For the networking, refer to the appendix. The AS on the left is the core AS and the AS on the right is the access AS. There are two ASBRs between the two ASs. EBGP is deployed for route information switching. The upstream AS on the left advertises only default BGP routes to the AS on the right. For some reason, only default IGP routes are advertised at 192.168.3.3.
default-route-advertise cost 100 type 1
The command to advertise BGP routes to 192.168.3.3 was deleted by mistake on 192.168.1.1.
command: undo peer 192.168.13.3 route-policy policy2 export.
As a result, the AS on the right cannot access the upstream AS. For example, the RR cannot successfully ping 192.168.5.5. After the command is re-configured, the AS on the right can access the upstream AS.
After the advertisement of default routes through EBGP was resumed on 192.168.1.1, the services were normal.
It is recommended that you make default routes advertised through IGP on 192.168.4.4 to implement redundancy configurations in the case of a single-homed host with multiple links.
There are two links between the two ASs. In theory, routes can be switched and forwarded through both links. However, it was not the case in practice. In tackling the problem, the engineer found that 192.168.5.5 could not learn default IGP routes and deduced that the problem was probably with 192.168.3.3. However, there were effective default routes in the routing table of 192.168.3.3 and the routes were learned through IBGP (peer 192.168.4.4).
dis ip rou
Routing Tables: Public
Destinations: 16 Routes: 17
Destination/Mask Proto Pre Cost NextHop Interface
0.0.0.0/0 BGP 255 0 192.168.4.4 Ethernet0/0/1
But, the OSPF LSDB did not generate related default routes.
dis ospf lsd ase
OSPF Process 1 with Router ID 192.168.3.3
Link State Database
Then the engineer determined that 192.168.3.3 did not regard the default routes learned through IBGP as a sufficient condition to trigger the advertisement of default routes by OSPF.
Generally, we think the condition for OSPF to advertise default routes is that the routing table of the router has effective default routes
Actually, there is the following description in section 2.2 "Importing BGP routes into OSPF" in RFC1364:
Routes learned via IBGP must not be imported into OSPF.
Therefore, learning the default routes advertised through IBGP cannot trigger the advertisement of default routes by OSPF.
Huawei and C also conform to this design rule.