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+Internet Engineering Task Force (IETF) S. Litkowski
+Request for Comments: 8541 Orange Business Service
+Category: Informational B. Decraene
+ISSN: 2070-1721 Orange
+ M. Horneffer
+ Deutsche Telekom
+ March 2019
+
+
+ Impact of Shortest Path First (SPF) Trigger and Delay Strategies
+ on IGP Micro-loops
+
+Abstract
+
+ A micro-loop is a packet-forwarding loop that may occur transiently
+ among two or more routers in a hop-by-hop packet-forwarding paradigm.
+
+ This document analyzes the impact of using different link state IGP
+ implementations in a single network with respect to micro-loops. The
+ analysis is focused on the Shortest Path First (SPF) delay algorithm
+ but also mentions the impact of SPF trigger strategies.
+
+Status of This Memo
+
+ This document is not an Internet Standards Track specification; it is
+ published for informational purposes.
+
+ This document is a product of the Internet Engineering Task Force
+ (IETF). It represents the consensus of the IETF community. It has
+ received public review and has been approved for publication by the
+ Internet Engineering Steering Group (IESG). Not all documents
+ approved by the IESG are candidates for any level of Internet
+ Standard; see Section 2 of RFC 7841.
+
+ Information about the current status of this document, any errata,
+ and how to provide feedback on it may be obtained at
+ https://www.rfc-editor.org/info/rfc8541.
+
+
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+Litkowski, et al. Informational [Page 1]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+Copyright Notice
+
+ Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of
+ the Trust Legal Provisions and are provided without warranty as
+ described in the Simplified BSD License.
+
+Table of Contents
+
+ 1. Introduction ....................................................3
+ 2. Problem Statement ...............................................4
+ 3. SPF Trigger Strategies ..........................................6
+ 4. SPF Delay Strategies ............................................6
+ 4.1. Two-Step SPF Delay .........................................7
+ 4.2. Exponential Back-Off Delay .................................7
+ 5. Mixing Strategies ...............................................9
+ 6. Benefits of Standardized SPF Delay Behavior ....................13
+ 7. Security Considerations ........................................14
+ 8. IANA Considerations ............................................14
+ 9. References .....................................................14
+ 9.1. Normative References ......................................14
+ 9.2. Informative References ....................................15
+ Acknowledgements ..................................................15
+ Authors' Addresses ................................................15
+
+
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+Litkowski, et al. Informational [Page 2]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+1. Introduction
+
+ Link state IGP protocols are based on a topology database on which
+ the SPF algorithm is run to find a consistent set of non-looping
+ routing paths.
+
+ Specifications like IS-IS [RFC1195] propose some optimizations of the
+ route computation (see Appendix C.1 of [RFC1195]), but not all
+ implementations follow those non-mandatory optimizations.
+
+ In this document, we refer to the events that lead to a new SPF
+ computation based on the topology as "SPF triggers".
+
+ Link state IGP protocols, like OSPF [RFC2328] and IS-IS [RFC1195],
+ use multiple timers to control the router behavior in case of churn:
+ SPF delay, Partial Route Computation (PRC) delay, Link State Packet
+ (LSP) generation delay, LSP flooding delay, and LSP retransmission
+ interval.
+
+ Some of the values and behaviors of these timers are standardized in
+ protocol specifications, and some are not. The SPF computation-
+ related timers have generally remained unspecified.
+
+ Implementations are free to implement non-standardized timers in any
+ way. For some standardized timers, implementations may offer
+ dynamically adjusted timers to help control the churn rather than use
+ static configurable values.
+
+ "SPF delay" refers to the timer in most implementations that
+ specifies the required delay before running an SPF computation after
+ an SPF trigger is received.
+
+ A micro-loop is a packet-forwarding loop that may occur transiently
+ among two or more routers in a hop-by-hop packet-forwarding paradigm.
+ These micro-loops are formed when two routers do not update their
+ Forwarding Information Bases (FIBs) for a certain prefix at the same
+ time. The micro-loop phenomenon is described in [MICROLOOP-LSRP].
+
+ Two micro-loop mitigation techniques have been defined by IETF. The
+ mechanism in [RFC6976] has not been widely implemented, presumably
+ due to the complexity of the technique. The mechanism in [RFC8333]
+ has been implemented. However, it does not prevent all micro-loops
+ that can occur for a given topology and failure scenario.
