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OSPF External path selection: External Type-2 (E2) VS NSSA Type-2 (N2)- Scenario 2

As we discuss earlier on the scenario 1, lets now discuss on the scenario 2, In this scenario, we will be looking at what behaviour is observed when using RFC 1587 for external path calculation. RFC 3101 compliance is automatically enabled on IOS-XE devices. 

 To replace RFC 3101 compatibility with RFC 1587 compatibility for route selection in not-so-stubby area (NSSA) Area Border Routers (ABRs), use the compatible RFC 1587 command in router configuration mode or address family configuration mode. To restore RFC 3101 compatibility, use the no form of this command.

We will be interested in prefix 192.0.2.100/32 which is redistributed on both R3 and R2. First we must enable RFC 1587 compatibility on R1.

Fig 1.1- Network Topology Scenario 2
RouteXP_R1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
RouteXP_R1(config)#router ospf 1
RouteXP_R1(config-router)#compatible rfc1587

RouteXP_R1#show ip ospf  | in RFC
 Supports NSSA (compatible with RFC 1587)

Once we have enabled Compatibility RFC 1587 on R1 we can check what paths are in our database and what LSA is preferred: 

RouteXP_R1#show ip ospf database external

            OSPF Router with ID (1.1.1.1) (Process ID 1)
                Type-5 AS External Link States

  LS age: 115
  Options: (No TOS-capability, DC, Upward)
  LS Type: AS External Link
  Link State ID: 192.0.2.100 (External Network Number )
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000003
  Checksum: 0xBDAD
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 0.0.0.0
        External Route Tag: 0


RouteXP_R1#show ip ospf database nssa-external

            OSPF Router with ID (1.1.1.1) (Process ID 1)
                Type-7 AS External Link States (Area 50)

  LS age: 48
  Options: (No TOS-capability, Type 7/5 translation, DC, Upward)
  LS Type: AS External Link
  Link State ID: 192.0.2.100 (External Network Number )
  Advertising Router: 2.2.2.2
  LS Seq Number: 80000005
  Checksum: 0x2CBF
  Length: 36
  Network Mask: /32
        Metric Type: 2 (Larger than any link state path)
        MTID: 0
        Metric: 20
        Forward Address: 192.168.12.2
        External Route Tag: 0

Now lets check what LSA is preferred on R1: 

RouteXP_R1#show ip ospf rib 192.0.2.100

            OSPF Router with ID (1.1.1.1) (Process ID 1)
                Base Topology (MTID 0)

OSPF local RIB
Codes: * - Best, > - Installed in global RIB
LSA: type/LSID/originator

*>  192.0.2.100/32, Ext2, cost 20, fwd cost 1, tag 0
     SPF Instance 44, age 00:01:56
      contributing LSA: 7/192.0.2.100/2.2.2.2 (area 50)
      contributing LSA: 5/192.0.2.100/3.3.3.3
     Flags: RIB, HiPrio, IntraNonBB, PartialSPF
      via 192.168.13.3, GigabitEthernet2 label 1048578
       Flags: RIB
       LSA: 5/192.0.2.100/3.3.3.3

The Type-5 LSA is preferred.
In the above output, you might have also noticed that R1 is not translating Type-7 to Type-5, this is because only Type-7 routes that have been added to the routing table are candidates for translation.