[apops] Update to RIPE-210 - final draft

  • To: routing-wg at ripe dot net
  • Subject: [apops] Update to RIPE-210 - final draft
  • From: Philip Smith <pfs at cisco dot com>
  • Date: Sat, 29 Sep 2001 23:38:14 +1000
  • Sender: owner-apops@lists.apnic.net
    • bcc: nanog, apops


      Please find attached the final draft of the document which will replace RIPE-210, the RIPE Routing WG recommendation for coordinated route flap damping parameters.

      The authors would welcome any further comments between now and the meeting of the Routing WG on Wednesday 3rd October at the RIPE Meeting in Prague.



      RIPE Routing-WG Recommendation for coordinated route-flap damping parameters

      Philip Smith
      Cristina Vistoli
      Christian Panigl
      Joachim Schmitz

      This document Obsoletes: ripe-210, ripe-178


      Document status Version 2.0, September 28th, 2001


      This paper recommends a set of route-flap damping parameters which should
      be applied by all ISPs in the Internet and should be deployed as default
      values by BGP router vendors.

      Table of Contents

          1. Introduction
          1.1 Motivation for route-flap damping
          1.2 What is route-flap damping ?
          1.3 "Progressive" versus "flat&gentle" approach
          1.4 Motivation for coordinated parameters
          1.5 Aggregation versus damping
          1.6 "Golden Networks"
          2. Recommended damping parameters
          2.1 Motivation for recommendation
          2.2 Description of recommended damping parameters
          3. Other Features contributing to Internet Stability
          3.1 BGP Route Refresh
          3.2 Soft Reconfiguration
          3.3 Tuning External BGP Failover
          4. Potential problems
          4.1 Multiplication of flaps between ASes with multiple interconnections
          4.2 Non-recommended flap damping parameters
          5. References
          6. Acknowledgements
          7. Changes over Previous Versions
          8. Authors
          A.1 "Golden Networks" Reference
          A.2 Sample Configurations Reference
          A.3 Study of Flap Damping Operation

      1. Introduction

      Route-flap damping is a mechanism for (BGP) routers which is aimed at
      improving the overall stability of the Internet routing table and reducing
      the load on the CPUs of the core routers.

      1.1 Motivation for route-flap damping

      In the early 1990s the accelerating growth in the number of prefixes being
      announced to the Internet (often due to inadequate prefix-aggregation),
      the denser meshing through multiple inter-provider paths, and increased
      instabilities started to cause significant impact on the performance and
      efficiency of the Internet backbone routers. Every time a routing prefix
      becomes unreachable because of a single line-flap, the withdrawal has to
      be advertised to the whole core Internet and dealt with by every single
      router which is carrying the full Internet routing table.

      To overcome this situation a route-flap damping mechanism was invented in
      1993 and has been integrated into several router software implementations
      since 1995 (for example, Cisco, Merit/RSd, GateD Consortium). The
      implementation is described in detail in RFC2439. The flap damping mechanism
      is now widely used to help keep severe instabilities under control and
      more localised in the Internet.

      And there is a second benefit: it is raising the awareness of the
      existence of instabilities because severe route/line-flapping problems
      lead to permanent suppression of the unstable area by means of holding
      down the flapping prefixes.

      Route-flap damping has its greatest and most consistent value if it
      is applied as near to the source of the problem as possible. Therefore
      flap-damping should be applied both at peering and upstream boundaries,
      as well as at customer boundaries (see 1.4 and 1.5 for details).

      1.2 What is route-flap damping ?

      When BGP route-flap damping is enabled in a router the router starts
      to collect statistics about the announcement and withdrawal of
      prefixes. Route-flap damping is governed by a set of parameters with
      vendor-supplied default values which may be modified by the router
      manager. The names, semantic and syntax of these parameters differ
      between the various implementations; however, the behaviour of the
      damping mechanism is basically the same.

       Each time a prefix is withdrawn, the router will increment the damping
       penalty by a fixed amount. When the number of withdrawals/announcements
       (=flap) is exceeded in a given time frame (cutoff threshold) the
       path is no longer used and not advertised to any BGP neighbour for a
       predetermined period starting from when the prefix stops flapping. Any
       more flaps happening after the prefix enters suppressed state will
       attract additional penalty. Once the prefix stops flapping, the penalty
       is decremented over time using a half-life parameter until the penalty is
       below a reuse threshold. Once below this reuse threshold the suppressed
       path is then re-used and re-advertised to BGP neighbours.

