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OSPF Fails to Deliver a Default Route

Publication Date:  2012-07-27 Views:  172 Downloads:  0

Issue Description

 Version: NE80E Version 5.50 V300R003C02B697
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There are two NE80Es (NE80E-1 and NE80E-2) at the core layer at a site. EBGP runs between NE80E-1 and its upper layer network devices; NE80E-1 has learnt a default route through EBGP. IBGP runs between NE80E-2 and NE80E-1; OSPF is used as an IGP between NE80E-2 and NE80E-1. The NE40E is dual-homed to NE80E-1 and NE80E-2 and OSPF runs between the NE40E and NE80Es. The NE80E-1 and NE80E-2 deliver default routes to the NE40E through OSPF in a non-forcible manner. OSPF defines a backbone area, namely, Area 0.
Fault phenomenon: There is only one default route learnt from NE80E-1 in the routing table of the NE40E. NE80E-2 fails to deliver an OSPF default route to the NE40E as configured. 


Alarm Information



Handling Process

 In the preceding networking, if the link between the NE40E and NE80E-1 fails, the NE40E can still learn the default route advertised by NE80E-1 in a non-forcible manner through NE80E-2, and services are not interrupted. As a result, no action is required to address this problem. If OSPF is not running between NE80E-1 and NE80E-2, the services on the NE40E will be interrupted. In this case, you need to configure OSPF to advertise default routes in forcible mode and then change the OSPF cost or the weight of the next hop used between the NE40E and NE80E-2 so that the NE40E can directly communicate with NE80E-1. This prevents traffic from traveling from the NE40E to NE80E-2 and then to NE80E-1. 


Root Cause

 1. Run the display ip routing-table command on NE80E-1 and NE80E-2, and you can find that there are active default routes on both NE80E-1 and NE80E-2. The next hop of the default route of NE80E-2 is NE80E-1 and this default route is learnt through IBGP. Note that it is planned that the local IBGP preference is 100 and the external OSPF route preference defaults to 150. 2. Run the display ospf routing command on the NE40E to view the OSPF routing table, and you can find that there is still only one default route with the next hop being NE80E-1.
3. Run the display ospf lsdb ase0.0.0.0 command on the NE40E, and you can find that there is only one LSA whose AdvRouter is NE80E-1. This indicates that NE80E-2 fails to generate an LSA that describes the default route.
4. OSPF delivers default routes in a non-forcible manner in the following conditions:
(1) There is an active default route in the routing table and the default route is not learnt through OSPF that is configured to deliver default routes in a non-forcible manner.
(2) There is an active default route in the routing table and the default route is not an IBGP route.
OSPF delivers default routes in a non-forcible manner only when both the preceding conditions are met.
As a result, because the default route in the routing table of NE80E-2 is learnt through IBGP, NE80E-2 does not advertise the default route even if it is configured with the function that OSPF advertises default routes in a non-forcible manner.