[c-nsp] CEF Tuning with ECMP?

Mon Oct 4 00:07:44 EDT 2010

Hi Mack,

An example is shown in figure 16 here:
http://www.cisco.com/application/pdf/en/us/guest/netsol/ns431/c649/ccmigration_09186a008093b876.pdf

Hope that helps,
Tim

At 03:18 PM 10/3/2010, Mack O'Brian remarked:
>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 
><<mailto:tstevens at cisco.com>tstevens at cisco.com> 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
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>Tim Stevenson, <mailto:tstevens at cisco.com>tstevens at cisco.com
>Routing & Switching CCIE #5561
>Distinguished Technical Marketing Engineer, Cisco Nexus 7000
>Cisco - <http://www.cisco.com>http://www.cisco.com
>IP Phone: 408-526-6759
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Tim Stevenson, tstevens at cisco.com
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.