      Pointers to some more detailed and vendor specific documents are listed
      in "5. References".

      1.3 "Progressive" versus "flat&gentle" approach

      One easy approach would be to just apply the current default-parameters
      which are treating all prefixes equally ("flat&gentle") everywhere. However,
      there is a major concern to penalise longer prefixes (=smaller aggregates)
      more than well aggregated short prefixes ("progressive"), because the
      number of short prefixes in the routing table is significantly lower and
      it seems in general that those are tending to be more stable and also are
      tending to affect more users.

      Another aspect is that progressive damping might increase the awareness
      of aggregation needs. However, it has to be accompanied by a careful
      design which doesn't force a rush to request and assign more address space
      than needed.

      A significant number of important services is sitting in long prefixes
      (e.g. root name servers), so the progressive approach has to exclude the
      strong penalisation for these so-called "golden" prefixes.

      With this recommendation we are trying to make a compromise and it is
      therefore called "graded damping".

      1.4 Motivation for coordinated parameters

      There is a strong need for the coordinated use of damping parameters for
      several reasons:

      Coordination of "progressiveness":

      If penalties are not coordinated throughout the Internet, route-flap damping
      could lead to additional flapping or inconsistent routing because longer
      prefixes might already be re-announced through some parts of the Internet
      where shorter prefixes are still held down through other paths.

      Coordination of hold-down and reuse-threshold parameters between ISPs:

      If an upstream or peering provider would be damping more aggressively
      (e.g. triggered by less flaps or applying longer hold-down timers) than an
      access-provider towards his customers it will lead to a very inconsistent
      situation, where a flapping network might still be able to reach "near-line"
      parts of the Internet. Debugging of such instabilities is then much harder
      because the effect for the customer leads to the assumption that there
      is a problem "somewhere" in the "upstream" Internet instead of making him
      just call his ISPs hot-line and complain that he can't get out any longer.

      Further, after successful repair of the problem the access-provider
      can easily clear the flap-damping for his customer on his local router
      instead of needing to contact upstream NOCs all over the Internet to get
      the damping cleared.

      Vendor Defaults:

      As with most software implementations, there need to be some default values
      set when route-flap damping is enabled on routers. Vendors choosing
      different default values will result in a similar situation to that
      described above, where the more aggressive values will result in "black
      spots" in the Internet. Coordinated values will ensure consistency in
      dealing with instabilities.

      1.5 Aggregation versus damping

      If a customer of an ISP is only using Provider Aggregated addresses,
      the aggregating upstream provider doesn't need to apply damping on these
      prefixes towards his customer, because instabilities of such prefixes
      will not propagate into the Internet. However, if a customer insists on
      announcing prefixes which can't be aggregated by its provider, damping
      should be applied. Reasons for leaking prefixes might include dual-homing
      (to different providers) of a customer, or customer's reluctance to renumber
      into the provider's aggregated address range.

      1.6 "Golden Networks"

      Even though damping is strongly recommended, in some cases it may make sense
      to exclude certain networks or even individual hosts from damping.  This is
      especially true if damping would cut off the access to vital infrastructure
      elements of the Internet. A most prominent example are the root name servers.

      At least in principle, there should be enough redundancy for root name
      servers. However we are still facing a situation where, at least outside USA,
      large parts of the Internet are seeing all of them through the same one or
      two backbone/upstream links (undersea cable) and any instability of those
      links which is triggering damping would unnecessarily prolong the
      inaccessibility of the root name servers for an hour (at least those sitting
      in a /24 or longer prefix).

      Other examples of inclusions in the "Golden Networks" might be the Global
      Top Level Domain (gTLD) name servers, and possibly overseas or "special"
      networks the local ISP wishes to have continued connectivity to regardless
      of the instability of the infrastructure inbetween.

      Appendix A.1 references a website which the authors believe represent an
      example of suitable Golden Networks. While the authors will endeavour to
      keep the website current, network managers are strongly encouraged to
      check that the networks listed are indeed still being announced and the
      hosts therein are still being used before implementation of route flap
      damping using the quoted Golden Networks. This can be done by matching
      BGP table announcements with the published addresses for the listed

      These exceptions must only be made if there are strong and identifiable
      needs for them - the rule should be to apply coordinated route flap
      damping throughout.