+
+
+
+
+
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+Litkowski, et al. Informational [Page 3]
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+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ In multi-vendor networks, using different implementations of a link
+ state protocol may favor micro-loop creation during the convergence
+ process due to discrepancies in timers. Service providers already
+ know to use timers with similar values and behaviors for all of the
+ network as a best practice, but this is sometimes not possible due to
+ the limitations of implementations.
+
+ This document presents reasons for service providers to have
+ consistent implementation of link state protocols across vendors. In
+ particular, this document analyzes the impact of using different link
+ state IGP implementations in a single network with regard to micro-
+ loops. The analysis focuses on the SPF delay algorithm.
+
+ [RFC8405] defines a solution that partially addresses this problem
+ statement, and this document captures the reasoning of the provided
+ solution.
+
+2. Problem Statement
+
+ S ---- E
+ | |
+ 10 | | 10
+ | |
+ D ---- A
+ | 2
+ Px
+
+ Figure 1: Network Topology Experiencing Micro-loops
+
+ Figure 1 represents a small network composed of four routers (S, D,
+ E, and A). Router S primarily uses the SD link to reach the prefixes
+ behind router D (named Px). When the SD link fails, the IGP
+ convergence occurs. If S converges before E, S will forward the
+ traffic to Px through E; however, because E has not converged yet, E
+ will loop the traffic back to S, leading to a micro-loop.
+
+ The micro-loop appears due to the asynchronous convergence of nodes
+ in a network when an event occurs.
+
+ Multiple factors (or a combination of factors) may increase the
+ probability of a micro-loop appearing:
+
+ o Delay of failure notification: The greater the time gap between E
+ and S being advised of the failure, the greater the chance that a
+ micro-loop may appear.
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 4]
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+RFC 8541 SPF Impact on IGP Micro-loops March 2019
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+ o SPF delay: Most implementations support a delay for the SPF
+ computation to catch as many events as possible. If S uses an SPF
+ delay timer of x ms, E uses an SPF delay timer of y ms, and x < y,
+ E would start converging after S, leading to a potential micro-
+ loop.
+
+ o SPF computation time: This is mostly a matter of CPU power and
+ optimizations like incremental SPF. If S computes its SPF faster
+ than E, there is a chance for a micro-loop to appear. Today, CPUs
+ are fast enough to consider the SPF computation time as negligible
+ (on the order of milliseconds in a large network).
+
+ o SPF computation ordering: An SPF trigger can be common to multiple
+ IGP areas or levels (e.g., IS-IS Level 1 and Level 2) or to
+ multiple address families with multi-topologies. There is no
+ specified order for SPF computation today, and it is
+ implementation dependent. In such scenarios, if the order of SPF
+ computation done in S and E for each area, level, topology, or SPF
+ algorithm is different, there is a possibility for a micro-loop to
+ appear.
+
+ o RIB and FIB prefix insertion speed or ordering: This is highly
+ dependent on the implementation.
+
+ Even if all of these factors increase the probability of a micro-loop
+ appearing, the SPF delay plays a significant role, especially in case
+ of churn. As the number of IGP events increases, the delta between
+ the SPF delay values used by routers becomes significant; in fact, it
+ becomes the dominating factor (especially when one router increases
+ its timer exponentially while another one increases it in a smoother
+ way). Another important factor is the time to update the FIB. As of
+ today, the total FIB update time is the major factor for IGP
+ convergence. However, for micro-loops, what matters is not the total
+ time but the difference in installing the same prefix between nodes.
+ The time to update the FIB may be the main part for the first
+ iteration but not for subsequent IGP events. In addition, the time
+ to update the FIB is very implementation specific and difficult or
+ impossible to standardize, while the SPF delay algorithm may be
+ standardized.
+
+ As a consequence, this document will focus on an analysis of SPF
+ delay behavior and associated triggers.
+
+
+
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 5]
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+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+3. SPF Trigger Strategies
+
+ Depending on the change advertised in the LSP or LSA (Link State
+ Advertisement), the topology may or may not be affected. An
+ implementation may avoid running the SPF computation (and may only
+ run an IP reachability computation instead) if the advertised change
+ does not affect the topology.
+
+ Different strategies can trigger the SPF computation:
+
+ 1. An implementation may always run a full SPF for any type of
+ change.
