type: 2
Sub-Length: 8 (0x0008)
Sub-Value: 1733789400 (0x00000000675786D8)
Hexadecimal encoding:
00 1F 18 C0 00 02 01 00 18 C6 33 64 01 00 00 FD E9 01
00 02 00 03 02 00 08 00 00 00 00 67 57 86 D8
3.6.2. Multiple Reporters
Example: 192.0.2.0/24 unreachable, reported by two speakers
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NLRI Length: 58 (0x003A)
Prefix Length: 24
Prefix: 192.0.2.0
Reporter TLV #1:
Type: 1
Length: 24
Reporter Identifier: 198.51.100.1
Reporter AS: 65001
Sub-TLV (Reason): 3 (RPKI Invalid)
Sub-TLV (Timestamp): 1733789400
Reporter TLV #2:
Type: 1
Length: 24
Reporter Identifier: 198.51.100.2
Reporter AS: 65002
Sub-TLV (Reason): 1 (Policy Blocked)
Sub-TLV (Timestamp): 1733789410
Total NLRI length (excludes the NLRI Length field):
4 + 27 + 27 = 58 octets
4. Operation
4.1. Generating Unreachability Information
A BGP speaker MAY generate an Unreachability Information NLRI when
local processing determines, for any reason, that a prefix is to be
reported as unreachable. The triggering condition is conveyed by the
Reason Code Sub-TLV (Section 3.5.1). This document does not mandate
the set of local conditions that cause generation; that set is
implementation- and deployment-specific, constrained only by the
Reason Codes defined in this document or subsequently registered.
The Reporting Speaker MUST populate the Reporter TLV with its own BGP
Identifier and AS Number. The speaker SHOULD include a Reason Code
Sub-TLV and SHOULD include a Timestamp Sub-TLV to facilitate temporal
correlation.
4.2. NLRI Processing and Aggregation
When a BGP speaker receives an UPDATE message with Unreachability
Information SAFI:
1. It MUST NOT install or remove any routes in the Loc-RIB based on
this information
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2. It MUST maintain a separate Unreachability Information RIB (UI-
RIB) for this SAFI
3. It SHOULD apply standard BGP path selection to UI-RIB entries for
consistency
4. It MAY propagate the information according to standard BGP rules,
local policy, and aggregation procedures defined in Section 4.2.1
5. It MUST NOT mix Unreachability Information NLRI with other SAFIs
in the same UPDATE message
4.2.1. Aggregation Procedures
When multiple UPDATE messages arrive advertising unreachability for
the same prefix from different neighbors, a BGP speaker supporting
aggregation (A bit set in capability) SHOULD combine the Reporter
TLVs according to the following procedure:
1. Perform standard BGP path selection on the received updates. The
"best path" is determined based on standard BGP decision process,
considering only standard BGP attributes (AS_PATH length,
LOCAL_PREF, etc.), NOT the content of Reporter TLVs.
2. Extract all Reporter TLVs from the best path.
3. For each non-selected path that would be feasible (not filtered
by policy):
* Extract all Reporter TLVs from that path
* For each Reporter TLV, check if a Reporter with the same
Reporter Identifier and Reporter AS already exists in the
aggregated set
* If not present, add the Reporter TLV to the aggregated set
* If already present, compare timestamps (if present) and keep
the more recent report, or keep the existing entry if
timestamps are equal or absent
4. Create a new NLRI containing all unique Reporter TLVs
5. Advertise this aggregated NLRI to appropriate neighbors, with BGP
path attributes taken from the best path
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A speaker performing aggregation (A=1) MUST place the Reporter TLV
corresponding to the best path in the first position of the resulting
NLRI. Reporter TLVs drawn from non-selected feasible paths MAY
follow in any order. Except for the first position, receivers MUST
NOT derive meaning from Reporter TLV ordering; implementations MUST
tolerate any ordering of Reporter TLVs past the first.
