Validating multi-path in production?

[Adam Thompson <athompson () merlin mb ca>]

Hello all.
Over time, we've run into occurrences of both bugs and human error, both in our own gear and in our partner networks' 
gear, specifically affecting multi-path forwarding, at pretty much all layers: Multi-chassis LAG, ECMP, and BGP MP.  
(Yes, I am a corner-case magnet.  Lucky me.)

Some of these issues were fairly obvious when they happened, but some were really hard to pin down.

We've found that typical network monitoring tools (Observium & Smokeping, not to mention plain old ping and traceroute) 
can't really detect a hashing-related or multi-path-related problem: either the packets get through or they don't.

Can anyone recommend either tools or techniques to validate that multi-path forwarding either is, or isn't, working 
correctly in a production network?  I'm looking to add something to our test suite for when we make changes to critical 
network gear.  Almost all the scenarios I want to test only involve two paths, if that helps.

The best I've come up with so far is to have two test systems (typically VMs) that use adjacent IP addresses and 
adjacent MAC addresses, and test both inbound and outbound to/from those, blindly trusting/hoping that hashing 
algorithms will probably exercise both paths.

Some of the problems we've seen show that merely looking at interface counters is insufficient, so I'm trying to find 
an explicit proof, not implicit.

Any suggestions?  Surely other vendors and/or admins have screwed this up in subtle ways enough times that this 
knowledge exists?  (My Google-fu is usually pretty good, but I'm striking out - maybe I'm using the wrong terms.)

-Adam

Adam Thompson
Consultant, Infrastructure Services
[1593169877849]
100 - 135 Innovation Drive
Winnipeg, MB, R3T 6A8
(204) 977-6824 or 1-800-430-6404 (MB only)
athompson () merlin mb ca<mailto:athompson () merlin mb ca>
www.merlin.mb.ca<http://www.merlin.mb.ca/>