Internet-Draft PUA July 2021
Wang, et al. Expires 1 February 2022 [Page]
LSR Working Group
Intended Status:
Standards Track
A. Wang
China Telecom
G. Mishra
Verizon Inc.
Z. Hu
Huawei Technologies
Y. Xiao
Huawei Technologies

Prefix Unreachable Announcement


This document describes a mechanism to solve an existing issue with Longest Prefix Match (LPM), that exists where an operator domain is divided into multiple areas or levels where summarization is utilized. This draft addresses a fail-over issue related to a multi areas or levels domain, where a link or node down event occurs resulting in an LPM component prefix being omitted from the FIB resulting in black hole sink of routing and connectivity loss. This draft introduces a new control plane convergence signaling mechanism using a negative prefix called Prefix Unreachable Announcement (PUA), utilized to detect a link or node down event and signal the RIB that the event has occurred to force immediate control plane convergence.

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

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 1 February 2022.

Table of Contents

1. Introduction

As part of an operator optimized design criteria, a critical requirement is to limit Shortest Path First (SPF) churn which occurs within a single OSPF area or ISIS level. This is accomplished by sub-dividing the IGP domain into multiple areas for flood reduction of intra area prefixes so they are contained within each discrete area to avoid domain wide flooding.

OSPF and ISIS have a default and summary route mechanism which is performed on the OSPF area border router or ISIS L1-L2 node. The OSPF summary route is triggered to be advertised conditionally when at least one component prefix exists within the non-zero area. ISIS Level-L1-L2 node as well generate a summary prefix into the level-2 backbone area for Level 1 area prefixes that is triggered to be advertised conditionally when at least a single component prefix exists within the Level-1 area. ISIS L1-L2 node with attach bit set also generates a default route into each Level-1 area along with summary prefixes generated for other Level-1 areas.

Operators have historically relied on MPLS architecture which is based on exact match host route FEC binding for single area. [RFC5283] LDP inter-area extension provides the ability to LPM, so now the RIB match can now be a summary match and not an exact match of a host route of the egress PE for an inter-area LSP to be instantiated. SRV6 routing framework utilities the IPv6 data plane standard IGP LPM. When operators start to migrate from MPLS LSP based host route bootstrapped FEC binding, to SRv6 routing framework, the IGP LPM now comes into play with summarization which will influence the forwarding of traffic when a link or node event occurs for a component prefix within the summary range resulting in black hole routing of traffic.

The motivation behind this draft is based on either MPLS LPM FEC binding, or SRv6 BGP service overlay using traditional unicast routing (uRIB) LPM forwarding plane where the IGP domain has been carved up into OSPF or ISIS areas and summarization is utilized. In this scenario where a failure conditions result in a black hole of traffic where multiple ABRs exist and either the area is partitioned or other link or node failures occur resulting in the component prefix host route missing within the summary range. Summarization of inter-area types routes propagated into the backbone area for flood reduction are made up of component prefixes. It is these component prefixes that the PUA tracks to ensure traffic is not black hole sink routed due to a PE or ABR failure. The PUA mechanism ensures immediate control plane convergence with ABR or PE node switchover when area is partitioned or ABR has services down to avoid black hole of traffic.

2. Conventions used in this document

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 [RFC2119] .

3. Scenario Description

Figure 1 illustrates the topology scenario when OSPF or ISIS is running in multi areas or multi levels domain. R0-R4 are routers in backbone area, S1-S4,T1-T4 are internal routers in area 1 and area 2 respectively. R1 and R3 are area border routers or ISIS Level 1-2 border nodes between area 0 and area 1. R2 and R4 are area border routers between area 0 and area 2.