The interoperability case of an aggregating sender paired with a non-
aggregating receiver is resolved by capability negotiation
(Section 2.2): a speaker that advertises A=0 will not receive
aggregated NLRIs on that session, since its peer MUST NOT send them.
The first-position rule above applies regardless, so that any speaker
transiting a single aggregated NLRI (for local inspection, BMP
export, or display) has a deterministic "first Reporter TLV = best
path" invariant to rely on.
The maximum number of Reporter TLVs that can be aggregated in a
single NLRI is limited by the maximum BGP UPDATE message size (4096
octets). Implementations SHOULD limit the number of Reporter TLVs to
prevent NLRI size from becoming unwieldy. A RECOMMENDED maximum is
50 Reporter TLVs per prefix, which allows for comprehensive multi-
vantage-point monitoring while maintaining reasonable message sizes.
If the maximum is reached and a new reporter must be added,
implementations SHOULD remove the oldest Reporter TLV (based on
Timestamp Sub-TLV if present), unless this is the reporter of the
best path. In that case the second oldest reporter should be
removed.
4.3. Withdrawal Procedures
Withdrawal of unreachability information operates at two levels:
4.3.1. Individual Reporter Withdrawal
When a BGP speaker determines that a specific reporter no longer
considers a prefix unreachable (e.g., receives an UPDATE from that
reporter's AS that doesn't include the unreachability NLRI, or local
policy determines the report is stale), it SHOULD:
1. Remove the corresponding Reporter TLV from the NLRI
2. If other Reporter TLVs remain, re-advertise the NLRI with the
remaining Reporter TLVs
3. If no Reporter TLVs remain, withdraw the entire NLRI as described
in Section 4.3.2
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To facilitate individual reporter withdrawal, implementations MUST
track the source of each Reporter TLV (which BGP neighbor or local
process it came from).
4.3.2. Complete NLRI Withdrawal
A BGP speaker MUST withdraw an Unreachability Information NLRI (send
the prefix in MP_UNREACH_NLRI) when:
* All Reporter TLVs have been removed
* The prefix is explicitly withdrawn by all upstream sources
* Local policy dictates the information should no longer be
propagated
The MP_UNREACH_NLRI NLRI consists of the 2-octet NLRI Length followed
by the Prefix Length and Prefix, with no Reporter TLVs. The NLRI
Length value equals the sum of the Prefix Length field (1 octet) and
the Prefix (the number of octets needed to encode the prefix for the
negotiated AFI).
4.3.3. Stale Reporter Detection
Implementations SHOULD implement aging mechanisms to remove stale
Reporter TLVs:
* If a Timestamp Sub-TLV is present and indicates the report is
older than a configurable threshold (RECOMMENDED default: 24
hours), the Reporter TLV MAY be removed
* If the BGP session to the neighbor that provided a Reporter TLV
goes down, implementations SHOULD mark associated Reporter TLVs as
potentially stale and MAY remove them after a grace period
4.4. Path Selection for Aggregation
The path selection for Unreachability Information SAFI follows
standard BGP best path selection (RFC 4271 Section 9.1) with the
following clarifications:
* Weight/Local Preference: Apply normally based on local policy.
* AS_PATH Length: Shorter AS_PATH is preferred. This represents the
path the UPDATE message took.
* ORIGIN: IGP preferred over EGP over INCOMPLETE.
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* MED: Apply if comparing paths from the same neighboring AS.
* BGP Identifier: Use for tie-breaking.
The content of Reporter TLVs (number of reporters, reason codes,
etc.) MUST NOT influence path selection. Path selection determines
which UPDATE's BGP attributes are used for propagation, while
aggregation combines Reporter TLVs from multiple paths.