      2. Recommended damping parameters

      2.1 Motivation for recommendation

      At RIPE26 and 27 Christian Panigl presented the following network backbone
      maintenance example from his own experience, which was triggering flap
      damping in some upstream and peering ISPs routers for all his and his
      customers /24 prefixes for more than 3 hours because of too "aggressive"

      scheduled SW upgrade of backbone router failed:

         - reload after SW upgrade       1 flap
         - new SW crashed                1 flap
         - reload with old SW            1 flap
                                         3 flaps within 10 minutes

      which resulted in the following damping scenario at some boundaries with
      progressive route-flap damping enabled:

      Prefix length:      /24     /19     /16
      suppress time:      ~3h     45-60'  <30'

      Therefore, in the Routing-WG session at RIPE27, it was agreed that
      suppression should not start until the 4th flap in a row and that the
      maximum suppression should in no case last longer than 1 hour from the
      last flap.

      It was agreed that a recommendation from RIPE would be desirable. Given that
      the current allocation policies are expected to hold for the foreseeable
      future, it was suggested that all /19's or shorter prefixes are not
      penalised harder (longer) than current Cisco default damping does. More
      recently, this recommendation has been altered so that only prefixes longer
      than a /21 are now damped more aggresively. The registries' minimum
      allocation is currently a /20, and a /21 announcement is quite feasible for
      a multihoming situation.

      With these suggestions in mind, Tony Barber (UUNET) designed the following
      set of route-flap damping parameters which have proved to work smoothly in his
      environment for a couple of months prior to the publication of RIPE-178 (the
      original version of this document).

      2.2 Description of recommended damping parameters

      Basically the recommended values do the following with harsher treatment
      for /24 and longer prefixes:

         * don't start damping until the 4th flap
         * /24 and longer prefixes: max=min outage 60 minutes
         * /22 and /23 prefixes: max outage 45 minutes; min outage of 30 minutes
         * all other prefix lengths: max outage 30 minutes; min outage 10 minutes

      If a specific damping implementation does not allow configuration of
      prefix-dependent parameters the least aggressive set should be used:

         * don't start damping before the 4th flap in a row
         * max outage 30 minutes; min outage 10 minutes

      Sample configurations for different vendors are referenced in Appendix A.2.
      These samples can be used as a basis for a configuration on other router
      platforms not listed there.

      3. Other Features contributing to Internet Stability

      3.1 BGP Route refresh

      RFC2918 describes a Route Refresh Capability for BGP-4. Prior to this, there
      was no mechanism to reset or refresh a BGP peering session without tearing
      it down and waiting for it to re-establish. This process is destructive -
      prefixes being exchanged between the two peering routers are withdrawn from
      their respective ASes, and this withdrawal can potentially pass through
      the whole Internet causing the burden and increased instability discussed
      earlier. Usually all that an ISP wishes when reseting a BGP session is to
      implement new or revised policy - destroying a BGP session carrying a large
      or the full routing table has severe impact on the ISP and his neighbours
      on the Internet. Furthermore, reset of a BGP session means the withdrawal
      of reachability information from the ISP's customers, and they have the
      perception that the Internet has "vanished" - the impression left with
      the end-user is that of an unreliable network.

      Route Refresh implements a messaging system whereby a router wishing to
      refresh or reset its BGP peering with its neighbour simply has to send the
      notification. When the neighbour receives the notification, it will send
      its entire announcement to its peer (obtained from BGP best path table and
      applicable outbound policy).

      To find out if your neighbour supports Route Refresh, using Cisco IOS as an
      example, enter:

      Router# sho ip bgp neigh w.x.y.c | include refresh
        Received route refresh capability(new) from peer
        Route refresh request: received 0, sent 0

      If your router and your peer router support Route Refresh, you can use:

      Router# clear ip bgp w.x.y.c in

      for requesting a route refresh without clearing the BGP session.

      For an outbound route refresh without clearing the BGP session use

      Router# clear ip bgp w.x.y.c out

      It is recommended that all users of BGP use the route refresh capability
      when implementing new BGP policy.

      3.2 Soft-Reconfiguration

      Where the neighbour does not support RFC 2918 Route Refresh, router
      vendors have implemented functionality to all the alternatiom of BGP
      policy without resetting the BGP session.

      In Cisco IOS this functionality is called "Soft Reconfiguration". This
      reserves additional memory in the router to store the BGP table exactly
      as it was received from the peer, prior to any inbound policy being
      applied. The advantage of this is that the ISP can then change any
      inbound policy on the router without reseting the BGP session - the router
      simply uses the "raw" BGP table it has received from its peer.
      Disadvantage is that this functionality could potentially consume almost
      twice the amount of memory required for the BGP table heard from the peer.

