Re: Sites unreachable while traversing Dallas IXP

[Tom Beecher via NANOG <nanog () lists nanog org>]

I think folks are mixing up concepts a bit in this thread.  The *hashing
algorithm* is not the same thing as the *load balancing* algorithm.

I have a LACP bundle with 4 member links. The *load balancing* algorithm
determines if traffic is balanced per packet or per flow across the member
links. The *hashing* algorithm is what is used to decide which link to use
for each traffic element, with the goal being even distribution across all
possible paths.

LACP establishment doesn't depend on these two things at all. There is also
no requirement that both sides use the same LB or hashing algorithms. (
There are cases where you absolutely DON'T want that to happen anyways.)

What can occur is that one side uses a given LB/hashing combo such that
traffic hotspots to one of the member links, running it over, and some
stuff gets dropped. What can also occur is that if one side does per-packet
balancing, which can create all kinds of out of order packet problems.



On Fri, Sep 26, 2025 at 10:33 AM Mel Beckman via NANOG <
nanog () lists nanog org> wrote:

> Nick,
>
> From
>
> https://www.exam-labs.com/blog/configuring-lacp-between-cisco-ios-and-juniper-junos-a-step-by-step-guide
>
> Understanding LACP Failures and Common Pitfalls
>
> Link aggregation, although a robust feature, is susceptible to various
> issues. To resolve these problems effectively, network administrators must
> first understand the potential causes of LACP failures. Here are some of
> the most common causes for LACP issues:
>
>   1.  Mismatched Configurations:
> Often, the primary reason for LACP failure is a misalignment in
> configurations between the two devices. This might include differences in
> LACP modes (active vs. passive), inconsistent port-channel settings, or
> mismatched VLANs. Proper alignment of settings across both devices is
> crucial to the successful negotiation of LACP.
>
>  -mel
>
> On Sep 26, 2025, at 7:10 AM, Nick Hilliard <nick () foobar org> wrote:
>
> Mel Beckman via NANOG wrote on 26/09/2025 06:33:
> Instead you configure one of the available hash-based distribution
> functions the two endpoints have in common.
> [...]
> In my own experience, packet loss on Cisco-Juniper LACP links has
> arisen from inconsistent or incompatible configurations.
> this analysis is straight down wrong. The hashing algos on each side of a
> LAG bundle are entirely independent of each other and there is no problem
> whatever with using different hashing algos on each side.
>
> Nick
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