+
+ 2. An implementation may run a full SPF only when required. For
+ example, if a link fails, a local node will run an SPF for its
+ local LSP update. If the LSP from the neighbor (describing the
+ same failure) is received after SPF has started, the local node
+ can decide that a new full SPF is not required as the topology
+ has not changed.
+
+ 3. If the topology does not change, an implementation may only
+ recompute the IP reachability.
+
+ As noted in Section 1, SPF optimizations are not mandatory in
+ specifications. This has led to the implementation of different
+ strategies.
+
+4. SPF Delay Strategies
+
+ Implementations of link state routing protocols use different
+ strategies to delay SPF computation. The two most common SPF delay
+ behaviors are the following:
+
+ 1. Two-step SPF delay
+
+ 2. Exponential back-off delay
+
+ These behaviors are explained in the following sections.
+
+
+
+
+
+
+
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+Litkowski, et al. Informational [Page 6]
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+RFC 8541 SPF Impact on IGP Micro-loops March 2019
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+
+4.1. Two-Step SPF Delay
+
+ The SPF delay is managed by four parameters:
+
+ o rapid delay: the amount of time to wait before running SPF after
+ the initial SPF trigger event.
+
+ o rapid runs: the number of consecutive SPF runs that can use the
+ rapid delay. When the number is exceeded, the delay moves to the
+ slow delay value.
+
+ o slow delay: the amount of time to wait before running an SPF.
+
+ o wait time: the amount of time to wait without detecting SPF
+ trigger events before going back to the rapid delay.
+
+ Figure 2 displays the evolution of the SPF delay timer (based on a
+ two-step delay algorithm) upon the reception of multiple events.
+ Figure 2 considers the following parameters for the algorithm: rapid
+ delay (RD) = 50 ms, rapid runs = 3, slow delay (SD) = 1 s, wait time
+ = 2 s.
+
+ SPF delay time
+ ^
+ |
+ |
+ SD- | x xx x
+ |
+ |
+ |
+ RD- | x x x x
+ |
+ +---------------------------------> Events
+ | | | | || | |
+ < wait time >
+
+ Figure 2: Two-Step SPF Delay Algorithm
+
+4.2. Exponential Back-Off Delay
+
+ The algorithm has two modes: fast mode and back-off mode. In fast
+ mode, the SPF delay is usually delayed by a very small amount of time
+ (fast reaction). When an SPF computation is run in fast mode, the
+ algorithm automatically moves to back-off mode (a single SPF run is
+ authorized in fast mode). In back-off mode, the SPF delay increases
+ exponentially in each run. When the network becomes stable, the
+ algorithm moves back to fast mode. The SPF delay is managed by four
+ parameters:
+
+
+
+Litkowski, et al. Informational [Page 7]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ o first delay: amount of time to wait before running SPF. This
+ delay is used only when SPF is in fast mode.
+
+ o incremental delay: amount of time to wait before running SPF.
+ This delay is used only when SPF is in back-off mode and
+ increments exponentially at each SPF run.
+
+ o maximum delay: maximum amount of time to wait before running SPF.
+
+ o wait time: amount of time to wait without events before going back
+ to fast mode.
+
+ Figure 3 displays the evolution of the SPF delay timer (based on an
+ exponential back-off delay algorithm) upon the reception of multiple
+ events. Figure 3 considers the following parameters for the
+ algorithm: first delay (FD) = 50 ms, incremental delay (ID) = 50 ms,
+ maximum delay (MD) = 1 s, wait time = 2 s
+
+ SPF delay time
+ ^
+ MD- | xx x
+ |
+ |
+ |
+ |
+ |
+ | x
+ |
+ |
+ |
+ | x
+ |
+ FD- | x x x
+ ID |
+ +---------------------------------> Events
+ | | | | || | |
+ < wait time >
+ FM->BM -------------------->FM
+
+ Figure 3: Exponential Back-Off Delay Algorithm
+
+
+
+
+
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 8]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+5. Mixing Strategies
+
+ Figure 1 illustrates a flow of packets from S to D. S uses optimized
+ SPF triggering (full SPF is triggered only when necessary) and two-
+ step SPF delay (rapid delay = 150 ms, rapid runs = 3, slow delay = 1
+ s). As the implementation of S is optimized, PRC is available. For
+ PRC delay, we consider the same timers as for SPF delay. E uses an
+ SPF trigger strategy that always computes a full SPF for any change
+ and uses the exponential back-off strategy for SPF delay (first delay
+ = 150 ms, incremental delay = 150 ms, maximum delay = 1 s).
