IBGP cannot import prefixs with its as_path

20 Sep 2021

      Hi,
         I use ibgp to transport full tables in my network. iBGP delete my
ASN but I use my ASN to identify my prefixes.
        So I added this code to my ibgp import filter
if bgp_path ~ [= =] then {
#bgp_path.prepend(LOCAL_ASN);
if my_opt_prefix() then bgp_path.prepend(LOCAL_ASN);
}
It works when 2 servers connect, but when I have 3 servers, the third
server can not import my first server's prefixed with my ASN.

At the first server

bird> show route export HOME_1_ARCH
Table master6:
2a0c:e640:1017::/48  unreachable [BGP_Prefix 22:12:23.507] * (200)
2602:feda:ab9::/48   unreachable [BGP_Prefix 22:12:23.507] * (200)

At the second server
bird> show route for 2a0c:e640:1017:: export Aperture_IX all
Table master6:
2a0c:e640:1017::/48  unicast [ZHYZ_PC 14:53:29.056] * (100) [AS141011i]
       via 2a0c:e640:1017::1 on zhyz_pc
       Type: BGP univ
       BGP.origin: IGP
       BGP.as_path: 141011
       BGP.next_hop: 2a0c:e640:1017::1
       BGP.local_pref: 100
At the third server
bird> show route for 2a0c:e640:1017::/48 protocol HOME_1_ARCH all
Table master6:
::/0                 unicast [HOME_1_ARCH 06:10:14.802] * (100) [AS134835i]
       via 2602:feda:ab5::1 on home_1_arch
       Type: BGP univ
       BGP.origin: IGP
       BGP.as_path: 134835
       BGP.next_hop: 2602:feda:ab5::1
       BGP.local_pref: 100
       BGP.community: (65530,2000)
HOME_1_ARCH BGP        ---        up     06:10:14.802  Established
 Description:    HOME_1_ARCH BGP
 BGP state:          Established
   Neighbor address: 2602:feda:ab5::1%home_1_arch
   Neighbor AS:      141011
   Local AS:         141011
   Neighbor ID:      192.168.117.52
   Local capabilities
     Multiprotocol
       AF announced: ipv6
     Route refresh
     Graceful restart
       Restart time: 120
       AF supported: ipv6
       AF preserved:
     4-octet AS numbers
     Enhanced refresh
     Long-lived graceful restart
   Neighbor capabilities
     Multiprotocol
       AF announced: ipv6
     Route refresh
     Graceful restart
       Restart time: 120
       AF supported: ipv6
       AF preserved:
     4-octet AS numbers
     Enhanced refresh
     Long-lived graceful restart
   Session:          internal route-reflector AS4
   Source address:   2602:feda:ab7::1
   Hold timer:       183.267/240
   Keepalive timer:  63.087/80
 Channel ipv6
   State:          UP
   Table:          master6
   Preference:     100
   Input filter:   (unnamed)
   Output filter:  (unnamed)
   Routes:         2560 imported, 129220 exported, 2557 preferred
   Route change stats:     received   rejected   filtered    ignored
  accepted
     Import updates:         266057          0          0       5170
    260887
     Import withdraws:       261009          0        ---       5382
    255627
     Export updates:         681596      17838     129330        ---
    534428
     Export withdraws:         3020        ---        ---        ---
    131521
   BGP Next hop:   2602:feda:ab7::1

You could see the third server cannot import  2a0c:e640:1017::/48
But when I deleted this code on the second server.
if bgp_path ~ [= =] then {
#bgp_path.prepend(LOCAL_ASN);
if my_opt_prefix() then bgp_path.prepend(LOCAL_ASN);
}

it has been successful import
bird> show route for 2a0c:e640:1017::/48 protocol HOME_1_ARCH all
Table master6:
2a0c:e640:1017::/48  unicast [HOME_1_ARCH 07:14:07.526] * (100) [AS141011i]
       via 2602:feda:ab5::1 on home_1_arch
       Type: BGP univ
       BGP.origin: IGP
       BGP.as_path: 141011
       BGP.next_hop: 2602:feda:ab5::1
       BGP.local_pref: 100
       BGP.originator_id: 172.16.229.1
       BGP.cluster_list: 192.168.117.52

I know that maybe no one will use ibgp like this, but why it won't work?

IBGP cannot import prefixs with its as_path

智慧雨泽