LSA 5 showing unexpectedly

[NaveenJakhmola]

Hi All,

I was designing a OSPF lab for virtual link but even before creating the virtual link between R3<---->R4. i checked the ospf database for R5 and found that LSA 5 was present there.

i just wanna know how the LSA 5 was injected in area 24. Is there any rule/concept i'm missing here???

PS :1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4 are external route redistributed by R1 and 10.10.10.10, 20.20.20.20...... so on are the router id for the respective numbered routers.

R4 ospf config:

R4#sh run | sec ospf
ip ospf 1 area 24
router ospf 1
router-id 40.40.40.40
log-adjacency-changes
network 10.1.3.0 0.0.0.255 area 34

R4#sh run int f0/0
Building configuration...

Current configuration : 112 bytes
!
interface FastEthernet0/0
ip address 10.1.4.1 255.255.255.0
ip ospf 1 area 24
duplex auto
speed auto
end

Regards

Naveen Jakhmola

[Meheretab Mengistu · Accepted solution]

Hi Naveen,

i just wanna know how the LSA 5 was injected in area 24. Is there any rule/concept i'm missing here???

PS :1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4 are external route redistributed by R1 and 10.10.10.10, 20.20.20.20...... so on are the router id for the respective numbered routers.

---

According to the RFC 2328, "... External routing information is flooded unaltered throughout the AS..."

In your case, R4 and R5 exchanged DBD packets to make sure that their LSDBs are the same. The DBD packets from R4 contained 4 AS-External-LSAs for each LSA-type 5 routes (1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4). 

Remember:- Even if you see the routes in the LSDB (with show ip ospf database), they are not reachable as LSA-type 4 (which provides a route to reach the ASBR) is missing until you add the Virtual-link.

HTH,

Meheretab 

[NaveenJakhmola]

Hi Meheretab,

Thanks for your prompt response.

But something is still bothering me.
Like you said "R4 and R5 exchanged DBD packets to make sure that their LSDBs are the same. The DBD packets from R4 contained 4 AS-External-LSAs for each LSA-type 5 routes (1.1.1.1, 2.2.2.2, 3.3.3.3, 4.4.4.4)."

Maybe I sound a bit silly But R4 LSDB also contain LSA3, LSA4 why not exchange those.

Regards
Naveen Jakhmola

[Meheretab Mengistu]

Hi Naveen,

If you look closely, those LSA-type 1 to 4, which are not shared, belong to
different area, area 34. As a result, could not be shared with area 24.

Feel free to ask further.

HTH,
Meheretab

[NaveenJakhmola]

Hi Meheretab,

I think i got it now.
Please Correct me if i am wrong LSA 5 are flooded to all the OSPF areas unaltered regardless of that if the area is directly connected to backbone or not.

[Meheretab Mengistu]

That is correct.