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Route not getting installed on the routing table even though present on the ospf database

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TitleRoute not getting installed on the routing table even though present on the ospf database
Symptoms
  • OSPF E2 external route not installed in MLX1 IP routing table:
SSH@MLX1#sh ip route 10.142.33.101
Type Codes - B:BGP D:Connected I:ISIS O:OSPF R:RIP S:Static; Cost - Dist/Metric
BGP  Codes - i:iBGP e:eBGP
ISIS Codes - L1:Level-1 L2:Level-2
OSPF Codes - i:Inter Area 1:External Type 1 2:External Type 2 s:Sham Link
STATIC Codes - d:DHCPv6
        Destination        Gateway         Port           Cost          Type Uptime src-vrf
1       10.142.33.101/32   10.15.22.3      ve 15          200/10        Bi   20h53m - 
SSH@MLX1#
  • OSPF type 7 LSAs appear in link state database of MLX3 and MLX4:
Ospf link-state by link-state ID 10.142.33.101 are in the following:

Area ID         Aging  LS ID           Router          Seq(hex) Chksum   SyncState   Type 
1               316    10.142.33.101   10.5.222.3      80000300 0000b615 Done       NSSA 
  LSA Header:  age: 316, options: ------P---, seq-nbr: 0x80000300, length: 36
  NetworkMask: 255.255.255.255
  TOS 0:  metric_type: 2, metric: 10
          forwarding_address: 10.14.1.201
          external_route_tag: 0

Area ID         Aging  LS ID           Router          Seq(hex) Chksum   SyncState   Type 
1               57     10.142.33.101   10.14.1.200     8000014e 00003857 Done       NSSA 
  LSA Header:  age: 57, options: ------P---, seq-nbr: 0x8000014e, length: 36
  NetworkMask: 255.255.255.255
  TOS 0:  metric_type: 2, metric: 10
          forwarding_address: 10.14.1.200
          external_route_tag: 0

 
  • debug ip ospf route 10.142.33.101 executed on MLX1 and MLX2 displays the output below:
MLX1
Aug  5 15:28:56.407 OSPF: calc ext route 10.142.33.101
Aug  5 15:28:56.407 OSPF: ext route, net = 10.142.33.101, mask = 255.255.255.255 advrtr = 10.14.1.33, fwd = 10.14.1.201
Aug  5 15:28:56.407 OSPF: FA is reachable thru NH:10.14.245.100
Aug  5 15:28:56.407 OSPF: Abort [NSSA] rfc 2.5.3 net=10.142.33.101 FA:10.14.245.100 AreaId:0.0.0.1 NSSA_Area:1


MLX2
Aug  5 15:28:56.209 OSPF: calc ext route 10.142.33.101
Aug  5 15:28:56.209 OSPF: ext route, net = 10.142.33.101, mask = 255.255.255.255 advrtr = 10.14.1.32, fwd = 10.14.1.201
Aug  5 15:28:56.209 OSPF: FA is reachable thru NH:10.14.245.104
Aug  5 15:28:56.209 OSPF: Abort [NSSA] rfc 2.5.3 net=10.142.33.101 FA:10.14.245.104 AreaId:0.0.0.1 NSSA_Area:1
 

 
Environment
  • MLXe
  • NetIron OS 6.2.00e code
  • OSPF route convergence
  • E2 external routes are static routes redistributed into OSPF at MLX3 and MLX4:
ip route 10.142.33.101/32 96.115.72.12
  • Network topology is shown below:
  MLX1 (ABR) ----- MLX2 (ABR)
         |                                 |
         |    AREA 1 (NSSA)   |   
         |                                 |    
MLX3 (ASBR) ----- MLX4 (ASBR)
Cause
Defect NI-21809 (redistribute route learned through Type-7 LSA in NSSA area is not installed in the routing table)
Resolution
Upgrade to 6.2.00f code
Additional notes
The root cause of this issue is due to the inability of an area border router (ABR) to find an intra/inter area path to the forwarding address.  When a static route is redistributed into an NSSA area, the LSA gets flooded to the ABRs (MLX1 and MLX2) as type 7. Upon receipt, the ABRs start external route computation for this destination.  Based on the forwarding address advertised in the type 7 LSA, the ABR finds an intra-area path through the NSSA area and installs the route in the routing table.  Since MLX1/MLX2 are ABRs and the LSA is type 7, a check is made to determine if the ABR has to take on the translator role to translate the type 7 LSA to a type 5 LSA and originate and flood the type 5 LSA into the domain with the same next hop as that in the type 7 LSA.  While computing the route, the ABR checks to see if it has any intra/inter area path to the forwarding address. The issue occurs when it can't find this path (due to the fact that routes to type 5 LSAs are not allowed in an NSSA area). When this occurs, the function returns without proceeding further, and it prints the debug log message below:

Aug  5 15:28:56.407 OSPF: Abort [NSSA] rfc 2.5.3 net=10.142.33.101 FA:10.14.245.100 AreaId:0.0.0.1 NSSA_Area:1

 

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