=============================================================================== RIPE Routing-WG Recommendation for coordinated route-flap dampening paramters Tony Barber Sean Doran Daniel Karrenberg Christian Panigl Joachim Schmitz Document status: DRAFT 2.0 26-JAN-1998 to be released as an official RIPE document shortly after RIPE29 meeting ABSTRACT This paper recommends a set of route-flap dampening parameters which should be applied by all ISPs in the Internet and should be deployed as new default values by BGP router vendors. Table of Contents ABSTRACT 1. Introduction 1.1 Motivation for route-flap dampening 1.2 What is route-flap dampening ? 1.3 "Progressive" versus "flat&gentle" approach 1.4 Motivation for coordinated parameters 1.5 Aggregation versus dampening 2. Recommended dampening parameters 2.1 Motivation for recommendation 2.2 Description of recommended dampening parameters 2.3 Example configuration for Cisco IOS 2.4 No BGP fast-external-fallover (Cisco IOS) 2.5 Clear IP BGP soft (Cisco IOS) 3. Further problems 3.1 Multiplication of flaps through multiply interconnected ASes 3.2 Software bug counts flaps twice 4. References 1. Introduction Route-flap dampening is a mechanism for (BGP) routers which is aimed at improving the overall stability of the Internet routing table and offloading core-routers CPUs. In the Routing WG session of RIPE26 Christian Panigl asked whether people are interested to participate in a BOF on route flap dampening. The BOF session was held after the plenary session of RIPE26. The discussion was continued in the Routing WG session of RIPE27 and led to a task-force directed to write a proposal document for coordinated route-flap dampening parameters. 1.1 Motivation for route-flap dampening In the early 1990s the massive growth of the Internet with regard to the number of announced prefixes (often due to inadequate prefix-aggregation), multiple paths and instabilities started to do significant harm to the efficiency of the core routers of the Internet. Every single line-flap at the periphery which makes a routing prefix unreachable has to be advertised to the whole core Internet and has to be dealt by every single router by means of updates of the routing-table. To overcome this situation a route-flap dampening mechanism was invented in 1993 and has been integrated into several router code since 1995 (Cisco, ISI/RSd, GateD Consortium). It significantly helps now with keeping severe instabilities more local. And there's a second benfit: it's 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 dampening is at its best value and most consistent and helpfull if applied as near to the source of the problem as possible. Therefore flap-dampening should not only be applied at peering and upstream boundaries but even more at customer boundaries (see 1.4 and 1.5 for details). 1.2 What is route-flap dampening ? When BGP route-flap dampening is enabled in a router the router starts to collect statistics about the announcement and withdrawal of prefixes. Route-flap dampening 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 dampening mechanism is basically the same: If a threshold of the number of pairs of withdrawals/announcements (=flap) is exceeded in a given timeframe (cutoff threshold) the prefix is held down for a calculated period (penalty) which is further incremented with every subsequent flap. The penalty is then decremented by using a half-life parameter until the penalty is below a reuse threshold. Therefore, after beeing stable up for a certain period the hold-down is released from the prefix and it is re-used and re-advertised. Pointers to some more detailed and vendor specific documents: Cisco BGP Case Studies: Route Flap Dampening http://www.cisco.com/warp/public/459/16.html ISI/RSd Configuration: Route Flap Dampening http://www.isi.edu/div7/ra/RSd/doc/dampen.html GateD Configuration: Weighted Route Dampening Statement http://www.gated.org/new_web/code/doc/gated-uni/config_guide/wrd.html See also "4. 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 effect more users. Another aspect is that progressive dampening 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. Because a significant number of important services is sitting in long prefixes (e.g. root nameservers) the progressive approach has to exclude the strong penalisation for those long but "golden" prefixes. With this recommendation we are trying to make a compromise and call it therefore "graded dampening". 1.4 Motivation for coordinated parameters There is a strong need for the coordinated use of dampening parameters because of several reasons: Coordination of "progressiveness": If the boundaries for different treatment of longer prefixes and the penalties are not coordinated throughout the Internet, route-flap dampening could even 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-thershold parameters: If an upstream or peering provider would be dampening 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 hotline 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-dampening for his customer on his local router instead of needing to contact upstream NOCs all over the Internet to get the dampening cleared. 