Thanks Tim for explaining the terminologies. That was really beneficial. I have a question on your comments under polarization........ In a 'multi-tier' network, using the same hash input on each tier results in traffic after the 1st tier polarizing to a subset ................What is 'multi-tier' network? Can you shed some light or point to a URL etc. Thanks again.
Mack On Sun, Oct 3, 2010 at 10:08 AM, Tim Stevenson <[email protected]> wrote: > Hi John, > Let's get everyone agreed on the terminology first, then we can try to > solve the problem. > > * ECMP = Equal cost multipath, it is most typically a term used around > unicast routing where for a given IP prefix you have multiple equal cost > next hops and you load share traffic among them based on a hash (or less > commonly per packet). The hash can be based on several criteria, ie, IPs & > L4 ports in various combinations. > > * CEF = it's interchangeable with 'ECMP' - CEF-based load sharing & ECMP > mean the same thing. > > * Multicast multipath = Uses a hash to select the RPF interface for a > particular multicast flow when there are ECMP paths back to the source/RP. > There are options to determine the input values (G, S,G, S,G+NH). This > feature is not on by default in IOS. If it is not enabled, then IOS will > choose ONE of the ECMP paths as the RPF (highest neighbor IP) and ALL > multicast will be pulled over that link. > > * Polarization = In a 'multi-tier' network, using the same hash input on > each tier results in traffic after the 1st tier polarizing to a subset of > the available links. It's accomodated for by adding a unique ID at each hop > to the hash input for unicast; for multicast multipath, by including the > next hop IP as hash input. Whether this really comes into play depends on > the depth of the network routing topology. > > Ok - so given all of the above, with ECMP routing between the 7600s & the > 4948s, and with multicast multipath already enabled on the 7600 and using > S,G basic hashing: if the traffic flow is from the 4k->7600, the only option > you have to improve things is to use S,G + next-hop. I'm not entirely > convinced it will have a major impact, it depends on whether you have > multiple levels of routing, one which is getting RPF hash selection pretty > evenly but then at this layer, polarization is occurring since only a subset > of traffic is reaching it and the hash input is the same (so only a subset > of links is being selected as RPF). Based on your description I can't tell > if that's a possibility in your setup. > > Regardless of all that, changing CEF/ECMP hash input on the 4948 will not > have any significant impact, since that wouldn't affect multicast traffic at > all, any particular S,G will still have only ONE of those four interfaces as > an OIF, and that is driven by where the PIM join came in from the 7600, > which in turn is driven by whether mcast multipath is enabled, and what hash > is used to select the RPF interface. > > Also, clearly, changing CEF/ECMP hash input on the 7600 would have not any > impact since you're worried about traffic flowing the other direction > anyway. > > Hope that helps, > Tim > > At 09:09 AM 10/3/2010, John Neiberger remarked: > > I'm starting another thread because the topic is migrating. To >> simplify, we have a 7600 with SUP720-3BXL connected via four routed >> links to a 4948. The bulk of the traffic on this network is multicast >> traffic flowing from the 4948 to the 7600 (and onward from there). In >> order to get the best load sharing over those four links, what is the >> recommended CEF tuning and ECMP configuration? >> >> I ask because we seem to be running into ECMP polarization and/or CEF >> polarization. We have already decided that we need to be using >> s-g-hash next-hop-based for ECMP. We're using s-g-hash basic right >> now. But what about CEF? Do we need to tune CEF along with tuning ECMP >> for this to work properly? We want the most even distribution of >> traffic possible. As it is right now, we're seeing serious unequal >> load sharing. In some cases all of the traffic is going over one link >> and not even using the other three. >> >> Do any of you know the recommended CEF parameters in a situation like >> this? >> >> Thanks, >> John >> _______________________________________________ >> cisco-nsp mailing list [email protected] >> <https://puck.nether.net/mailman/listinfo/cisco-nsp> >> https://puck.nether.net/mailman/listinfo/cisco-nsp >> archive at <http://puck.nether.net/pipermail/cisco-nsp/> >> http://puck.nether.net/pipermail/cisco-nsp/ >> > > > > > Tim Stevenson, [email protected] > Routing & Switching CCIE #5561 > Distinguished Technical Marketing Engineer, Cisco Nexus 7000 > Cisco - http://www.cisco.com > IP Phone: 408-526-6759 > ******************************************************** > The contents of this message may be *Cisco Confidential* > and are intended for the specified recipients only. > > > > _______________________________________________ > cisco-nsp mailing list [email protected] > https://puck.nether.net/mailman/listinfo/cisco-nsp > archive at http://puck.nether.net/pipermail/cisco-nsp/ > _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
