I could lab it but eff it... I know you love BGP questions

[wintermute000]

As per the title


What exactly is the intersection between BGP add-path and ECMP via maximum-paths?


Add-Paths merely sends the additional number of BGP paths specified between BGP neighbours BUT ECMP behaviour is still governed by maximum-paths plus the usual criteria? (i.e. matching attributes blah blah and blah can't remember exactly but y'all know what I mean)

[LynK]

ECMP follows the rules/criteria of BGP best path selection. There would need to be a tie (or an explicit allowance, based on allowed metric). One would think that if you were using as-path prepend, and you were matching on as-path for routes that you would have an issue because it would break ECMP.

Am I right or crazy?

[burnyd]

There is more to the story.  Add-path historically has been used so that the downstream neighbor will advertise everything to its upstream neighbor.  So for example if you have 3 paths in you're fib to get to 1.1.1.1/32 and you turn on add-path it will advertise all routes.  Without it it will simply advertise its only best known route.

[LynK]

So in the common folk term:

Maximum Paths affect local routers routing table allowance of multiple entries

Add-path can affect downstream routers and advertise back up to upstream routers

I guess the question remains: Which one is transitive? (too lazy to look it up)

[wintermute000]

Add path but it's not really an attribute so the term doesn't really apply

[srg]

None are attributes of the paths, they're only different local router decisions (addpath might be "local" as it depends on the neighbor having addpath capability to be used, but nothing is added to additional path(s) being sent)

As to the original post; yes, addpath can send multiple paths for the same prefix to a neighbor that would otherwise be only one due to BGP path selection. The receiving neighbor still needs the correct maximum-paths settings to do ECMP since normal BGP path selection applies there as well.