ospf network types and neighbor discovery

[sarahr202]

Hi everybody

This is what my book says:

Ospf  elects DR/bdr on NBMA int.  Dynamic neighbor discovery is not permitted therefore we need to use " neighbor A.B.C.D " command.

I found something diffferent:

R1-----f0/0----sw------f0/0----R2

Above I changed the ospf network type from broadcast to NBMA on both routers:

R1

f0/0

200.200.200.1/24

ip ospf network non-broadcast

router ospf 1

network 200.200.200.0 0.0.0.255 area 0

R2;

f0/0

200.200.200.2/24

ip ospf network non-broadcast

router ospf 1

network 200.200.200.0 0.0.0.255 area 0

===================================================

In absence of neighbor  command, no hellos are sent by both routers.

Next I configure " neighbor 200.200.200.2" on R1 only.

Strange things happens. As expected, R1 starts sending hellos  to R2. R2 does not have any neighbor command but yet starts sending hellos once it verifies all the hellos' parameters received match its own. i.e all the timers , hello, dead interval etc.

I did another lab to make sure it is indeed the timers

R1f0/0--------sw------f0/0 R2

I only change f0/0 on R1 to default ospf network type i.e broadcast.

R1 starts sending hello to R2.

R2 generates a log message " mismatch hello parameters" (  default for NBMA  hello 30sec, dead timer 120)

R1 's hello carries hello 10 sec, dead timer 40 sec.

But when I change the timers on R2 as:

ip ospf hello 10

ip ospf dead-interval 40.

R2 starts sending hello to R1 and neighbor relationship forms even though R2 is not configured with any " neighbor 200.200.200.1 " command.

It appears to me a router will form a neighbor relationship over a nbma int as long as the hello parameters received in hello  match the hello parameters of its NBMA int without the need of " neighbor A.B.C.D" command .

I appreciate your input

Happy New Year !!!!

[Nagendra Kumar Nainar · Accepted solution]

Hi,

Yes. As you might be aware, OSPF on receiving hello will just check hello, dead intervals, area, subnet (skip in some case), auth (if enabled). There is no field as such in hello packet to carry wht network type the interface is enabled with.

If you have different network type on same link but above parameters matches, neighborship may come up. But OSPF database will have differnt link type in Router LSA and so the topology will be incomplete which in turn will reflect as incomplete RIB.

HTH,

Nagendra

[sarahr202]

Thanks Nagendra

This is exactly I observed in my lab. Incomplete ospf database when there is inconssitency in router lsa.

Happy New year to you. This is my first post for 2013!!!

[Hardik Vaidh]

Dear nagendra,

u r right

If you have different network type on same link but above parameters  matches, neighborship may come up. But OSPF database will have differnt  link type in Router LSA and so the topology will be incomplete ospf database..

Dear sarahr202

neighbourship may b up bt lsa type 5 will nt match bcoz lsa type 5 external LSA - these LSAs contain information imported into OSPF from  other routing processes. They are flooded to all areas unchanged(except  stub areas).