ASBRs advertise nonOSPF routes into OSPF domains so that they can pass through the OSPF routing domain. A router can function as an ASBR if one or more interfaces connects to a nonOSPF network, for example, RIP, BGP, or Exterior Gateway Protocol (EGP).
An ASBR imports external routes into the OSPF domain by using ASE LSAs (LSA type 5) originated by the ASBR.
ASE LSAs flood across area borders. When an ASBR imports external routes, it imports OSPF route information using external type 1 or type 2 metrics. The result is a four-level routing hierarchy, as shown in the following table, according to routing preference.
|
Level |
Description |
|---|---|
|
1 |
Intra-area routing |
|
2 |
Inter-area routing |
|
3 |
External type 1 metrics |
|
4 |
External type 2 metrics |
The use of these metrics results in a routing preference from most preferred to least preferred of
routing within an OSPF area
routing within the OSPF domain
routing within the OSPF domain and external routes with external type 1 metrics
routing within the OSPF domain and external routes with external type 2 metrics
For example, an ASBR can import RIP routes into OSPF with external type 1 metrics. Another ASBR can import Internet routes and advertise a default route with an external type 2 metric. This results in RIP-imported routes that have a higher preference than the Internet-imported default routes. In reality, BGP Internet routes must use external type 2 metrics, whereas RIP imported routes must use external type 1 metrics.
Routes imported into OSPF as external type 1 are from IGPs whose external metric is comparable to OSPF metrics. With external type 1 metrics, OSPF adds the internal cost of the ASBR to the external metric. EGPs, whose metric is not comparable to OSPF metrics, use external type 2 metrics. External type 2 metrics use only the internal OSPF cost to the ASBR in the routing decision.
To conserve resources, you can limit the number of ASBRs in your network or specifically control which routers perform as ASBRs to control traffic flow.
The following table explains the seven LSA types exchanged between areas. LSAs share link-state information among routers. LSAs typically contain information about the router and its neighbors. OSPF generates LSAs periodically to ensure connectivity or after a change in state of a router or link (that is, up or down).
|
LSA type |
Description |
Area of distribution |
|---|---|---|
|
1 |
A router originates type 1 LSAs (router LSAs) to describe its set of active interfaces and neighbors. |
Passed only within the same area |
|
2 |
Type 2 LSAs (network LSAs) describe a network segment such as broadcast or NBMA. In a broadcast network, the DR originates network LSAs. |
Passed only within the same area |
|
3 |
The ABR originates type 3 LSAs (network-summary LSAs) to describe the networks within an area. |
Passed between areas |
|
4 |
Type 4 LSAs (ASBR-summary LSAs) advertise the location of the ASBRs from area to area. |
Passed between areas |
|
5 |
Type 5 LSAs (ASE LSAs) describe networks outside of the OSPF domain. The ASBR originates type 5 LSAs. In stub and NSSA areas, a single default route replaces type 5 LSA routes. |
Passed between areas |
|
6 |
Type 6 LSAs (group membership LSAs) identify the location of multicast group members in multicast OSPF. |
Passed between areas |
|
7 |
Type 7 LSAs import external routes in OSPF NSSAs. |
Translated between areas |