OSPF

[Ratheesh mv]

R7 is receiving 100.100.100.100/32 from R8(78.0.0.8) over area 0 and R9(79.0.0.9)  over area 5 .

R7 always prefers R9(79.0.0.9)  over area 5  path .(Below out put)

Gateway of last resort is not set

100.0.0.0/32 is subnetted, 1 subnets
O 100.100.100.100 [110/11] via 79.0.0.9, 00:00:06, Ethernet0/1
R7#

If I shutdown link R7-R9 then R7 prefers R8(78.0.0.8) over area 0 path (Below output)

R7(config-if)#int ethernet0/1
R7(config-if)#shutdown

Gateway of last resort is not set

100.0.0.0/32 is subnetted, 1 subnets
O 100.100.100.100 [110/11] via 78.0.0.8, 00:00:24, Ethernet0/0
R7#

I would like to know that even R7 receives LSA 1 from both neighbor why does R7 always prefer R9 than R8(If both neighbors are up) ?

[MHM Cisco World]

OSPF prefere router in this order
Intra Area
Inter Area 
External Area 

[Ratheesh mv]

R7 is learning LSA 1 from both neighbour .When R9 goes down , R7 installs intra area route itself towards R8.

[MHM Cisco World]

OSPFv2 tie breaker is area ID highest win,
So to test change the config to be 
R7 connect to R8 in area ID 10
R7 connect to R9 in area ID 5

now you must see R7 select the loopback through the Area ID 10

this what I know about the tie breaker.

[paul driver]

Hello
Ospf should always prefer the backbone area to any other non backbone area

When both adjacencies are up can you post:

Show ip ospf interface brief

[Georg Pauwen]

Hello,

if you issue the command:

sh ip ospf rib detail

the route installed is the route with the lowest SFP instance. This seems to be the tiebreaker.

*> 100.100.100.100/32, Intra, cost 2, area 5
SPF Instance 12, age 00:10:15
Flags: RIB, HiPrio
via 79.0.0.2, GigabitEthernet0/1
Flags: RIB
LSA: 1/3.3.3.3/3.3.3.3
100.100.100.100/32, Intra, cost 2, area 0
SPF Instance 15, age 00:07:37
Flags: HiPrio
via 78.0.0.2, GigabitEthernet0/0
Flags: none
LSA: 1/2.2.2.2/2.2.2.2

[Martin L]

This is weird indeed. both routes are Intra as indicated by O (not IA), so default internal process of preferring Intra over Inter over External does not apply.  Therefore, area id is sort of a tie-breaker here, as mentioned by MHM, highest Area ID wins.  in your case, 5 over 0

Regards, ML
**Please Rate All Helpful Responses **

[paul driver]

Hello
@Martin L if you look at the design it isn't that weird
rtr 8 is the internal backbone rtr not rtr7 this is at present an ABR between area 5 and area 0, and rtr 9 is an internal rtr of area 5.
So rtr7 should take rtr9 for that prefix as its directly connected to area 5.

As it is presently if you removed the loopback from R8 and check the rib table on that rtr you should see an inter-area route from r9 because R8 is an internal backbone rtr.

Now If all three rtrs were in ospf area 0 and the loopback on r9 was in area 5 then it would be more valid, The route on R9 should be seen as an inter-area route because rtr9 would have a direct connection to area 0 and area 5 thus would become the ABR not rtr7.

[Georg Pauwen]

Good catch ! I changed area 0 to area 6, and indeed that link got selected. I could not really find that documented anywhere, but you are absolutely right !

[paul driver]

Hello

In the real world you wouldn't have two non backbone areas interconnected without any backbone area as both areas would be discontigous as such rtr 8-9 wouldn't be able to reach each other