OSPF Area 0 inactive and loopbacks

[tomc.pnnl]

Good day board -

Can one use a loopback to activate area 0? I have a setup where I desire a star topology and would prefer a loopback over other options. The state and configuration are:

sh ospf
...
Area BACKBONE(0) (Inactive)
        Number of interfaces in this area is 1
        SPF algorithm executed 6 times
        Number of LSA 1.  Checksum Sum 0x002843
        Number of opaque link LSA 0.  Checksum Sum 00000000
        Number of DCbitless LSA 0
        Number of indication LSA 0
        Number of DoNotAge LSA 0
        Flood list length 0
        Number of LFA enabled interfaces 0, LFA revision 0
        Number of Per Prefix LFA enabled interfaces 0
        Number of neighbors forming in staggered mode 0, 0 full

sh int loop1
interface Loopback1
 vrf VrfA
 ipv4 address 172.18.0.1 255.255.255.0
!

sh int loop1 br
               Intf       Intf        LineP              Encap  MTU        BW
               Name       State       State               Type (byte)    (Kbps)
--------------------------------------------------------------------------------
                Lo1          up          up           Loopback  1500          0

sh run router ospf 1
router ospf 1
 vrf VrfA
  router-id 0.0.0.1
  network point-to-point
  prefix-suppression
  area 0
   interface Loopback1
    network point-to-point
   !
  !
  area 10
   nssa default-information-originate metric 100 metric-type 2
   interface GigabitEthernet0/0/0/1.10
   !
  !
  area 12
   nssa default-information-originate
   interface GigabitEthernet0/0/0/1.12
   !
  !
 !
!
sh ospf vrf VrfA int 
Interfaces for OSPF 1, VRF VrfA

Loopback1 is up, line protocol is up 
  Internet Address 172.18.0.1/24, Area 0
  Label stack Primary label 0 Backup label 0 SRTE label 0
  Process ID 1, VRF VrfA, Router ID 0.0.0.1, Network Type LOOPBACK, Cost: 1
  Loopback interface is treated as a stub Host

[Giuseppe Larosa]

Hello @tomc.pnnl ,

OSPF is enough smart to understand what loopback interfaces are and I think that to activate area 0 you need a L3 interface in area 0 that can be a VLAN based subinterface in state of up/up. In simple words you need a true interface where OSPF hellos can be sent out and eventually an OSPF adjacency can be built.

To be noted until area 0 is inactive your router will not act as an ABR between area NSSA 10 and NSSA 12.

This can have an impact or not in your network scenario. Actually not as you are sending a  default route in each NSSA area.

Hope to help

Giuseppe

[ngkin2010 · Accepted solution]

Hi,

I've the similar question before, and I did some testing before. According to the Cisco's interpretation about ABR, even the router only has a loopback interface attached to area 0, it will consider as ABR.  

Area Border Router (ABR):

      Cisco Systems Interpretation:
         A router is considered to be an ABR if it has more than one
         area Actively Attached and one of them is the backbone area.

      IBM Interpretation:
         A router is considered to be an ABR if it has more than one
         Actively Attached area and the backbone area Configured.

I found this trick could be used to build a transit router between two non-backbone areas. If we don't cater the single point of failure, I think it's possible that @tomc.pnnl use the loopback interface to make the router become ABR. 

https://cdn2.hubspot.net/hubfs/3985396/Blog/Loop-Prevention-in-OSPF.pdf