S1/S4 and T2/T4 PEs peer to customer CEs for overlay VPNs. Ps1/Ps4 is the loopback0 address of S1/S4 and Pt2/Pt4 is the loopback0 address of T2/T4.

 | +--+        +--+   ++-+   ++-+    +-++   + -+        +--+|
 | |S1+--------+S2+---+R1+---|R0+----+R2+---+T1+--------+T2||
 | +-++Ps1     +-++   ++-+   +--+    +-++   ++++    Pt2 +-++|
 |   |           |     |               |     ||           | |
 |   |           |     |               |     ||           | |
 | +-++Ps4     +-++   ++-+           +-++   ++++     Pt4+-++|
 | |S4+--------+S3+---+R3+-----------+R4+---+T3+--------+T4||
 | +--+        +--+   ++-+           +-++   ++-+        +--+|
 |                     |               |                    |
 |                     |               |                    |
 |         Area 1      |     Area 0    |      Area 2        |

Figure 1: OSPF Inter-Area Prefix Unreachable Announcement Scenario

3.1. Inter-Area Node Failure Scenario

If the area border router R2/R4 does the summary action, then one summary address that cover the prefixes of area 2 will be announced to area 0 and area 1, instead of the detail address. When the node T2 is down, Pt2 bgp next hop becomes unreachable while the LPM summary prefix continues to be advertised into the backbone area. Except the border router R2/R4, the other routers within area 0 and area 1 do not know the unreachable status of the Pt2 bgp next hop prefix. Traffic will continue to forward LPM match to prefix Pt2 and will be dropped on the ABR or Level 1-2 border node resulting in black hole routing and connectivity loss. Customer overlay VPN dual homed to both S1/S4 and T2/R4, traffic will not be able to fail-over to alternate egress PE T4 bgp next hop Pt4 due to the summarization.

4. PUA (Prefix Unreachable Advertisement) Procedures

[RFC7794] and [I-D.ietf-lsr-ospf-prefix-originator] draft both define one sub-tlv to announce the originator information of the one prefix from a specified node. This draft utilizes such TLV for both OSPF and ISIS to signal the negative prefix in the perspective PUA when a link or node goes down.

ABR detects link or node down and floods PUA negative prefix advertisement along with the summary advertisement according to the prefix-originator specification. The ABR or ISIS L1-L2 border node has the responsibility to add the prefix originator information when it receives the Router LSA from other routers in the same area or level.

When the ABR or ISIS L1-L2 border node generates the summary advertisement based on component prefixes, the ABR will announce one new summary LSA or LSP which includes the information about this down prefix, with the prefix originator set to NULL. The number of PUAs is equivalent to the number of links down or nodes down. The LSA or LSP will be propagated with standard flooding procedures.

If the nodes in the area receive the PUA flood from all of its ABR routers, they will start BGP convergence process if there exist BGP session on this PUA prefix. The PUA creates a forced fail over action to initiate immediate control plane convergence switchover to alternate egress PE. Without the PUA forced convergence the down prefix will yield black hole routing resulting in loss of connectivity.

When only some of the ABRs can't reach the failure node/link, as that described in Section 3.2, the ABR that can reach the PUA prefix should advertise one specific route to this PUA prefix. The internal routers within another area can then bypass the ABRs that can't reach the PUA prefix, to reach the PUA prefix.

5. MPLS and SRv6 LPM based BGP Next-hop Failure Application

In an MPLS or SR-MPLS service provider core, scalability has been a concern for operators which have split up the IGP domain into multiple areas to avoid SPF churn. Normally, MPLS FEC binding for LSP instantiation is based on egress PE exact match of a host route Looback0. [RFC5283] LDP inter-area extension provides the ability to LPM, so now the RIB match can now be a summary match and not an exact match of host route of the egress PE for an inter-area LSP to be instantiated. The caveat related to this feature that has prevented operators from using the [RFC5283] LDP inter-area extension concept is that when the component prefixes are now hidden in the summary prefix, and thus the visibility of the BGP next-hop attribute is lost.

In a case where a PE is down, and the [RFC5283] LDP inter-area extension LPM summary is used to build the LSP inter-area, the LSP remains partially established black hole on the ABR performing the summarization. This major gap with [RFC5283] inter-area extension forces operators into a workaround of having to flood the BGP next-hop domain wide. In a small network this is fine, however if you have 1000s PEs and many areas, the domain wide flooding can be painful for operators as far as resource usage memory consumption and computational requirements for RIB / FIB / LFIB label binding control plane state. The ramifications of domain wide flooding of host routes is described in detail in [RFC5302] domain wide prefix distribution with 2 level ISIS Section 1.2 - Scalability. As SRv6 utilizes LPM, this problem exists as well with SRv6 when IGP domain is broken up into areas and summarization is utilized.

