[c-nsp] CEF Tuning with ECMP?

[Tim Stevenson]

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, 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
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