4.5. Interaction with Route Reflection
Route Reflectors process Unreachability Information SAFI like any
other AFI/SAFI combination:
* Apply standard route reflection rules
* ORIGINATOR_ID and CLUSTER_LIST attributes apply normally to the
UPDATE message, not to individual reporters
* Route Reflectors SHOULD support aggregation to combine reports
from multiple clients
* When reflecting to clients, include all aggregated Reporter TLVs
The distinction between the ORIGINATOR_ID BGP attribute and the
Reporter Identifier field in Reporter TLVs is important:
* ORIGINATOR_ID identifies the originator of the BGP UPDATE message
for loop prevention
* Reporter Identifier identifies the speaker that observed and
reported the unreachability condition
* These MAY be different in aggregated scenarios
4.6. Communities and Attributes
Standard BGP communities and attributes apply to the UPDATE message:
* NO_EXPORT, NO_ADVERTISE, and NO_EXPORT_SUBCONFED work as defined
* Large Communities [RFC8092] MAY be used for policy control of
aggregation behavior
* AS_PATH is constructed normally for the UPDATE message path
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* ORIGIN SHOULD be set to INCOMPLETE for locally generated
information, reflecting that the information does not originate
from routing protocol state
4.7. Interaction with Graceful Restart
BGP Graceful Restart (GR) as defined in [RFC4724] applies to the
Unreachability Information SAFI. This section describes how UI-RIB
is handled during restart scenarios.
4.7.1. Graceful Restart Capability
A BGP speaker that supports Graceful Restart for Unreachability
Information SAFI MUST include the AFI/SAFI pair (AFI=1 or 2, SAFI=81)
in the Graceful Restart Capability advertisement as defined in RFC
4724.
The "Forwarding State" (F) bit for this SAFI:
* Since Unreachability Information does not affect the forwarding
plane (Loc-RIB or FIB), there is no forwarding state to preserve.
* The F bit SHOULD be set to 0 in the Graceful Restart Capability
for this AFI/SAFI combination.
* Receiving Speakers MUST NOT interpret the F bit for this SAFI as
indicating preservation of forwarding state. The F bit, if set,
has no defined meaning for this SAFI and MUST be ignored.
* Implementations MAY use the F bit in future extensions to signal
UI-RIB preservation capabilities, but such usage is outside the
scope of this document.
4.7.2. Restarting Speaker Behavior
When a BGP speaker restarts and has negotiated GR for the
Unreachability Information SAFI:
1. The speaker SHOULD set the F bit to 0 in the Graceful Restart
Capability for this AFI/SAFI pair, as there is no forwarding
state associated with this SAFI.
2. If the speaker preserved its UI-RIB across the restart, it SHOULD
re-advertise all retained UI-RIB entries to its peers as soon as
possible after restart, but MAY do so gradually to avoid
overwhelming the network.
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3. If the speaker did NOT preserve its UI-RIB across the restart, it
SHOULD rebuild the UI-RIB from local information sources before
re-advertising entries.
4. The speaker MUST send an End-of-RIB (EoR) marker for this SAFI
after completing the re-advertisement of UI-RIB entries. The EoR
marker is an UPDATE message with no NLRI and the MP_UNREACH_NLRI
attribute containing the AFI/SAFI pair but no withdrawn routes.
4.7.3. Receiving Speaker Behavior
When a BGP speaker detects that a peer has restarted with GR
capability for Unreachability Information SAFI:
1. The speaker MUST mark all UI-RIB entries learned from the
restarting peer as stale. Stale marking occurs regardless of the
F bit value, since the F bit has no defined semantics for this
SAFI.
2. Stale entries MUST NOT be immediately withdrawn. They MUST be
retained for the duration of the Restart Time advertised in the
peer's GR Capability, or until the End-of-RIB marker is received,
whichever comes first.
3. During the Restart Time period:
* Stale entries MAY be used for monitoring and correlation
purposes
* Implementations MAY mark stale entries distinctly in display
and APIs
* Stale entries SHOULD NOT be propagated to other peers unless
explicitly configured to do so
4. Upon receiving the End-of-RIB marker from the restarting peer:
* All stale entries that were not refreshed MUST be removed from
the UI-RIB
* Reporter TLVs from the restarted peer that were part of
aggregated NLRIs MUST be removed if not refreshed
* If removal of Reporter TLVs leaves other Reporter TLVs for the
same prefix, the NLRI SHOULD be re-advertised with the
remaining Reporter TLVs
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5. If the Restart Time expires before receiving the End-of-RIB
marker, all stale entries MUST be removed immediately.
