External Routes and Metric Types 1 and 2

What it is

External routes are routes that come from outside OSPF, such as static, connected or other protocol routes, injected by an AS boundary router.

The ASBR takes these routes, gives them a metric, and floods them as external LSAs. The rest of the network learns them as OSPF routes and can use them like any other.

An external route has one of two metric types. They differ in how the cost to reach the destination is calculated.

How it works

The ASBR redistributes routes with a chosen metric type, and every other router adds up the cost in a way that depends on that type.

Redistribution in RouterOS v6

The instance has a setting for each source of routes. The value chooses the metric type, or turns redistribution off.

/routing ospf instance set [find default=yes] redistribute-connected=as-type-1 redistribute-static=as-type-2
/routing ospf instance set [find default=yes] originate-default=if-installed-as-type-2

The values are as-type-1, as-type-2 and no. There are similar settings for routes from RIP, BGP and other OSPF instances. A metric for the redistributed routes can also be set in the instance.

The originate-default setting makes the router advertise a default route into OSPF. The values are never, if-installed-as-type-1, if-installed-as-type-2, always-as-type-1 and always-as-type-2. With an if-installed value, it advertises the default route only while it has one of its own. With an always value, it advertises one regardless. The type in the value is the external metric type of that default route.

In RouterOS v7, redistribution is configured on the instance with a list of sources, and the details of the metric type differ. Check the v7 menu before relying on the v6 names.

External type 1

With type 1, the cost of the route is the external metric plus the internal OSPF cost to reach the ASBR. The cost grows as the route travels further from the ASBR.

This is the right choice when several ASBRs advertise the same destination and routers should choose the nearest exit.

External type 2

With type 2, the cost is the external metric only. The internal cost to the ASBR is not added, so every router sees the same cost for the route.

If two type 2 routes have the same external metric, the router compares the internal cost to the ASBR as a tie breaker. Type 2 suits a single exit point, where the path inside OSPF does not matter.

Preference between types

When a router learns the same prefix in different ways, it prefers them in a fixed order. Routes inside the area win, then routes from other areas, then external type 1, then external type 2.

This means a type 1 route beats a type 2 route to the same destination, whatever the metric values.

In this example the router picks ASBR 1 for a type 1 route, because the total is lower. With type 2, both would show a cost of 20, and the tie breaker would pick the nearer ASBR.

Where external routes do not go

External LSAs of type 5 are flooded across the whole domain, but not into stub areas. In an NSSA, the ASBR uses type 7 LSAs instead. These cases are covered in the page on stub and NSSA areas.

Verifying

/routing ospf lsa print
/ip route print

Look at the cost and the type shown for the route. The LSA list shows the external LSAs that the ASBR originated.