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Abnormal Advertisement of Default OSPF Routes Caused by Improper Configurations

Publication Date:  2012-07-27 Views:  4 Downloads:  2
Issue Description
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

ospf 1
default-route-advertise cost 100 type 1
The command to advertise BGP routes to was deleted by mistake on
command: undo peer route-policy policy2 export.
As a result, the AS on the right cannot access the upstream AS. For example, the RR cannot successfully ping After the command is re-configured, the AS on the right can access the upstream AS. 

Alarm Information
Handling Process
After the advertisement of default routes through EBGP was resumed on, the services were normal.
It is recommended that you make default routes advertised through IGP on to implement redundancy configurations in the case of a single-homed host with multiple links. 

Root Cause
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 could not learn default IGP routes and deduced that the problem was probably with However, there were effective default routes in the routing table of and the routes were learned through IBGP (peer
dis ip rou
Routing Tables: Public
Destinations: 16 Routes: 17
Destination/Mask    Proto  Pre  Cost       NextHop         Interface   BGP    255  0     Ethernet0/0/1
But, the OSPF LSDB did not generate related default routes.
dis ospf lsd ase
OSPF Process 1 with Router ID
Link State Database
Then the engineer determined that 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.