[j-nsp] Verifying Juniper ECMP

[Masood Ahmad Shah]

See inline, prefixed [Masood] ...


On Thu, Apr 17, 2014 at 1:09 AM, John Neiberger <jneiberger at gmail.com>wrote:

> ​Another question: if a link in a ECMP "bundle" goes down and then comes
> back up later, do things end up hashed and balanced the same way they were
> prior to the link going down, or is there some amount of randomness to it?
>

[Masood] You may not see traffic balanced instantly, because existing flow
will NOT move to the newly added member. Only new flows will get hashed
across the members and then new member will have his fair share of good
luck :) However, the following things may happen and make load balancing
more fun:

1. incorrect load balancing by aggregate next hops
2. incorrect packet hash computation
3. insufficient variance in the packet flow
4. incorrect pattern selection

You may look for "Adaptive Load Balancing", a Juniper method to balance
traffic across LAG members (that focus more on the weights, the bandwidth
and packet stream of link) but that has it's on pros and cons.


> If I check a certain flow and see that it is hashed to a particular link,
> is it a fair bet that it was hashed to that same link prior to the link
> going down?
>

[Masood] AFAIK, #Junos does not keep track of it and I wonder if any other
vendor would do that.


>
> Thanks,
> John​
>
>
> On Tue, Apr 15, 2014 at 12:07 PM, John Neiberger <jneiberger at gmail.com
> >wrote:
>
> > Holy cow. I never would have figured that one out, and the two Juniper
> > engineers I asked had no idea how to do it. I appreciate the help!
> >
> > Thanks,
> > John
> >
> >
> > On Tue, Apr 15, 2014 at 3:50 AM, Olivier Benghozi <
> > olivier.benghozi at wifirst.fr> wrote:
> >
> >> Hi John,
> >>
> >> as usual with Juniper it's ridiculously overcomplicated, David Roy wrote
> >> a fine article about that, at least for MX with DPC:
> >>
> >>
> http://www.junosandme.net/article-junos-load-balancing-part-3-troubleshooting-109382234.html
> >>
> >>
> >> Olivier
> >>
> >> Le 15 avr. 2014 à 04:01, John Neiberger <jneiberger at gmail.com> a écrit
> :
> >> > ​I know that ECMP is, by default, based on a hash of source and
> >> destination
> >> > IP address, and I know that we can see the available paths by doing
> >> "show
> >> > route forwarding-table destination <prefix>", but is there a way to
> >> > determine which path a particular flow is using?
> >> >
> >> > For those of you familiar with Cisco, I'm looking for an equivalent to
> >> > "show cef exact-route".
> >>
> >> _______________________________________________
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> >
> >
> >
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