[c-nsp] add-path on XR

[Sebastian Neuner]

Hi all,

I got no replies and that might be because nobody cares, or it might be 
because nobody knows how to do it on XR. Googling for something and 
finding posts without solution is always annoying, so here's what I 
found. This is all on 7.5.2.

> route-policy monitoring-out
>  set path-selection all advertise
> end-policy

=> can't be attached at the neighbor-out attach point

You can use "destination" to filter for the destination prefix, but you 
can also use it to match a path type (best, backup, multipath, all), 
like this: "if destination is-best-external then ...". Not sure why they 
chose a syntax with two meanings for "destination". Weird.

But: you can only advertise paths to neighbors that are marked for 
RIB-out. You can set a policy with "set path-selection all advertise" to 
mark all paths for RIB-out:

> router bgp X
>  address-family ipv6 unicast
>   additional-paths send
>   additional-paths selection route-policy send-all-paths

As soon as a neighbor negotiates the capability (neighbor has "receive" 
enabled), it will receive *all* paths by default, for all the prefixes 
you advertise. To filter this, you'd have something like this in your 
neighbor-out policy:

> route-policy CUSTOMER-out
>   if destination is-all then
>     drop  # "is-all" matches all *additional* paths, not the best-path
>   else
>     pass
>   endif
> end-policy 

Weird design choices all over the place IMO. But that might be just me. 
Is it a Cisco-thing to always advertise everything to everyone by default?!

So what if you want to advertise add-paths only to specific peers? I 
know, I know, very rare use case.

You can advertise the capability to all neighbors and filter 
("destination is-all") in all neighbor-out policies where you don't want 
it. But that's a lot of work and changes, it's complex, and error prone.

The good news is: you can send the capability to specific neighbors and 
don't have to change filters for the others. But not how you'd think.

So, if we can enable it globally under the address-family, we might 
enable it for just one neighbor? What do you think this does?

> router bgp X
>  neighbor X
>   address-family ipv6 unicast
>    additional-paths send

Wrong, this doesn't exist.

Sooo, the next one exists, but what does this do?

> router bgp X
>  neighbor X
>   capability additional-paths send

Wrong, it doesn't do anything. At least in my lab it did nothing. Maybe 
I held it wrong? Maybe it's a bug? Idk.

> router bgp
>  neighbor X
>   capability additional-paths send disable

Now this one disables advertising the capability for this neighbor and 
address-family... idk? All? Probably all.

So you can globally enable "additional-paths send" and disable it for 
every neighbor (or in a neighbor-group, which might be convenient).

All in all this one-liner in JunOS translates to something like that on 
my IOS XR boxes:

> router bgp X
>  address-family ipv4 unicast
>   additional-paths send
>   additional-paths selection route-policy add-path-selection
>  address-family ipv6 unicast
>   additional-paths send
>   additional-paths selection route-policy add-path-selection
>  neighbor-group ibgp
>   capability additional-paths send disable
>  neighbor-group ebgp
>   capability additional-paths send disable
>  neighbor X
>   ! no "use neighbor-group ibgp" here!
>   description Looking Glass
> 
> route-policy add-path-selection
>   set path-selection all advertise
> end-policy


To find all this info, I needed to read the documentation, which is 
pretty much non-existent, open two TAC cases, read a troubleshooting 
guide[1] which is actually documentation and not a troubleshooting guide.

IMO several weird design choices combined with a lack of documentation 
makes this pretty frustrating. The good news is: nothing of this will 
reset neighbor sessions and you can manually clear them to renegotiate 
capabilities whenever.

Hope this saves someone a headache.

-Sebastian


[1] 
https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/217097-bgp-path-selection-in-ios-xr.html