4.7.4. Route Reflector Considerations
Route Reflectors implementing Graceful Restart for this SAFI:
* MUST properly handle stale marking of UI-RIB entries from
restarting clients
* SHOULD NOT reflect stale entries to other clients unless
configured with a specific policy to do so
* MUST correctly manage ORIGINATOR_ID and CLUSTER_LIST for entries
that transition through stale and refresh phases
* SHOULD send End-of-RIB markers to clients after the RR itself
completes restart processing
4.7.5. Implementation Recommendations
* Restart Time for this SAFI SHOULD be configurable independently
from other AFI/SAFI combinations, with a RECOMMENDED default of
120 seconds.
* Implementations SHOULD provide configuration options to:
- Enable/disable preservation of UI-RIB across restarts
- Control whether stale entries are propagated during GR
- Set the Restart Time for this SAFI
- Configure actions when End-of-RIB is not received in time
* Implementations SHOULD log GR events for this SAFI to aid in
debugging, including:
- Detection of peer restart
- Stale marking of entries
- Receipt of End-of-RIB marker
- Removal of stale entries
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4.8. Preventing State Explosion
To prevent unbounded growth of the UI-RIB:
1. Implementations SHOULD limit the number of Reporter TLVs per
prefix (RECOMMENDED maximum: 50)
2. Implementations SHOULD implement rate limiting for accepting new
unreachability information
3. Default maximum UI-RIB size SHOULD be configurable with a
reasonable default (e.g., 100,000 prefixes)
4. Implementations SHOULD implement memory limits for total Reporter
TLV storage
5. Error Handling
Error handling for the Unreachability Information SAFI follows
[RFC7606] and [RFC4760] Section 7. Per-class actions are specified
in Section 5.1 through Section 5.4.
The primary action for session-level errors is "session reset". A
receiver that supports "AFI/SAFI disable" per [RFC7606] Section 7 MAY
apply it in place of "session reset", confining the impact to the
Unreachability Information SAFI rather than the entire BGP session.
Checks in Section 5.1 are performed before those in Section 5.2; on
first error the corresponding action MUST be taken and further NLRI
parsing MUST cease. All error conditions MUST be logged.
5.1. NLRI Structural Errors
A receiver MUST apply "session reset" per [RFC7606] Section 4, or
"AFI/SAFI disable" per [RFC7606] Section 7 if supported, on any of
the following:
These checks are applied in the order listed; on the first failure
the specified action MUST be taken and NLRI parsing MUST cease.
* The NLRI Length field is absent or truncated (fewer than 2 octets
remain in the enclosing MP_REACH_NLRI or MP_UNREACH_NLRI
attribute), or the NLRI Length value is inconsistent with the
octets remaining in that attribute: the value either exceeds the
octets remaining or is below the minimum given in the next item.
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* The NLRI Length value is below the minimum for this SAFI: in
MP_REACH_NLRI, 1 octet (Prefix Length) plus the Prefix octets for
the negotiated AFI plus at least 11 octets for one Reporter TLV
(Section 3.4); in MP_UNREACH_NLRI, 1 octet (Prefix Length) plus
the Prefix octets.
* Prefix Length outside the range permitted for the negotiated AFI
(0-32 for AFI=1, 0-128 for AFI=2): the Prefix field boundary
cannot be determined, and any Reporter TLVs cannot be parsed.
5.2. Non-Key Field Errors
A receiver MUST apply "treat-as-withdraw" per [RFC7606] Section 2 on
any of the following:
* An MP_REACH_NLRI containing no Reporter TLVs after the Prefix
(Section 3 requires one or more).
* An MP_UNREACH_NLRI whose NLRI Length value exceeds 1 octet (Prefix
Length) plus the Prefix octets for the negotiated AFI (Section 4.3
prohibits Reporter TLVs in withdrawals).
