[c-nsp] add-path on XR

[Mark Tinka]

Very old thread, but I was digging around and found it, so thought I'd 
answer, in case no one did:

On 5/10/22 13:27, Sebastian Neuner wrote:

> But on IOS XR, I can only find global options to enable the capability 
> and set a general limit for the number of paths, like this:
>
>> router bgp 65000
>>  address-family ipv6 unicast
>>   additional-paths send
>>   maximum-paths ibgp 6

"maximum-paths" only applies to Multipath, not Add-Paths.


> So that looks like it enables the capability for all iBGP peers, and 
> in the process resets all sessions? (yay, fun)

Add-Paths is a non-default capability. If it has not been previously 
enabled, the session is torn down so it can be negotiated with the neighbor.


> But there is no configuration per neighbor. Am I missing something?

To support the capability, that is at the AF level.

After IOS XR 7.3.1, per-neighbor level is supported for path selection. 
But the capability remains at the AF level, AFAICT.


>
> There are some knobs in the route-policy. I found stuff like
>
>> route-policy monitoring-out
>>   set path-selection all advertise 
>
> But how does that work? Do all iBGP neighbors without "path-selection" 
> options in their out-policy continue to only receive the best path? 
> And the monitoring-servers receive "maximum-paths"-many paths?

It means that for any neighbors who are advertising their ability to 
receive additional paths, you will be able to send them all available 
paths with that policy.


>
> Would configuring that still reset all iBGP neighbors? Or just the 
> ones where the policy sets "path-selection" parameters?

No. Path selection policies and their advertisement does not reset BGP 
sessions. BGP sessions are only reset when you need to negotiate 
Add-Path capability exchange.


>
> And why is the documentation for those products of the last few years 
> in general such a catastrophe?

I'm guessing most people don't use Add-Paths as much as they do 
Multipath, so perhaps why.

Mark.