      To configure soft-reconfiguration in IOS, simply add the extra line to
      the BGP peer configuration as below.  Soft-reconfiguration is configured
      on a per-neighbour basis.

      router bgp 65501
       neighbor remote-as 65502
       neighbor soft-reconfiguration inbound

      Without the keyword "soft" a "clear ip bgp x.x.x.x" will completely reset
      the BGP session and therefore always withdraw all announced prefixes from/to
      neighbour x.x.x.x and re-advertise them (= route-flap for all prefixes which
      are available before and after the clear). With "clear ip bgp x.x.x.x
      soft out" the router doesn't reset the BGP session itself but sends an
      update for all its advertised prefixes. With "clear ip bgp x.x.x.x soft
      in" the router just compares the already received routes (stored in the
      "received" data structures) from the neighbour against locally configured
      inbound policy statements.

      In Juniper's JunOS software, all the prefixes advertised by a peer are
      stored on the router, allowing the router to re-evaluate new policies on
      the set of routes advertised by the peer. So in the event of a peer not
      supporting the route-refresh capability, JunOS default configuration
      will compensate for this in the same way the optional "soft-reconfiguration"
      support in IOS.

      It is recommended to use soft-reconfiguration with all peers which do not
      support RFC2918 Route Refresh Capability to avoid tearing down and
      restarting BGP peerings when new BGP policies need to be implemented.

      3.3 Tuning External BGP Failover

      Cisco IOS by default implements a feature known as "fast-external-fallover".
      This feature immediately clears the BGP session whenever the line-protocol
      to the external neighbour goes down. This feature is desirable so that
      there is fast failover in case of link failures - the router can withdraw
      paths as soon as the line goes down, rather than waiting for BGP keepalive
      timers.  The drawback of this, however, is that circuits which are prone
      to unreliability will cause BGP sessions to drop and return (i.e. flap),
      resulting in instability within the ISP's network, and the potential for
      flap damping by upstreams or peers.

      If fast-external-fallover is turned off, the BGP sessions will survive
      these short line-flaps as it will use the longer BGP keepalive/hold timers
      (default 60/180 seconds). The drawback of turning it off - and currently
      it has to be done for a whole router and can not be selected peer-by-peer -
      is that the switch-over to an alternative path will take longer.

      We recommend turning off fast-external-fallover whenever possible:

      router bgp 65501
       no bgp fast-external-fallover

      Alternatively it might be considered acceptable to retain
      "fast-external-fallover" and to turn off "interface keepalives" on unreliable
      circuits to overcome the immediate BGP resets on any significant CRC
      error period.

      Another potentially more satisfactory alternative would be to use a shorter
      per-neighbour BGP keepalive timer which has to be applied on both routers
      (e.g. 10 seconds which gives a hold-timer of 30 seconds):

      router bgp 65501
       neighbor w.x.y.z timers 10

      In JunOS, this instability can be avoided by using the following

       - out-delay  <second>; applicable to all BGP peers, all peers in a
         group, or an individual peer. This implements a delay between when the
         routing table receives the routing information and when the
         information is exported to BGP peers.

       - hold-time sec; applicable to all BGP peers, all peers in a group or an
         individual peer. This allows a shorter per neighbor holdtimer to be
         applied on both routers (30 sec will gives keepalives of 10 sec).

       - hold-time msec; to be configured in the router interfaces where the BGP
         peering wil be established. This delays the propagation of the
         interfaces-down events to the routing protocol.

      4. Potential problems

      4.1 Multiplication of flaps between ASes with multiple interconnections

      Christian Panigl experienced the following during a circuit upgrade of an
      Ebone customer:

       - Only ONE flap was generated as a result of the upgrade process (disconnect
         router-port from modem A, reconnect to modem B). Nevertheless the
         customer's prefix was damped in all ICM routers.

       - The flap statistics in the ICM routers stated *4* flaps !!!

       - The only explanation would be that the multiple interconnections
         between Ebone/AS1755 and ICM/AS1800 did multiply the flaps
         (advertisements/withdrawals arrived time-shifted at ICM routers through
         the multiple circuits).

       - This would then potentially hold true for any meshed topology because
         of the propagation delays of advertisements/withdrawals.

      There are two potential solutions to workaround this problem. The first
      one is operational, the second one is a software configuration feature
      (for Cisco IOS and possibly other implementations as well).

