At 2:34 PM +0000 7/25/03, p b wrote: >Here's some text from CCO regarding CEF and using source >and destination IPs to map a packet to one of a set of >load sharing links: > >Configuring Per-Destination Load Balancing > >Per-destination load balancing is enabled by default when you enable CEF. To >use per-destination load balancing, you do not perform any additional tasks >once you enable CEF. > >Per-destination load balancing allows the router to use multiple paths to >achieve load sharing. Packets for a given source-destination host pair are >guaranteed to take the same path, even if multiple paths are available. >Traffic destined for different pairs tend to take different paths. >Per-destination load balancing is enabled by default when you enable CEF, >and is the load balancing method of choice for most situations. > >The URL for the above is (watch wrap): > >http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a00800ca6ca.html#1000956
I think the problem here is the documentation author, not IOS. It seems fairly clear to me that said author doesn't understand the difference between traditional destination cache and source-destination hash. This hasn't been the first time strange things have happened. I remember that I looked at the original description of OSPF demand circuits and just blinked. First, I knew from the OSPF Working Group how they were supposed to work. Second, I knew the developer who wrote the code, and also wrote English very well. I sent him an email, and he responded he had absolutely no idea, either, what the documentation was trying to say. He observed his C code, even uncommented, was more readable. > > > >John Neiberger wrote: >> >> Priscilla Oppenheimer wrote: >> > >> > John Neiberger wrote: >> > > >> > > Tim Champion wrote: >> > > > >> > > > Could someone please confirm the following to be true >> (taken >> > > > from CCO): >> > > > >> > > > "Per-destination load balancing allows the router to >> > > distribute >> > > > packets >> > > > based on the destination address, and uses multiple paths >> to >> > > > achieve load >> > > > sharing. Packets for a given source-destination host pair >> > are >> > > > guaranteed to >> > > > take the same path, even if multiple paths are available. >> > For >> > > > example, given >> > > > two paths to the same network, all packets for >> destination1 >> > on >> > > > that network >> > > > go over the first path, all packets for destination2 on >> that >> > > > network go over >> > > > the second path, and so on. Per-destination load balancing >> > is >> > > > enabled by >> > > > default when you start the router, and is the preferred >> load >> > > > balancing for >> > > > most situations." >> > > > >> > > > It was my understanding that per-destination load >> balancing >> > > was >> > > > based on the >> > > > destination address only and not on the source/destination >> > > pair. >> > > > >> > > > If someone could clarify it would be much appreciated. >> > > > >> > > > Cheers >> > > > Tim >> > > >> > > This probably depends on the switching mechanism in place. >> > Fast >> > > switching, as I recall, simply caches the outgoing interface >> > > for any given destination so it's relying on the destination >> > > information only. >> > >> > Yes, fast-switching caches the outgoing interface for a >> > destination. All packets to a particular destination go out >> the >> > same interface. CEF works that way too if you use the default. >> > >> > > CEF uses both the source and destination. >> > >> > I don't think that is true? CEF doesn't look at source >> > addresses. >> >> I just checked this on our 7513 running 12.2(17a). If you use >> the command "show ip cef exact-route sourceip destinationip" >> you'll see the cached exit interface. If you do this with >> several source addresses going to the same destination when >> there are multiple paths you'll see that they use different >> exit interfaces. >> >> I wonder if the default behavior has changed as CEF has evolved? > > >> John Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=73031&t=72944 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
