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Default Routes Are Advertised in the OSPF NSSA on an NE20E-S4, But OSPF Neighbors Fail to Learn the Default Routes

Publication Date:  2019-04-19 Views:  41 Downloads:  0
Issue Description

Default routes are advertised in the OSPF NSSA on an NE20E-S4, but OSPF neighbors (third-party devices) fail to learn the default routes.

Key configuration:
#
interface GigabitEthernet0/2/1
 undo shutdown
 ip address 2.106.191.145 255.255.255.252
 dcn
#
interface LoopBack0
 ip address 100.73.61.124 255.255.255.255
#
ospf 1 router-id 100.73.61.124
 default-route-advertise always
 import-route direct
 area 0.0.0.0
  network 100.73.61.24 0.0.0.3
  network 100.73.61.124 0.0.0.0
 area 0.0.0.1
  network 10.106.191.128 0.0.0.15
  nssa default-route-advertise no-summary
 area 0.0.0.2
  network 2.106.191.144 0.0.0.15
  nssa default-route-advertise no-summary
#

Handling Process

Default BGP routes exist on the device, and the default-route-advertise parameter is specified.
<NE20E>display ip routing-table
Route Flags: R - relay, D - download to fib, T - to vpn-instance
------------------------------------------------------------------------------
Routing Table : _public_
         Destinations : 540      Routes : 540      

Destination/Mask    Proto   Pre  Cost        Flags NextHop         Interface

        0.0.0.0/0   IBGP    255  0             RD  100.73.59.254   GigabitEthernet0/2/7

 

The OSPF neighbor status is normal.
<NE20E>display ospf peer

          OSPF Process 1 with Router ID 100.73.61.124
                Neighbors

 Area 0.0.0.2 interface 2.106.191.145 (GE0/2/1)'s neighbors
 Router ID: 10.106.191.3     Address: 2.106.191.146   
   State: Full           Mode:Nbr is Slave      Priority: 1
   DR: 2.106.191.146     BDR: 2.106.191.145     MTU: 0
   Dead timer due in  32  sec
   Retrans timer interval: 5
   Neighbor is up for 00h19m48s
   Authentication Sequence: [ 0 ]

 

The routing table of the OSPF neighbor shows that default OSPF routes are not learned.
<MSR3610>disp ip routing-table

Destinations : 37       Routes : 37

Destination/Mask   Proto   Pre Cost        NextHop         Interface
0.0.0.0/0          Static  60  0           2.106.191.145   GE0/4

 

The OSPF neighbor status is normal.
<MSR3610>display ospf peer

         OSPF Process 1 with Router ID 10.106.191.3
               Neighbor Brief Information

 Area: 0.0.0.2       
 Router ID       Address         Pri Dead-Time  State             Interface
 100.73.61.124   2.106.191.145   1   32         Full/BDR          GE0/4

 

The OSPF neighbor LSDB does not contain the entry 0.0.0.0.
<MSR3610>display ospf lsdb

         OSPF Process 1 with Router ID 10.106.191.3
                 Link State Database

                         Area: 0.0.0.2
 Type      LinkState ID    AdvRouter       Age  Len   Sequence  Metric
 Router    100.73.61.124   100.73.61.124   1329 36    80001584  0  
 Router    10.106.191.3    10.106.191.3    1329 60    80001CCB  0  
 Network   2.106.191.146   10.106.191.3    1323 32    80000002  0  
 NSSA      2.106.132.0     10.106.191.3    354  36    80000003  1  
 NSSA      172.19.212.16   10.106.191.3    354  36    8000139B  1  
 NSSA      2.106.191.152   10.106.191.3    354  36    80000003  1  
 NSSA      2.106.191.148   10.106.191.3    334  36    80000002  1  
 NSSA      2.106.191.144   10.106.191.3    182  36    80000002  1  
 NSSA      2.106.191.3     10.106.191.3    326  36    80000002  1  
 NSSA      10.106.191.148  100.73.61.124   1330 36    8000099B  1  
 NSSA      10.106.191.132  100.73.61.124   1330 36    8000099B  1  
 NSSA      10.106.191.128  100.73.61.124   1330 36    80000F28  1  
 NSSA      10.106.191.176  100.73.61.124   1330 36    8000099B  1  
 NSSA      100.73.61.124   100.73.61.124   1330 36    80001567  1  
 NSSA      100.73.61.12    100.73.61.124   1330 36    80000887  1  
 NSSA      100.73.61.8     100.73.61.124   1330 36    80001564  1  
 NSSA      2.106.191.144   100.73.61.124   1330 36    80000004  1  


The conditions for generating default type-7 LSAs to the NSSA are as follows:
1. The backbone area must have neighbors in Full state and interfaces in Up state.
2. The default-route-advertise parameter is configured.
3. The local routing table contains the route destined for 0.0.0.0 (that is, another default routes exists).
If condition 1 is met, the ABR can generate default type-7 LSAs in the NSSA. If conditions 2 and 3 are met, the ASBR can generate default type-7 LSAs in the NSSA.

(1) Because no other routes are imported, these conditions do not apply to the ASBR. If default type-7 LSAs need to be generated, use BGP in OSPF and configure route filtering so that only default routes are imported. In addition, after a third-party devices learns type-7 LSAs, because external routes have a lower priority than static routes, the external routes cannot be added to the routing table. Therefore, the static route needs to be deleted from the third-party device.

(2) According to condition 1, the backbone area does not have neighbors in Full state. Therefore, default type-3 LSAs cannot be generated.
ospf 1 router-id 100.73.61.124
default-route-advertise always
import-route direct
area 0.0.0.0
  network 100.73.61.24 0.0.0.3         //The interface does not exist.
  network 100.73.61.124 0.0.0.0        //This is a loopback interface, and a neighbor does not exist.
area 0.0.0.1
  network 10.106.191.128 0.0.0.15
  nssa default-route-advertise no-summary
area 0.0.0.2
  network 2.106.191.144 0.0.0.15
  nssa default-route-advertise no-summary

(3) To allow interfaces in the backbone area to be in Up state, run the nssa default-route-advertise backbone-peer-ignore no-summary command. If interfaces in the Up state exist in the backbone area, the ABR generates both the default type-7 LSAs and type-3 LSAs regardless of whether a neighbor in Full state exists. The default type-3 LSAs take effect preferentially.

Root Cause

The conditions for generating default type-7 LSAs in the NSSA are not met.

Solution

1. Use BGP in OSPF. Allow the ASBR to generate default type-7 LSAs in the NSSA.

2. Configure the interface in the backbone area to keep in Up state. After the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates both the default type-7 LSAs and default type-3 LSAs in the NSSA.

END