+
+ Consider the following sequence of events:
+
+ o t0=0 ms: A prefix is declared down in the network. This event
+ happens at time=0.
+
+ o 200 ms: The prefix is declared up.
+
+ o 400 ms: The prefix is declared down in the network.
+
+ o 1000 ms: S-D link fails.
+
+ +---------+-------------------+------------------+------------------+
+ | Time | Network Event | Router S Events | Router E Events |
+ +---------+-------------------+------------------+------------------+
+ | t0=0 | Prefix DOWN | | |
+ | 10 ms | | Schedule PRC (in | Schedule SPF (in |
+ | | | 150 ms) | 150 ms) |
+ | | | | |
+ | | | | |
+ | 160 ms | | PRC starts | SPF starts |
+ | 161 ms | | PRC ends | |
+ | 162 ms | | RIB/FIB starts | |
+ | 163 ms | | | SPF ends |
+ | 164 ms | | | RIB/FIB starts |
+ | 175 ms | | RIB/FIB ends | |
+ | 178 ms | | | RIB/FIB ends |
+ | | | | |
+ | 200 ms | Prefix UP | | |
+ | 212 ms | | Schedule PRC (in | |
+ | | | 150 ms) | |
+ | 214 ms | | | Schedule SPF (in |
+ | | | | 150 ms) |
+ | | | | |
+ | | | | |
+ | 370 ms | | PRC starts | |
+ | 372 ms | | PRC ends | |
+ | 373 ms | | | SPF starts |
+
+
+
+Litkowski, et al. Informational [Page 9]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ | 373 ms | | RIB/FIB starts | |
+ | 375 ms | | | SPF ends |
+ | 376 ms | | | RIB/FIB starts |
+ | 383 ms | | RIB/FIB ends | |
+ | 385 ms | | | RIB/FIB ends |
+ | | | | |
+ | 400 ms | Prefix DOWN | | |
+ | 410 ms | | Schedule PRC (in | Schedule SPF (in |
+ | | | 300 ms) | 300 ms) |
+ | | | | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | 710 ms | | PRC starts | SPF starts |
+ | 711 ms | | PRC ends | |
+ | 712 ms | | RIB/FIB starts | |
+ | 713 ms | | | SPF ends |
+ | 714 ms | | | RIB/FIB starts |
+ | 716 ms | | RIB/FIB ends | RIB/FIB ends |
+ | | | | |
+ | 1000 ms | S-D link DOWN | | |
+ | 1010 ms | | Schedule SPF (in | Schedule SPF (in |
+ | | | 150 ms) | 600 ms) |
+ | | | | |
+ | | | | |
+ | 1160 ms | | SPF starts | |
+ | 1161 ms | | SPF ends | |
+ | 1162 ms | Micro-loop may | RIB/FIB starts | |
+ | | start from here | | |
+ | 1175 ms | | RIB/FIB ends | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | 1612 ms | | | SPF starts |
+ | 1615 ms | | | SPF ends |
+ | 1616 ms | | | RIB/FIB starts |
+ | 1626 ms | Micro-loop ends | | RIB/FIB ends |
+ +---------+-------------------+------------------+------------------+
+
+ Table 1: Route Computation When S and E Use Different Behaviors and
+ Multiple Events Appear
+
+
+
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 10]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ In Table 1, due to discrepancies in the SPF management and after
+ multiple events of different types, the values of the SPF delay are
+ completely misaligned between node S and node E, leading to the
+ creation of micro-loops.
