routing-wg

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routing-wg@ripe.net

January 1997

10 participants
11 discussions

Progressive BGP route flap dampening
by Christian Panigl, ACOnet/UniVie +43 1 4065822-383 02 Jan '97

02 Jan '97

Dear Routing WG members, sorry for not having had the opportunity to participate in the last few meetings. I'm trying hard for the January meeting ! In the meantime let me initiate an e-mail discussion wrt route flap dampening (I'm appending the relevant part of the minutes of the Sept. meeting, where further discussion was requested). Recently, following a scheduled router-maintenance on an Ebone backbone router, I had problems to communicate with NORDUnet (haven't explicitely tried PIPEX ;-) from /24 networks for about 2 hours ! This reminded me of the "progressive route flap dampening" discussion, and increased my motivation for throwing my 0.02 EUROs into the routing-wg list: - I can't beleive that it's really necessary and reasonable to kill a network (regardless of prefix-length) for two hours if it's flapping twice maybe within half an hour but not more frequently within a month ! - Imagine you're SW-upgrading a router and (very unlikely, as we all know ;-} detect that you have to step back ... BINGO, it's perfectly and innocently electrocuted :-( - I'd suggest that dampening (regardless of prefix length) shouldn't start before AT LEAST three flaps are happening in a row (let's say within half an hour). - Dampening should lockout real network instabilities not make worse even scheduled maintenance ! - Besides, I know applications where it might be perfectly reasonable to announce a single *providerindependent* /24 and where it's contraproductive and politically incorrect to include it into an ISP aggregate ! A solution could be to ask Internic/RIPE to define "PI" address-ranges which can and should be excluded from the /24 hostility acts. Kind regards Christian ===== Following quoted from = = RIPE 25, Amsterdam = Routing Working Group = Report of Meeting, 23rd September 1996 = = [...] = =6. Progressive BGP route flap dampening = ftp://ftp.ripe.net/ripe/presentations/ripe-m25-tbarber-bgp-damp.html = = Tony Barber gave a presentation on the strategies used by = UUnet-Pipex to reduce the effects of route flapping and to = try to prevent router table overflow. These were: = = - route dampening = - prefix filtering = - more router memory = = They had encountered many instabilities from peers and found = that many ISPs had not deployed CIDR; this gave rise to more = flapping as more routes, and particularly more specific ones, = were advertised. = = Tony explained the parameters used for route dampening on a = Cisco router. He had arrived at the following re-use times = for various route sizes: = = /24 and greater ~160 minutes = /23 and /22 ~60 minutes = /21 and less ~30 minutes = = He recommended filtering out all prefixes more specific than /24. = = While route dampening consumed router memory, this was more or = less balanced by a reduction in routing CPU cycles. = = He recommended that if route dampening was to be widely = deployed in Europe, consistency was important. In this sense, = the Routing WG should agree on guidelines for parameters to = be used. = = In discussion, the following points were made: = = - aggregation works in reducing router load and route = flapping. = = - route flapping is often a feature of certain autonomous = systems rather than a function of prefix length. = = - much instability was due to configuration changes and = errors as distinct from link failures. = = - making dampening dependent on prefix length could = penalise many stable /24s. = = - it might be useful to discriminate against /24s in the = 192.0.0.0/8 block (the swamp). = = - the focus should be on keeping noise out of the system = rather than trying to mitigate against it once in the = system. = = In summary, it was agreed that route dampening was an = important topic and that more discussion was needed. = ===== End quote

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