North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Load balancing in routers
A few comments: > I don't think flow-caching is necessarily due to CEF. CEF, afaik, is unaware of flows. > Even on dinky 2500 & 2600 series where you don't run CEF, Many people run CEF on 2600's, it's about the only way to get to the cisco-advertised PPS on the box. > load balancing over multiple links uses a flow-hashed method. If you > want per-packet load distribution you have to specifically enable it by > saying "no ip route-cache" on each interface. That is very deadly, please, don't anyone actually try that. CEF load balancing, IIRC, had two options, specifyable on a per-interface basis -- 'per-packet', and 'per-destination'. Both have obvious meanings. Newer IOS's seem to have a defaulting mechanism available in global config mode, but being a weirdo, I don't trust it. I still specify on the per-interface. We use this in several scenerios, specifically for load-balancing T1's, and it amazingly works well, with the links often being in balance to the tune of 1 to 3%. I've seen similar performance at DS3 rates. > Paul's statement about CEF is interesting. It's probably the first public > statement I've ever heard where someone was praising CEF. Usually > discussions about CEF are accompanied by liberal amounts of swearing... I dunno; except for some silliness in 12.1(8a)E[1-4] on a MSFC2, we've seen general goodness from CEF from 2600, 3600, 4700, 5300, 7200, 7500. Then again, we're not UU or Sprint, and don't have the traffic loading they do. > Joe > > > On 4/8/02 9:03 AM, "Iljitsch van Beijnum" <[email protected]> wrote: > > > > > On 8 Apr 2002, Paul Vixie wrote: > > > >>> I seem to remember fast switching was per-destination, and CEF was > >>> round robin. But it seems CEF is now per-destination as well in IOS 12.2. > >>> Round robin is optional. > > > >> CEF is flow-hashed, and the hash seems to include both source and > >> destination, and seems to include the port numbers. This is by observing > >> the behaviour of flows hitting various members of the F.ROOT-SERVERS.NET > >> set, each of whom sends F's address to several upstream routers using OSPF. > >> CEF works like a charm -- the load is never split by more than 45-55 and > >> that's damn good for wire speed hashing in my view. > > > >> We used CEF in 11.x and it behaved the same way. It was never round-robin > >> in any way we could observe. > > > > You're right. I was thinking of process switching. > > > > According to: > > http://www.ils.unc.edu/dempsey/186s00/reorderingpaper.pdf > > > > packet reordering at MAE East was extremely common a few years ago. Does > > anyone have information whether this is still happening? > > > > > > -- Alex Rubenstein, AR97, K2AHR, [email protected], latency, Al Reuben -- -- Net Access Corporation, 800-NET-ME-36, http://www.nac.net --
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