PUA is now able to provide the negative prefix component flooded across the backbone to the other areas along with the summary prefix, which is now immediately programmed into the RIB control plane. MPLS LSP exact match or SRv6 LPM match over fail over path can now be established to the alternate egress PE. No disruption in traffic or loss of connectivity results from PUA. Further optimizations such as LFA and BFD can be done to make the data plane convergence hitless. The PUA solution applies to MPLS or SR-MPLS where LDP inter-area extension is utilized for LPM aggregate FEC, as well a SRv6 IPv6 control plane LPM match summarization of BGP next hop.

6. Implementation Consideration

Considering the balances of reachable information and unreachable information announcement capabilities, the implementation of this mechanism should set one MAX_Address_Announcement (MAA) threshold value that can be configurable. Then, the ABR should make the following decisions to announce the prefixes:

1. If the number of unreachable prefixes is less than MAA, the ABR should advertise the summary address and the PUA.

2. Else if the number of reachable address is less than MAA, the ABR should advertise the detail reachable address only.

3. Else, the number of reachable prefixes and unreachable prefixes both exceed MAA, then advertise the summary address with MAX metric.

7. Deployment Considerations

To support the PUA advertisement, the ABRs should be upgraded according to the procedures described in Section 4. The PEs that want to accomplish the BGP switchover that described in Section 3.1 and Section 5 should also be upgraded to act upon the receive of the PUA message. Other nodes within the network should ignore such PUA message if they don't care or don't support it. The routers within the IGP domain should not install erroneously the route to the prefixes when they receives PUA message.

As described in Section 4, the ABR will advertise the PUA message once it detects there is link or node down within the summary address. In order to reduce the unnecessary advertisements of PUA messages on ABRs, the ABRs should support the configuration of the protected prefixes. Based on such information, the ABR will only advertise the PUA message when the protected prefixes(for example, the loopback addresses of PEs that run BGP) that within the summary address is missing.

The advertisement of PUA message should only last one configurable period to allow the services that run on the failure prefixes are converged or switchover. If one prefix is missed before the PUA mechanism takes effect, the ABR will not declare its absence via the PUA mechanism.

8. Security Considerations

Advertisement of PUA information follow the same procedure of traditional LSA. The action based on the PUA is clearly defined in this document for ABR or Level1/2 router and the receiver that run BGP.

There is no changes to the forward behavior of other internal routers.

9. IANA Considerations

This document has no IANA actions.

10. Acknowledgement

Thanks Peter Psenak, Les Ginsberg, Acee Lindem, Shraddha Hegde, Robert Raszuk, Tonly Li, Jeff Tantsura, Tony Przygienda and Bruno Decraene for their suggestions and comments on this draft.

11. Normative References

Wang, A., Lindem, A., Dong, J., Psenak, P., and K. Talaulikar, "OSPF Prefix Originator Extensions", Work in Progress, Internet-Draft, draft-ietf-lsr-ospf-prefix-originator-12, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Moy, J., "OSPF Version 2", STD 54, RFC 2328, DOI 10.17487/RFC2328, , <>.
Decraene, B., Le Roux, JL., and I. Minei, "LDP Extension for Inter-Area Label Switched Paths (LSPs)", RFC 5283, DOI 10.17487/RFC5283, , <>.
Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix Distribution with Two-Level IS-IS", RFC 5302, DOI 10.17487/RFC5302, , <>.
Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF for IPv6", RFC 5340, DOI 10.17487/RFC5340, , <>.
Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M., Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic Authentication", RFC 5709, DOI 10.17487/RFC5709, , <>.
Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and S. Shaffer, "Extensions to OSPF for Advertising Optional Router Capabilities", RFC 7770, DOI 10.17487/RFC7770, , <>.
Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794, , <>.
Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions for Advertising Router Information", RFC 7981, DOI 10.17487/RFC7981, , <>.

Authors' Addresses

Aijun Wang
China Telecom
Beiqijia Town, Changping District
Gyan Mishra
Verizon Inc.
Zhibo Hu
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.
Yaqun Xiao
Huawei Technologies
Huawei Bld., No.156 Beiqing Rd.