       * Schedule a downtime for at least 3-5 minutes which should be enough
         time for the prefix withdrawals to have propagated through all paths
         before reconnection and re-advertisement of the prefix.  Avoid clearing
         BGP sessions as this also could generate a 30 minute outage through
         flap damping!

       * Configure a permanent static route pointing to the customer interface.
         Even if the interface goes down, there is still an entry in the routing
         table for the customer network, and BGP will therefore still announce
         the prefix. Example, using Cisco IOS:

         router bgp 65500
          network mask
         ip route serial 5/0 permanent

         If migrating the customer from one router port to another, simply enter
         the second static route pointing to the new interface. Move the cable
         between ports - BGP continues to announce the prefix as the entry is
         still in the routing table.

         Note: this solution only applies to customers who connect using
         static routes. If the customer connects using BGP, first disable
         fast-external-fallover on both the customer and ISP router, and then
         move the cable in a time period less than the BGP hold-timer.

      4.2 Non-recommended flap damping parameters

      There are situations where service providers would like to design their
      own route flap damping parameters for local needs or conditions. If this is
      really desired, then it is important to pay attention to how flap damping
      parameters are configured, whether the values are feasible or not, etc.

      For example, in Cisco IOS, it is perfectly possible to configure flap damping
      parameters which do nothing, with IOS not giving any warning about them
      being "unfeasible" parameters.

       * One example might be the configuration "set dampening 15 500 3000
         30". Here the reuse limit is 500, maximum suppress time is 30 minutes
         and the half life is 15 minutes. Using these three parameters gives a
         maximum possible penalty value of 2000, well below the suppress limit of
         3000. So even though this can be successfully configured on the router,
         no damping will take place.

       * Another example might be the configuration "set dampening 15 750 3000 30".
         Here the reuse limit is 750, maximum suppress time is 30 minutes and the
         half life is 15 minutes. Using these three parameters gives a maximum
         possible penalty of 3000, exactly the same as the suppress-limit. In
         Cisco IOS, the penalty is decayed every 5 seconds, so flap damping will
         only be take place if the update follows the withdraw within that 5
         second time frame. 99% of the time no flap damping will take place.

      5. References

      RIPE/Routing-WG Minutes dealing with Route Flap Damping:


      Curtis Villamizar, Ravi Chandra, Ramesh Govindan
      RFC2439: BGP Route Flap Damping (Proposed Standard)

      Enke Chen
      RFC2918: Route Refresh Capability for BGP-4 (Proposed Standard)

      Merit/IPMA: Internet Routing Recommendations

      Cisco BGP Case Studies: Route Flap Damping

      Cisco Documentation: Configuring BGP / Route Damping / Soft Reset

      ISI/RSd Configuration: Route Flap Damping

      GateD Configuration: Weighted Route Damping Statement

      Juniper Configuration: Configuring Dumping parameters

      6. Acknowledgements

      Thanks go to all the contributors to this updated version and to the
      RIPE NCC for hosting the "Golden Networks" web-site.
      7. Changes over previous versions

      This document is a significant rewrite and update of RIPE-210. The "Golden
      Networks" have now been moved from this document on to a website dedicated
      to listing them as they are frequently changing. The authors have come
      across several instances of providers implementing the recommendations
      without actually checking that the Root Nameserver networks were still
      as listed in the document.

      Updates to the Cisco IOS configuration have been made, and the parameters
      chosen for /24 networks have been corrected to make them feasible.
      Juniper JunOS configuration samples have been added to this document.

      Examples of flap damping in operation have been added to Appendix 3.

      Router configurations for the recommended route flap damping parameters
      have been moved out of this document to the web-site.

      8. Authors

      The authors can be contacted as follows:

      Philip Smith      <pfs at cisco dot com>
      Cristina Vistoli  <cvistoli at juniper dot net>
      Christian Panigl  <panigl at cc.univie dot ac dot at>
      Joachim Schmitz   <schmitzjo at aol dot com>


      A.1 "Golden Networks"

      Examples of Golden Networks can be found on a website which has been set
      up specifically for them. Please consult http://www.golden-networks.net
      for a sample list of current golden networks and the equivalent router
      configuration for these networks.

      A.2 Sample Configurations

      Sample Router configurations which have been contributed to this project
      can be found at the http://www.golden-networks.net website.
      Contributions of working configurations from other routing software
      should be sent to the authors for inclusion in the website.

      A.3 Study of Flap Damping Operation

      It is instructive to observe how route flap damping actually works on
      a router - doing so will help the reader understand how the particular
      values described in Section 2.2 were chosen. The tests were carried out
      using both Cisco IOS and JunOS.

