Metric type in OSPF

[nguyenvancan110]

Hi everybody,

I have problem about metric type in OSPF. In some books it write as below:
"type 1 route is always preferred over a type 2 route for the same destination" 

I don't understand why type 1 is preferred over type 2. Can anybody explain help me?

Thank so much.

[eakdeniz83]

Hi Nguyenvancan110,

Routes are redistributed in OSPF as either type 1 (E1) routes or type 2 (E2) routes, with type 2 being the default.

A type 1 route has a metric that is the sum of the internal OSPF cost and the external redistributed cost.

A type 2 route has a metric equal only to the redistributed cost.

If routes are redistributed into OSPF as type 2 then every router in the OSPF domain will see the same cost to reach the external networks.

If routes are redistributed into OSPF as type 1, then the cost to reach the external networks could vary from router to router.

The order of preference for OSPF as per RFC 2328 is :

• intra-area routes, O

• interarea routes, O IA

• external routes type 1, O E1

• external routes type 2, O E2

This rule of preference cannot be changed.

I hope this helps.

[nguyenvancan110]

Hi eakdeniz83,

I understood this point. Your answer is very clearly.

Thank so much.