Detailed explanation of OSPF OE2 and OE1 external routes

External routes that are redistributed from the OSPF ASBR will generate two types of routes for OE1 and OE2 to be delivered in the OSPF area.

The type of Cisco default external route is OE2. The default metric=20 is not cumulative based on the cost of the link.

The OE1 must be manually specified before it can be passed within the OSPF area. In this document will be OE1 and OE2 through the experiment in detail, and then see where the difference, and finally there is a OE2 routing problem is forward metric, this knowledge point is rarely used, but need to understand. It's a high level knowledge point.

Well, here's a look at the difference between OE1 and OE2.

The following is an experimental topology: (The experimental configuration is not provided, here is the main principle, in view of this document at least need to understand the basic OSPF configuration)

In this topology, let's first introduce the situation. R1 in area 0 backbone region, R2 is ABR, respectively connected to R1 and R3,R3 in Zone 10, R4 is ASBR, and the network behind is EIGRP.

In such cases, if the configuration above the R4 is:

Note that the yellow configuration distributes the EIGRP 100 to OSPF 1, so R4 will spread to all areas of OSPF with the 5 LSA. Finally R1 will receive the relevant routing messages, and finally the routing table synchronization, R1 above show IP route can see the route to 172.16.0.0/16. (For the 1-5 categories of LSA experiment detailed please refer to http://361531.blog.51cto.com/351531/687142)

Finally, you can see the route 3 OE2 received above the R1.

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