On Thu, Dec 16, 2004 at 11:43:12PM -0500, Jared Mauch wrote:
On Fri, Dec 17, 2004 at 12:42:21AM +0100, Iljitsch van Beijnum wrote:
On 17-dec-04, at 0:21, Jerry Pasker wrote:
ie: does dampening cause more problems than it tries to solve/avoid these days.
I don't know what takes more router resources; dampening enabled doing the dampening calculations, or no dampening and constantly churning the BGP table. I would assume dampening generally saves router resources, or operators wouldn't chose to enable it.
One reason to be careful with dampening is that flaps can be multiplied. (Connect to routeviews and see the different flap counts under different peers for the same flap at your end to observe this.)
There have been numerous people who have spoken and released research on this topic.
I think with the "better" routing code out there these days, that most people can quickly handle a large number of next-hop changes, etc.. in their hw/sw that disabling dampening would allow the networks to reconverge fairly quickly without (much) trouble. (going to respond to the streaming video/audio/whatnot issue seperately).
oops, i thought their reply was cc:ed to the list, but i guess not. i'm going to summarize a private reply i got: a streaming video provider saw that when dampening was enabled, it reduced the number of complaints related to rebuffering and other similar issues by 15%. I think that having a more stable/reliable network switchover to a forwarding path should happen at a more moderate rate.. Obviously, if a prefix is flapping badly via one connection, it should be less prefered, but if it's the only path to that prefix, what harm is caused by reinstalling it fairly quickly? (eg: after 60 seconds of stability instead of a lot longer). This would possibly create a few transient cases of lack of reachability that would be a bit harder to diagnose if max dampening timers were a lot lower and a route was still oscilating.. In the case of lack of reachability to an anycast prefix (i know that .org and ultradns were being picked on, so i'll use them as an example.. sorry Rodney :) ) having the shorter times would increase availability. Obviously the ideal case is that you would exempt these prefixes from dampening, but creating such policies may actually be more work for the router in the long term than just disabling dampening totally. you might see more transient cpu usage as the network goes flappity-flap, but the cost of not evaluating a more complex dampening policy might see some savings as well. (considering the cpu time spent to evaluate it, allocate memory to store state, count flaps, etc..) - jared -- Jared Mauch | pgp key available via finger from jared@puck.nether.net clue++; | http://puck.nether.net/~jared/ My statements are only mine.