5.3. Reporter TLV Errors
Consistent with the principle in [RFC9552] Section 5.1, unknown or
malformed Reporter TLVs MUST NOT cause the enclosing NLRI to be
considered malformed.
* Unrecognized Reporter TLV Type: the TLV MUST be ignored; parsing
resumes at the next TLV boundary computed from the Length field.
* Malformed Reporter TLV (Length inconsistent with the remaining
NLRI data, or less than the 8-octet minimum required to carry the
Reporter Identifier and Reporter AS Number): the TLV MUST be
discarded. If one or more well-formed Reporter TLVs remain, they
MUST be processed; otherwise "treat-as-withdraw" MUST be applied
to the NLRI.
* Duplicate Reporter TLVs (identical Reporter Identifier and
Reporter AS Number within a single NLRI): only the first
occurrence is processed, per Section 3.4.
* Reporter TLV count exceeding the implementation limit
(Section 4.8): excess TLVs MUST be discarded.
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5.4. Sub-TLV Errors
Unrecognized Sub-TLV Types within a Reporter TLV MUST be silently
ignored per Section 3.5. A Sub-TLV whose Sub-Length is inconsistent
with the available data within the enclosing Reporter TLV MUST be
discarded; processing of the enclosing Reporter TLV and any remaining
Sub-TLVs continues. Duplicate Sub-TLVs of the same Sub-Type within a
single Reporter TLV are handled per Section 3.5: only the first
occurrence is processed.
6. Deployment Considerations
6.1. Incremental Deployment
The Unreachability Information SAFI can be deployed incrementally:
* Speakers that don't support it simply don't negotiate the
capability
* Mixed environments work correctly with normal BGP capability
negotiation
* Can be enabled on specific sessions for testing
* Aggregation support (A bit in capability) is OPTIONAL; speakers
without it can still propagate single-reporter NLRIs
* Speakers MAY use BGP dynamic capabilities to enable or disable
this SAFI without resetting the BGP session
6.2. Use Cases
Example deployment scenarios:
Inter-AS Debugging: Enable between cooperating ASes for
troubleshooting. Aggregation provides comprehensive view of why
different ASes find a prefix unreachable.
Route Collectors: Deploy on route collector sessions for enhanced
telemetry. Collectors can aggregate reports from multiple feeders
to provide consolidated unreachability view.
DDoS Coordination: Share attack target information without null-
routing. Multiple reports from different locations confirm attack
patterns.
Security Monitoring: Track suspicious unreachability patterns.
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Correlation of reports from multiple vantage points aids in
distinguishing localized issues from widespread problems.
RPKI Validation Monitoring: Track RPKI validation failures across
different ASes. Aggregation shows consensus or disagreement on
RPKI status.
6.3. Operational Recommendations
* Enable aggregation on route collectors and monitoring systems to
maximize visibility
* Configure reasonable Reporter TLV limits based on expected number
of reporters
* Use Timestamp Sub-TLVs to facilitate debugging of temporal aspects
of unreachability
* Monitor UI-RIB size and Reporter TLV counts for capacity planning
7. Security Considerations
The Unreachability Information SAFI introduces new security
considerations:
1. Information Leakage: Unreachability information might reveal
network topology or operational issues. Reporter TLVs explicitly
identify reporting ASes and routers. Operators SHOULD carefully
consider peering policies for this SAFI.
2. State Exhaustion: Malicious peers could attempt to exhaust memory
by advertising large numbers of unreachable prefixes or including
excessive Reporter TLVs. Implementations SHOULD enforce limits
as described in Section 4.8.
3. False Information: Peers could advertise false unreachability
information or spoof Reporter TLVs. This SAFI SHOULD only be
enabled with trusted peers. Consider validating Reporter
Identifiers and AS Numbers against known valid values.
4. Prefix Hijacking: The SAFI could be used to claim prefixes are
unreachable when they're not. Since this doesn't affect routing,
the impact is limited to monitoring systems.
