Unexpected behavior with "remove-private-AS all" in config

[rsingha108]

Description

Route is not getting advertised properly if "remove-private-AS all" is mentioned in config.

Version

FRR version 10.1

How to reproduce

Topology:

exabgp (3.0.0.3) ----- (3.0.0.2) frr2 (4.0.0.2) ------------ (4.0.0.3) frr3
[AS 65535]          [confedAS 64512, subAS 257]     [confedAS 422, subAS 256] 

Exabgp config:

process announce-routes {  
    run python exabgp/example.py;
    encoder json;
}

neighbor 3.0.0.2 {                 # Remote neighbor to peer with
    router-id 3.0.0.3;              # Our local router-id
    local-address 3.0.0.3;          # Our local update-source
    local-as 65535;                    # Our local AS
    peer-as 64512;                     # Peer's AS

    api {
        processes [announce-routes];
    }
}
    

And I am sending the following route using ExaBGP:

announce route 100.10.1.0/24 next-hop self 

frr2 config:

router bgp 257  
  no bgp ebgp-requires-policy
  bgp confederation identifier 64512
  neighbor 3.0.0.3 remote-as 65535
  neighbor 4.0.0.3 remote-as 422 remove-private-AS all
exit
!

frr3 config:

router bgp 256
  no bgp ebgp-requires-policy
  bgp confederation identifier 422
  neighbor 4.0.0.2 remote-as 64512
exit
!

Expected behavior

frr3 should receive the route with prefix 100.10.1.0/24

Actual behavior

frr3 is not receiving the route with prefix 100.10.1.0/24

Additional context

If we do not mention "remove-private-AS all" in the config of frr2 then frr3 receives the route with prefix 100.10.1.0/24

Checklist

• I have searched the open issues for this bug.
• I have not included sensitive information in this report.

[ton31337]

How do you imagine the route should look? With your configuration frr2 has ebgp sessions (both)... If removing private ASNs... aspath is empty, that is not allowed for ebgp sessions.

[rsingha108]

For this scenario, we get the following results:

• if both R2 & R3 are FRR routers: R3 doesn't receive the route
• if both R2 & R3 are GoBGP routers: R3 receives the route with as-path [64512 65535]
• Testing the same configuration with batfish simulation tool also gives same result as GoBGP.

Why using FRR I am getting different result?

[ton31337]

Maybe it's because GoBGP/Batfish do not follow RFCs?