1.5 Aggregation versus dampening Of course, if a customer is just using Provider Aggregated addresses, the aggregating upstream provider doesn't need to apply dampening on these prefixes towards his customer, because instabilities of such prefixes wouldn't propagate into the Internet. However, if a customer insists to announce prefixes which can't be aggregated by its provider dampening should be applied for the reasons given in 1.4. Reasons might be dual-homing (to different providers) of a customer or customers reluctance to renumber into the providers aggregated address range. 2. Recommended dampening 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 dampening in some upstream and peering ISPs routers for all his and his customers /24 prefixes for more than 3 hours because of too "aggressive" paramters: 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 dampening scenario at some boundaries with progressive route-flap dampening 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 dampening does (see: 2.3). Those suggestions in mind Tony Barber designed the following set of route-flap dampening parameters which have prooved to work smoothly in his environment for a couple of months. 2.2 Description of recommended dampening parameters Basically the recommended values do the following with harsher treatment for /24 and longer prefixes: - don't start dampening before the 4th flap in a row (suppress-value = 3000) - /24 and longer prefixes: max=min outage 60 minutes - /22 and /23 prefixes: max outage 45 minutes but potential for less because of half life value - minimum of 30 minutes outage - all else prefixes: max outage 30 minutes min outage 10 minutes 2.3 Example configuration for Cisco IOS ! Parameters are : ! set dampening <half life> <reuse-at> <supress-at> <max suppress time> ! There is a 1000 penalty for each flap ! Penalty decays at granularity of 5 seconds ! Unsuppressed at granularity of 10 seconds ! Dampening info kept until penalty becomes < half of reuse limit. ! ! current Cisco/IOS value-ranges and defaults: ! ! <half-life-time> (range is 1-45 min, current default is 15 min). ! <reuse-value> (range is 1-20000, default is 750). ! <suppress-value> (range is 1-20000, default is 2000). ! <max-suppress-time> (maximum duration a route can be suppressed, range ! is 1-255 min, default is 30 min ). ! router bgp 65500 !no bgp damp bgp damp route-map graded-flap-dampening ! ! don't dampen candidate default routes ! OPTIONAL (not part of recommendation) ! access-list 189 is the candidate default routes ! no route-map graded-flap-dampening deny 5 route-map graded-flap-dampening deny 5 match ip address 189 ! ! don't dampen root nameserver nets ! no route-map graded-flap-dampening deny 7 route-map graded-flap-dampening deny 7 match ip address 180 ! ! - /24 and longer prefixes: max=min outage 60 minutes ! no route-map graded-flap-dampening permit 10 route-map graded-flap-dampening permit 10 match ip address 181 set dampening 30 750 3000 60 ! ! - /22 and /23 prefixes: max outage 45 minutes but potential for less ! because of shorter half life value - minimum of 30 minutes outage ! no route-map graded-flap-dampening permit 20 route-map graded-flap-dampening permit 20 match ip address 182 set dampening 15 750 3000 45 ! ! - all else prefixes: max outage 30 minutes min outage 10 minutes ! no route-map graded-flap-dampening permit 40 route-map graded-flap-dampening permit 40 set dampening 10 1500 3000 30 ! !----------------------------------------------------------------------- ! ACCESS LISTS 180-189 GO BELOW !----------------------------------------------------------------------- ! access-lists 180 to 189 used or reserved for progressive route flap dampening ! ! 180 - BGP dampening - root-nameservers.net networks are NOT dampened ! This filter stops these networks being dampened. ! Also DONT dampen routes used to derive default (see list 7) ! but this is handled in a separate route-map statement. ! in the file dampening-confg. ! Route map uses DENY to drop out of map on matching. ! no access-list 180 ! ! A.ROOT-SERVERS.NET. access-list 180 permit ip 198.41.0.0 0.0.0.0 255.255.252.0 0.0.0.0 ! ! B.ROOT-SERVERS.NET. access-list 180 permit ip 128.9.0.0 0.0.0.0 255.255.0.0 0.0.0.0 ! ! C.ROOT-SERVERS.NET. access-list 180 permit ip 192.33.4.0 0.0.0.0 255.255.255.0 0.0.0.0 ! ! D.ROOT-SERVERS.NET. access-list 180 permit ip 128.8.0.0 0.0.0.0 255.255.0.0 0.0.0.0 ! ! E.ROOT-SERVERS.NET. access-list 180 permit ip 192.203.230.0 0.0.0.0 255.255.255.0 0.0.0.0 ! ! F.ROOT-SERVERS.NET. access-list 180 permit ip 192.5.4.0 0.0.0.0 255.255.254.0 0.0.0.0 ! ! G.ROOT-SERVERS.NET. access-list 