OSPF Question

philth_123_2 · ‎10-24-2008

Hi,

I have a Lab Scenario as follows:

Two routers R2 & R4 are connected to a F/R cloud via Area 246. They are also connected to each other via Ethernet in Area 0. I have two loopbacks configured on R2 (2.2.2.2) and on R4 (4.4.4.4) in area 246 also.

The cost of reaching network 4.4.4.4 from R2 is therefore 10 + 1 (loopback) via Ethernet or 64 + 1 via F/R Serial link. When I bring the routers up from a cold start the preferred route is via the F/R link even though the cost is higher. Is this because OSPF prefers the inter-area route over the Intra-area route ?

If I shut R2's serial interface the route to 4.4.4.4 becomes IA over the Ethernet with cost 11 as expected. However, as I open the Serial interface the route to 4.4.4.4 is still over Ethernet and the F/R link isn't used ?

Can anyone help to explain ?

Giuseppe Larosa · ‎10-24-2008

Hello Philip,

in OSPF the hierarchy is:

O routes intraarea are preferred over O IA inter-area route.

Being the loopback advertised with a network area 246 command the intra area O route is preferred over the O IA route even if the metric of O IA via area 0 link is lower.

The same happens with O E1 and O E2: O E1 is preferred over O E2 even if the latter has a lower metric

When you shut the serial link you are changing the topology and area 246 is divided in two parts and so the O IA path is used.

As soon as the two parts of Area 246 are rejoined by enabling the serial link the O route is installed.

To be noticed it is the OSPF process itself here that makes this decision and then sends the updates to the IP route maintainer process.

So what you say is normal and you were pointing in the right direction : the different route types.

Hope to help

Giuseppe