+
+ The same issue can also appear with only a single type of event as
+ shown below:
+
+ +---------+-------------------+------------------+------------------+
+ | Time | Network Event | Router S Events | Router E Events |
+ +---------+-------------------+------------------+------------------+
+ | t0=0 | Link DOWN | | |
+ | 10 ms | | Schedule SPF (in | Schedule SPF (in |
+ | | | 150 ms) | 150 ms) |
+ | | | | |
+ | | | | |
+ | 160 ms | | SPF starts | SPF starts |
+ | 161 ms | | SPF ends | |
+ | 162 ms | | RIB/FIB starts | |
+ | 163 ms | | | SPF ends |
+ | 164 ms | | | RIB/FIB starts |
+ | 175 ms | | RIB/FIB ends | |
+ | 178 ms | | | RIB/FIB ends |
+ | | | | |
+ | 200 ms | Link DOWN | | |
+ | 212 ms | | Schedule SPF (in | |
+ | | | 150 ms) | |
+ | 214 ms | | | Schedule SPF (in |
+ | | | | 150 ms) |
+ | | | | |
+ | | | | |
+ | 370 ms | | SPF starts | |
+ | 372 ms | | SPF ends | |
+ | 373 ms | | | SPF starts |
+ | 373 ms | | RIB/FIB starts | |
+ | 375 ms | | | SPF ends |
+ | 376 ms | | | RIB/FIB starts |
+ | 383 ms | | RIB/FIB ends | |
+ | 385 ms | | | RIB/FIB ends |
+ | | | | |
+ | 400 ms | Link DOWN | | |
+ | 410 ms | | Schedule SPF (in | Schedule SPF (in |
+ | | | 150 ms) | 300 ms) |
+ | | | | |
+ | | | | |
+ | 560 ms | | SPF starts | |
+ | 561 ms | | SPF ends | |
+
+
+
+
+Litkowski, et al. Informational [Page 11]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ | 562 ms | Micro-loop may | RIB/FIB starts | |
+ | | start from here | | |
+ | 568 ms | | RIB/FIB ends | |
+ | | | | |
+ | | | | |
+ | 710 ms | | | SPF starts |
+ | 713 ms | | | SPF ends |
+ | 714 ms | | | RIB/FIB starts |
+ | 716 ms | Micro-loop ends | | RIB/FIB ends |
+ | | | | |
+ | 1000 ms | Link DOWN | | |
+ | 1010 ms | | Schedule SPF (in | Schedule SPF (in |
+ | | | 1 s) | 600 ms) |
+ | | | | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | 1612 ms | | | SPF starts |
+ | 1615 ms | | | SPF ends |
+ | 1616 ms | Micro-loop may | | RIB/FIB starts |
+ | | start from here | | |
+ | 1626 ms | | | RIB/FIB ends |
+ | | | | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | 2012 ms | | SPF starts | |
+ | 2014 ms | | SPF ends | |
+ | 2015 ms | | RIB/FIB starts | |
+ | 2025 ms | Micro-loop ends | RIB/FIB ends | |
+ | | | | |
+ | | | | |
+ +---------+-------------------+------------------+------------------+
+
+ Table 2: Route Computation upon Multiple Link Down Events When S and
+ E Use Different Behaviors
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 12]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+6. Benefits of Standardized SPF Delay Behavior
+
+ Table 3 uses the same event sequence as Table 1. Fewer and/or
+ shorter micro-loops are expected using a standardized SPF delay.
+
+ +---------+-------------------+------------------+------------------+
+ | Time | Network Event | Router S Events | Router E Events |
+ +---------+-------------------+------------------+------------------+
+ | t0=0 | Prefix DOWN | | |
+ | 10 ms | | Schedule PRC (in | Schedule PRC (in |
+ | | | 150 ms) | 150 ms) |
+ | | | | |
+ | | | | |
+ | 160 ms | | PRC starts | PRC starts |
+ | 161 ms | | PRC ends | |
+ | 162 ms | | RIB/FIB starts | PRC ends |
+ | 163 ms | | | RIB/FIB starts |
+ | 175 ms | | RIB/FIB ends | |
+ | 176 ms | | | RIB/FIB ends |
+ | | | | |
+ | 200 ms | Prefix UP | | |
+ | 212 ms | | Schedule PRC (in | |
+ | | | 150 ms) | |
+ | 213 ms | | | Schedule PRC (in |
+ | | | | 150 ms) |
+ | | | | |
+ | | | | |
+ | 370 ms | | PRC starts | PRC starts |
+ | 372 ms | | PRC ends | |
+ | 373 ms | | RIB/FIB starts | PRC ends |
+ | 374 ms | | | RIB/FIB starts |
+ | 383 ms | | RIB/FIB ends | |
+ | 384 ms | | | RIB/FIB ends |