180 permit ip 192.112.36.0 0.0.0.0 255.255.255.0 0.0.0.0 ! ! H.ROOT-SERVERS.NET. access-list 180 permit ip 128.63.0.0 0.0.0.0 255.255.0.0 0.0.0.0 ! ! I.ROOT-SERVERS.NET. access-list 180 permit ip 192.36.148.0 0.0.0.0 255.255.255.0 0.0.0.0 ! ! J.ROOT-SERVERS.NET. 198.41.0.10 same net as A ! ! K.ROOT-SERVERS.NET. access-list 180 permit ip 193.0.14.0 0.0.0.0 255.255.255.0 0.0.0.0 ! ! L.ROOT-SERVERS.NET. 198.32.64.12 access-list 180 permit ip 198.32.64.0 0.0.0.255 255.255.255.0 0.0.0.255 ! ! M.ROOT-SERVERS.NET. 198.32.65.12 access-list 180 permit ip 198.32.65.0 0.0.0.255 255.255.255.0 0.0.0.255 ! ! ! - 181 - dampens /24 and greater prefixes ! no access-list 181 ! access-list 181 permit ip 0.0.0.0 255.255.255.255 255.255.255.0 0.0.0.255 access-list 181 deny ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 ! ! - 182 - dampens /23 /22 and above ! no access-list 182 ! access-list 182 permit ip 0.0.0.0 255.255.255.255 255.255.252.0 0.0.3.255 access-list 182 deny ip 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 ! ! - 189 - Candidate default networks used in some customer bgp implementations ! no access-list 189 ! access-list 189 permit ip !!! put your defaults in here access-list 189 deny ip any any ! 2.4 No BGP fast-external-fallover (Cisco IOS) In Cisco IOS there is a BGP configuration parameter "fast-external-fallover" which when on (default) leads to an immediate clearing of a BGP neighbor whenever the line-protocol to this external neighbor goes down. If it is turned off the BGP sessions will survive short line-flaps as they 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 switchover to an alternative path will take longer. We are recommending to turn off fast-external-fallover whenever possible: ! router bgp 65501 no bgp fast-external-fallover ! 2.5 Clear IP BGP soft (Cisco IOS) There is a new "soft" mechanism for the clearing of BGP sessions available with newer versions of Cisco IOS. For beeing able to make use of the "clear ip bgp x.x.x.x soft inbound" command the router which should support it needs to be configured for additional data structures: ! router bgp 65501 neighbor 10.0.0.2 remote-as 65502 neighbor 10.0.0.2 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 neighbor 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 neighbor against locally configured inbound route-maps and filter-lists. 3. Open problems 3.1 Multiplication of flaps through multiply interconnected ASes Christian Panigl recently made the following experience with a line upgrade of an Ebone customer: - It is absolutely positive that through the upgrade process just ONE flap was generated (disconnect router-port from modem A reconnect to modem B), nevertheless the customers prefix was dampened in all ICM routers (ICM/AS1800 is US upstream for Ebone). - 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 paths). - This would then potentially hold true for any meshed topology because of the propagation delays of advertisements/withdrawals. - Workaround for scheduled actions like with the given example: Schedule a downtime for at least 3-5 minutes which should be enough for the prefix withdrawals to have propagated through all paths before reconnection and re-advertisement of the prefix. Avoid clearing BGP sessions as this is usually generating a 30" outage which might easily give the same result. - A solution has to be provided by the vendors ! 3.2 Software bug counts flaps twice A bug was identified in the dampening code of of some Cisco IOS releases where a penalty is assigned and the flap counter is incremented even when a withdrawn prefix is re-announced. This bug is said to be fixed in the following IOS versions and above: 11.1(16)CA 11.2(10)* 11.3(0.6) Everybody who has dampening enabled should verify to have a corrected IOS version running. 4. References RIPE/Routing-WG Minutes dealing with Route Flap Dampening: ftp://ftp.ripe.net/ripe/minutes/ripe-m-24.ps ftp://ftp.ripe.net/ripe/minutes/ripe-m-25.ps http://www.ripe.net/wg/routing/r25-routing.html http://www.ripe.net/wg/routing/r26-routing.html http://www.ripe.net/wg/routing/r27-routing.html Curtis Villamizar, Ravi Chandra, Ramesh Govindan Internet-Draft: BGP Route Flap Dampening ftp://ietf.org/internet-drafts/draft-ietf-idr-route-damp-01.txt (Expires July 8, 1998) Curtis Villamizar, ANS: Controlling BGP/IDRP Routing Overhead http://engr.ans.net/route-dampen/ NANOG-Feb-1995 Route Flap Dampening Presentation (slides): ftp://engr.ans.net/pub/papers/slides/nanog-feb-1995-route-dampen.ps Merit/IPMA: Internet Routing Recommendations http://www.merit.edu/~ipma/docs/help.html Cisco BGP Case Studies: Route Flap Dampening http://www.cisco.com/warp/public/459/16.html ISI/RSd Configuration: Route Flap Dampening http://www.isi.edu/div7/ra/RSd/doc/dampen.html GateD Configuration: Weighted Route Dampening Statement http://www.gated.org/new_web/code/doc/gated-uni/config_guide/wrd.html