Description

 This article describes the issue of being unable to export the default 0.0.0.0.0/0 route via eBGP.

Symptoms

  • It is possible to learn the 0.0.0.0/0 route via iBGP from the peer router.
  • However, it is not possible to export the same 0.0.0.0/0 route via eBGP to the eBGP peer.
  • By default, routes that are learned by iBGP should be able to be exported to eBGP and it should not require an external policy to export them.
  • Even adding a policy to export the exact 0.0.0.0/0 route is unsucessful.

Solution

 

Check the 0.0.0.0/0 route to identify which BGP attributes are being sent with the BGP community. There is a no-export BGP attribute, which some AS's may impose. If the route received has this configured, it can be advertised to IBGP peers while prevents to be exported outside the AS / EBGP peers.  

To determine this, issue the run show route 0.0.0.0/0 extensive command:

mhatch@co1-fw1-svc# run show route 0.0.0.0/0 extensive

inet.0: 132 destinations, 138 routes (131 active, 0 holddown, 1 hidden)
0.0.0.0/0 (2 entries, 1 announced)
TSI:
KRT in-kernel 0.0.0.0/0 -> {indirect(1048574), indirect(1048575)}
OSPF area : 0.0.0.0, LSA ID : 0.0.0.0, LSA type : Extern
        *BGP    Preference: 170/-101
                Next hop type: Indirect
                Address: 0x29f4298
                Next-hop reference count: 3
                Source: 208.92.200.252
                Next hop type: Router, Next hop index: 721
                Next hop: 208.92.200.252 via reth0.0, selected
                Next hop type: Router, Next hop index: 751
                Next hop: 208.92.200.253 via reth0.0
                Protocol next hop: 208.92.200.252
                Indirect next hop: 2780000 1048574
                Protocol next hop: 208.92.200.253
                Indirect next hop: 29ee270 1048575
                State: <Active Int Ext>
                Local AS: 46189 Peer AS: 46189
                Age: 1w1d 16:37:52      Metric2: 0
                Task: BGP_46189.208.92.200.252+179
                Announcement bits (3): 0-KRT 2-OSPF 8-Resolve tree 1
                AS path: 13790 I
                Communities: 65001:0 no-export  < Look here
                Accepted Multipath
                Localpref: 100
                Router ID: 208.92.200.252
                Indirect next hops: 2
                        Protocol next hop: 208.92.200.252
                        Indirect next hop: 2780000 1048574
                        Indirect path forwarding next hops: 1
                                Next hop type: Router
                                Next hop: 208.92.200.252 via reth0.0
                        208.92.200.248/29 Originating RIB: inet.0
                          Node path count: 1
                          Forwarding nexthops: 1
                                Next hop type: Interface
                                Nexthop: via reth0.0
                        Protocol next hop: 208.92.200.253
                        Indirect next hop: 29ee270 1048575
                        Indirect path forwarding next hops: 1
                                Next hop type: Router
                                Next hop: 208.92.200.253 via reth0.0
                        208.92.200.248/29 Originating RIB: inet.0
                          Node path count: 1
                          Forwarding nexthops: 1
                                Next hop type: Interface
The above output contains the no-export attribute; in such a scenario, check with the peer / AS source of the route to check if the BGP community no-export can be turn off.
One use of the community is to avoid your own AS to become a transit AS. 

Modification History

2022-09-01: Edited 1st and last paragraph to add clarity on who can and cannot receive an advertisement with this community and the reason for a possible usage.