5. Reporter Impersonation: A malicious speaker could include
Reporter TLVs claiming to represent other ASes. Implementations
SHOULD validate that Reporter AS Numbers in TLVs are consistent
with the AS_PATH of UPDATEs that introduced them.
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6. Aggregation Amplification: A malicious speaker could send many
UPDATEs with different Reporter TLVs for the same prefix, causing
downstream aggregating speakers to accumulate and propagate large
numbers of Reporter TLVs. Rate limiting and Reporter TLV limits
mitigate this.
Operators SHOULD:
* Use BGP TCP-AO [RFC5925] or MD5 for session protection
* Implement prefix filtering for unreachability information
* Monitor UI-RIB size and Reporter TLV counts
* Enable this SAFI only with explicitly trusted peers
* Validate Reporter AS Numbers against expected values
* Configure appropriate Reporter TLV limits per prefix
* Implement rate limiting on incoming unreachability UPDATEs
8. IANA Considerations
8.1. SAFI Assignment
IANA has assigned a SAFI value from the "Subsequent Address Family
Identifiers (SAFI) Parameters" registry within the "Border Gateway
Protocol (BGP) Parameters" registry group, applicable to AFI 1 (IPv4)
and AFI 2 (IPv6):
* Value: 81
* Description: Unreachability Information
* Reference: This document
8.2. BGP Capability Code
IANA is requested to assign a new BGP Capability Code from the
"Capability Codes" registry within the "Border Gateway Protocol (BGP)
Parameters" registry group:
* Value: TBD2
* Description: Enhanced Unreachability Information
* Reference: This document
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8.3. BGP Unreachability Information Reporter TLV Types
IANA is requested to create a new registry called "BGP Unreachability
Information Reporter TLV Types" under the "Border Gateway Protocol
(BGP) Parameters" registry page.
Registration Procedure: Standards Action
Initial registrations:
Value Description Reference
----- ------------------------------------ -----------
0 Reserved This document
1 Reporter TLV This document
2-254 Unassigned
255 Reserved This document
8.4. BGP Unreachability Information Sub-TLV Types
IANA is requested to create a new registry called "BGP Unreachability
Information Sub-TLV Types" under the "Border Gateway Protocol (BGP)
Parameters" registry page.
Registration Procedure: RFC Required
Initial registrations:
Value Description Reference
----- ------------------------------------ -----------
0 Reserved This document
1 Unreachability Reason Code This document
2 Timestamp This document
3-254 Unassigned
255 Reserved This document
8.5. BGP Unreachability Reason Codes
IANA is requested to create a new registry called "BGP Unreachability
Reason Codes" under the "Border Gateway Protocol (BGP) Parameters"
registry page.
Registration Procedure: RFC Required for values 0-64535, Reserved for
Private Use for values 64536-65535
Initial registrations:
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Value Description Reference
----- ------------------------------------ -----------
0 Unspecified This document
1 Policy Blocked This document
2 Security Filtered This document
3 RPKI Invalid This document
4 No Export Policy This document
5 Martian Address This document
6 Bogon Prefix This document
7 Maintenance This document
8 Local Administrative Action This document
9 Local Link Down This document
10-64535 Unassigned
64536-65535 Reserved for Private Use This document
9. References
9.1. Normative References
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
DOI 10.17487/RFC4724, January 2007,
<https://www.rfc-editor.org/info/rfc4724>.
[RFC4760] Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.
[RFC5492] Scudder, J. and R. Chandra, "Capabilities Advertisement
with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
2009, <https://www.rfc-editor.org/info/rfc5492>.
[RFC7606] Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <https://www.rfc-editor.org/info/rfc5925>.
[RFC7854] Scudder, J., Ed., Fernando, R., and S. Stuart, "BGP
Monitoring Protocol (BMP)", RFC 7854,
DOI 10.17487/RFC7854, June 2016,
<https://www.rfc-editor.org/info/rfc7854>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<https://www.rfc-editor.org/info/rfc7911>.
