4 ms·
> Oscillating between different available paths? The term for unwanted oscillation between different paths is, IIRC, "flapping". However, when working with EC
by zkms 8y ago
> Oscillating between different available paths?
The term for unwanted oscillation between different paths is, IIRC, "flapping".
However, when working with ECMP (equal-cost multi path), you actually do want to use all your paths (hence "equal cost") simultaneously (for load-balancing purposes, say). "Polarisation" refers to the unwanted condition when the hash algorithm that decides which path a flow/packet takes is not properly distributing flows/packets across links, leading to underutilisation and/or overutilisation of links/routes.
- bogomipz 8y agoSure, I meant when the same 5 tuple is oscillating between different paths which really should not happen. You mentioned there were plenty other ways to avoid polarization. I would be curious to hear what those are.
- alecbenzer 8y agoNew to most of this but couldn't you just salt the hashes being used per-router?
- bogomipz 8y agoYeah I think you could seed each router's hashing with something unique like a mac address hardware ID but you could also just use different hashing at each tier in your network like using source IP + dest IP on one tier and source IP + dest IP + IP protocol on another tier. This should achieve the same result.
- alecbenzer 8y agoYeah but then you need to keep coming up with new ways of hashing things as you add tiers. It's easier to generate a new salt than it is to figure out a new but still-valid way of hashing the packets.
- bogomipz 8y agoPractically speaking this isn't an issue though. You don't see many spine and leaf networks that are deeper than 3 tiers(edge, distribution, ToR) which means one link upstream and one down. It's not like people build networks arbitrarily deep.