+ | | | | |
+ | 400 ms | Prefix DOWN | | |
+ | 410 ms | | Schedule PRC (in | Schedule PRC (in |
+ | | | 300 ms) | 300 ms) |
+ | | | | |
+ | | | | |
+ | | | | |
+ | | | | |
+ | 710 ms | | PRC starts | PRC starts |
+ | 711 ms | | PRC ends | PRC ends |
+ | 712 ms | | RIB/FIB starts | |
+ | 713 ms | | | RIB/FIB starts |
+ | 716 ms | | RIB/FIB ends | RIB/FIB ends |
+ | | | | |
+ | 1000 ms | S-D link DOWN | | |
+
+
+
+Litkowski, et al. Informational [Page 13]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ | 1010 ms | | Schedule SPF (in | Schedule SPF (in |
+ | | | 150 ms) | 150 ms) |
+ | | | | |
+ | | | | |
+ | 1160 ms | | SPF starts | |
+ | 1161 ms | | SPF ends | SPF starts |
+ | 1162 ms | Micro-loop may | RIB/FIB starts | SPF ends |
+ | | start from here | | |
+ | 1163 ms | | | RIB/FIB starts |
+ | 1175 ms | | RIB/FIB ends | |
+ | 1177 ms | Micro-loop ends | | RIB/FIB ends |
+ +---------+-------------------+------------------+------------------+
+
+ Table 3: Route Computation When S and E Use the Same Standardized
+ Behavior
+
+ As displayed above, there can be other parameters, like router
+ computation power and flooding timers, that may also influence micro-
+ loops. In all the examples in this document comparing the SPF timer
+ behavior of router S and router E, we have made router E a bit slower
+ than router S. This can lead to micro-loops even when both S and E
+ use a common standardized SPF behavior. However, by aligning
+ implementations of the SPF delay, we expect that service providers
+ may reduce the number and duration of micro-loops.
+
+7. Security Considerations
+
+ This document does not introduce any security considerations.
+
+8. IANA Considerations
+
+ This document has no actions for IANA.
+
+9. References
+
+9.1. Normative References
+
+ [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
+ dual environments", RFC 1195, DOI 10.17487/RFC1195,
+ December 1990, <https://www.rfc-editor.org/info/rfc1195>.
+
+ [RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
+ DOI 10.17487/RFC2328, April 1998,
+ <https://www.rfc-editor.org/info/rfc2328>.
+
+
+
+
+
+
+
+Litkowski, et al. Informational [Page 14]
+
+RFC 8541 SPF Impact on IGP Micro-loops March 2019
+
+
+ [RFC8405] Decraene, B., Litkowski, S., Gredler, H., Lindem, A.,
+ Francois, P., and C. Bowers, "Shortest Path First (SPF)
+ Back-Off Delay Algorithm for Link-State IGPs", RFC 8405,
+ DOI 10.17487/RFC8405, June 2018,
+ <https://www.rfc-editor.org/info/rfc8405>.
+
+9.2. Informative References
+
+ [MICROLOOP-LSRP]
+ Zinin, A., "Analysis and Minimization of Microloops in
+ Link-state Routing Protocols", Work in Progress,
+ draft-ietf-rtgwg-microloop-analysis-01, October 2005.
+
+ [RFC6976] Shand, M., Bryant, S., Previdi, S., Filsfils, C.,
+ Francois, P., and O. Bonaventure, "Framework for Loop-Free
+ Convergence Using the Ordered Forwarding Information Base
+ (oFIB) Approach", RFC 6976, DOI 10.17487/RFC6976, July
+ 2013, <https://www.rfc-editor.org/info/rfc6976>.
+
+ [RFC8333] Litkowski, S., Decraene, B., Filsfils, C., and P.
+ Francois, "Micro-loop Prevention by Introducing a Local
+ Convergence Delay", RFC 8333, DOI 10.17487/RFC8333, March
+ 2018, <https://www.rfc-editor.org/info/rfc8333>.
+
+Acknowledgements
+
+ The authors would like to thank Mike Shand and Chris Bowers for their
+ useful comments.
+
+Authors' Addresses
+
+ Stephane Litkowski
+ Orange Business Service
+
+ Email: stephane.litkowski@orange.com
+
+
+ Bruno Decraene
+ Orange
+
+ Email: bruno.decraene@orange.com
+
+
+ Martin Horneffer
+ Deutsche Telekom
+
+ Email: martin.horneffer@telekom.de
+
+
+
+
+Litkowski, et al. Informational [Page 15]
+