[RFC8092] Heitz, J., Ed., Snijders, J., Ed., Patel, K., Bagdonas,
I., and N. Hilliard, "BGP Large Communities Attribute",
RFC 8092, DOI 10.17487/RFC8092, February 2017,
<https://www.rfc-editor.org/info/rfc8092>.
[RFC9552] Talaulikar, K., Ed., "Distribution of Link-State and
Traffic Engineering Information Using BGP", RFC 9552,
DOI 10.17487/RFC9552, December 2023,
<https://www.rfc-editor.org/info/rfc9552>.
Appendix A. Implementation Considerations
Implementers should consider the following aspects when implementing
the Unreachability Information SAFI with nested Reporter TLVs:
1. UI-RIB should store both the NLRI and associated Reporter TLVs
with tracking of which neighbor provided each Reporter TLV
2. Show commands should clearly display all reporters for a given
prefix, including Reporter ID, AS, Reason, and Timestamp
3. MIB/YANG models need structures to represent multiple reporters
per prefix
4. BMP [RFC7854] should be extended to convey Reporter TLV
information in monitoring messages
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5. Efficient data structures for Reporter TLV storage and lookup are
important for performance
6. Consider implementing a local database to track reporter history
for forensic analysis
Appendix B. Detailed Examples
B.1. Complete UPDATE Message Example
An UPDATE message advertising that 192.0.2.0/24 is unreachable,
reported by AS 65001 (Router 198.51.100.1) due to RPKI validation
failure:
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BGP UPDATE Message:
Withdrawn Routes Length: 0
Total Path Attribute Length: (calculated)
Path Attributes:
MP_REACH_NLRI (Type 14, Flags 0x90):
AFI: 1 (IPv4)
SAFI: 81 (Unreachability Information)
Next Hop Length: 0
Reserved: 0
NLRI:
NLRI Length: 31
Prefix Length: 24
Prefix: 192.0.2.0
Reporter TLV:
Type: 1
Length: 24
Reporter ID: 198.51.100.1
Reporter AS: 65001
Sub-TLV (Reason):
Sub-Type: 1
Sub-Length: 2
Reason: 3 (RPKI Invalid)
Sub-TLV (Timestamp):
Sub-Type: 2
Sub-Length: 8
Value: 1733789400
AS_PATH (Type 2):
Segment Type: AS_SEQUENCE
Segment Length: 1
AS: 65001
ORIGIN (Type 1):
Value: INCOMPLETE (2)
B.2. Aggregation Example
Router R1 receives two UPDATEs for 192.0.2.0/24:
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UPDATE 1 (from Neighbor N1, AS 65100):
AS_PATH: 65100
Reporter TLV:
Reporter ID: 198.51.100.1, AS: 65001
Reason: RPKI Invalid (3)
Timestamp: 1733789400
UPDATE 2 (from Neighbor N2, AS 65200):
AS_PATH: 65200
Reporter TLV:
Reporter ID: 198.51.100.2, AS: 65002
Reason: Policy Blocked (1)
Timestamp: 1733789410
R1 Path Selection:
- Compare AS_PATH length: both length 1
- Compare by BGP ID: UPDATE 1 wins
R1 Aggregation:
- Extract Reporter TLV from UPDATE 1 (best path)
- Extract Reporter TLV from UPDATE 2 (add to aggregated set)
- Result: NLRI with 2 Reporter TLVs
R1 Advertisement to downstream:
AS_PATH: 65100 (from best path)
NLRI for 192.0.2.0/24:
NLRI Length: 58
Prefix Length: 24
Prefix: 192.0.2.0
Reporter TLV #1:
Reporter ID: 198.51.100.1, AS: 65001
Reason: 3, Timestamp: 1733789400
Reporter TLV #2:
Reporter ID: 198.51.100.2, AS: 65002
Reason: 1, Timestamp: 1733789410
B.3. Withdrawal Example
Scenario: Reporter 198.51.100.1 (AS 65001) clears its unreachability
report, but Reporter 198.51.100.2 (AS 65002) maintains its report.