      A.3.1 Cisco IOS

      The test bed had two Cisco routers connected to each other. One router
      originated prefixes, the other one had the flap damping parameters described
      above in the text. The router originating the prefixes would withdraw a
      prefix, then reannounce, then withdraw, reannounce, etc. The BGP process
      in IOS checks every 60 seconds for any new or withdrawn prefixes in the
      local configuration - so on the source router, the withdraw and announce
      was done by removing and adding the BGP network statement for the prefix
      in question.  The router monitoring the flaps would see the prefix being
      withdrawn and then announced 60s later.

      A.3.1.1 For /24s

      Parameters used are "set dampening 15 820 3000 30"
        1st flap   1000   decay to 966, 982 at update
        2nd flap   1966   decay to 1894, 1926 at update
        3rd flap   2894   decay to 2787, 2846 at update
        4th flap   3280   decay to 3165, 3226 at update

      Maximum possible penalty is 3280 as defined by the flap parameters, so
      the penalty at the 4th flap was only incremented from 2787 to 3280, not
      3787 as might have been expected. At the 4th flap the prefix was marked as
      being suppressed for 59 minutes when the update message was received. If
      the update after the 4th flap was not received within 4 minutes and 20
      seconds, the penalty dropped below 3000, and the prefix was not suppressed.

      A.3.1.2 For /22s, /23s

      Parameters used are "set dampening 15 750 3000 45"
        1st flap   1000   decay to 921, 960 at update
        2nd flap   1921   decay to 1777, 1850 at update
        3rd flap   2777   decay to 2583, 2671 at update
        4th flap   3583   decay to 3311, 3451 at update

      Maximum possible penalty is 6000. At the 4th flap the prefix was marked as
      being suppressed for 33 minutes when the update message was received. If
      the update after the 4th flap was not received within 4 minutes and 40
      seconds, the penalty dropped below 3000, and the prefix was not suppressed.

      A.3.1.3 For remaining prefixes

      Parameters used are "set dampening 10 1500 3000 30"
        1st flap   1000   decay to 889, 946 at update
        2nd flap   1889   decay to 1679, 1781 at update
        3rd flap   2679   decay to 2367, 2526 at update
        4th flap   3367   decay to 3019, 3176 at update

      Maximum possible penalty is 12000. At the 4th flap the prefix was marked as
      being suppressed for 10 minutes when the update message was received. If the
      update after the 4th flap was not received within 2 minutes and 5 seconds,
      the penalty dropped below 3000, and the prefix was not suppressed.

      A.3.2 JunOS

      A similar test bed with two Juniper routers was set up using the damping
      parameters described in Appendix A.2.2 above. One router originated
      prefixes, the other router implemented the flap damping parameters. The
      router originating the prefixes would withdraw a prefix, then reannounce,
      then withdraw, reannounce, etc, with the effects being monitored on the
      second router.

      A.3.2.1 For /24s

      Parameters used are "set-high policy"
              half-life 30;
              reuse 1640;
              suppress 6000;
              max-suppress 60;

        1 up/down:  decay to 1946
        2 up/down:  decay to 3723
        3 up/down:  decay to 5575
        4 up/down:  decay to 6577

      At the 4th flap the prefix was marked as being suppressed for 1 hour when
      the update message was received.

      A.3.2.2 For /22s, /23s

      Parameters used are "set-medium policy"
              half-life 15;
              reuse 1500;
              suppress 6000;
              max-suppress 45;

        1 up/down:  decay to 1939
        2 up/down:  decay to 3269
        3 up/down:  decay to 3733
        4 up/down:  decay to 4944
        5 up/down:  decay to 6032

      At the 5th flap the prefix was marked as being suppressed for 30 min
      when the update message was received

      A.3.2.3 For remaining prefixes
      Parameters used are "set-normal policy"
              half-life 10;
              reuse 3000;
              suppress 6000;
              max-suppress 30;

        1 up/down:  decay to 1909
        2 up/down:  decay to 3503
        3 up/down:  decay to 5065
        4 up/down:  decay to 6556

      At the 4th flap the prefix was marked as being suppressed for 10 min
      when the update message was received

      A.3.3 Summary

      When analysing flap damping performance on the router or across the network,
      network managers should compare with the above lab tests. Note especially
      that slowly flapping prefixes are unlikely to be suppressed even though
      they show significant flapping history. A future version of this document
      may consider what to do in this instance.