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Initial State on Router R1:
UI-RIB Entry for 192.0.2.0/24:
Reporter TLV #1: 198.51.100.1/AS65001 (from Neighbor N1)
Reporter TLV #2: 198.51.100.2/AS65002 (from Neighbor N2)
Event: N1 sends MP_UNREACH_NLRI for 192.0.2.0/24
R1 Processing:
1. Identify that withdrawal came from N1
2. Find Reporter TLVs associated with N1
3. Remove Reporter TLV for 198.51.100.1/AS65001
4. Check remaining Reporter TLVs
5. Reporter TLV #2 still present
6. Re-advertise NLRI with remaining Reporter TLV
R1 Advertisement to downstream:
MP_REACH_NLRI for 192.0.2.0/24:
NLRI Length: 31
Prefix Length: 24
Prefix: 192.0.2.0
Reporter TLV #2:
Reporter ID: 198.51.100.2, AS: 65002
Reason: 1, Timestamp: 1733789410
Later Event: N2 also sends MP_UNREACH_NLRI for 192.0.2.0/24
R1 Processing:
1. Remove Reporter TLV for 198.51.100.2/AS65002
2. No Reporter TLVs remain
3. Send MP_UNREACH_NLRI for 192.0.2.0/24
R1 Advertisement to downstream:
MP_UNREACH_NLRI:
AFI: 1, SAFI: 81
NLRI Length: 4
Withdrawn Route: 192.0.2.0/24
Appendix C. Comparison with ADD-PATH Approach
This nested TLV approach differs from using BGP ADD-PATH [RFC7911] in
several fundamental ways:
C.1. Architectural Differences
ADD-PATH Approach: Maintains multiple complete BGP paths for the
same prefix, each with full set of BGP attributes (AS_PATH,
communities, etc.). Each reporter's information travels as a
separate BGP UPDATE path.
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Nested TLV Approach: Aggregates multiple reporter perspectives into
a single BGP path. Only one set of BGP attributes (representing
the advertising path), but multiple Reporter TLVs within the NLRI.
C.2. Advantages of Nested TLV Approach
* No ADD-PATH capability negotiation required - works with all BGP
implementations
* More compact representation - single UPDATE can carry reports from
many speakers
* Explicit aggregation model designed for consolidating multiple
perspectives
* Lower BGP state - one path entry instead of multiple
* Easier correlation - all reports for a prefix in one NLRI
C.3. Disadvantages of Nested TLV Approach
* More complex specification and implementation
* BGP attribute ambiguity - AS_PATH represents the path of the
UPDATE message, not the original paths reporters observed
* Withdrawal complexity - requires tracking which neighbor provided
each Reporter TLV
* NLRI size can grow large with many reporters
* Non-standard pattern in BGP protocol design
* Path selection doesn't consider reporter content
* New capability bit and processing logic required
C.4. When to Use Each Approach
Use ADD-PATH when:
* Full BGP path information per reporter is important
* Independent lifecycle management is critical
* ADD-PATH is already widely deployed in the network
* Standard BGP mechanisms are preferred
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Use Nested TLV when:
* ADD-PATH support is limited or unavailable
* Compact aggregation is desired
* Monitoring systems want single consolidated view
* Lower BGP state is important
Acknowledgements
The authors would like to thank the IDR working group for their
valuable feedback and suggestions on this proposal. Special thanks
to Pooja Jagadeesh Doijode for her review and to Ketan Talaulikar for
his guidance.
Authors' Addresses
Jeff Tantsura
Nvidia
Email: jefftant.ietf@gmail.com
Donald Sharp
Nvidia
Email: sharpd@nvidia.com
Vivek Venkatraman
Nvidia
Email: vivek@nvidia.com
Karthikeya Venkat Muppalla
Nvidia
Email: kmuppalla@nvidia.com
Maciej Rzehak
CoreWeave
Email: mrzehak@coreweave.com
Abderrahman Jouhari
Oracle
Email: jouharii@gmail.com
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Smit Parikh
Oracle
Email: smit.parikh@oracle.com
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