OE1, OE2, ON1, and ON2 routing differences

Types of routes for OSPF:

1, O Domain routing

2. OIA Routing between domains

3, OE1 extraterritorial route, will accumulate metric value (default 20)

4, OE2 extraterritorial routing, not cumulative metric value (default 20), by the external redistribution in the default use of OE2.

5, ON1 and ON2 similar OE1 and OE2, by Nssa ASBR redistribute, NSSA area routers do not LSA5, with LSA7 calculated external route, marked as ON1/2

The difference between OE1 and OE2:

They represent external Route 1 and external Route 2, and the difference is whether to add internal routes (measure expenses). The default is OE2 is not the internal route, assuming that my network has only one export, then use OE1 and OE2 are the same;

A, if there is more than one ASBR to declare an external route to the same external as to use only the need to compare the extraterritorial overhead, just consider the external overhead is smaller, no need to consider the internal cost. So choose OE2 First.

B, single export (ASBR), the calculation of the cost of the domain is no longer meaningful, so the default OE2.

C, if we have only one exit then OE2 can help us solve all the problems, if we have multiple exits then we can use OE1, which allows us to make routing decisions in a more accurate way. Therefore, it is recommended to use OE1 for multiple exits.

The following article reproduced to deepen the impression:

If there is only one ASBR in an OSPF domain that can reach a particular external route (commonly known as a one-way redistribution), either the E1 or the E2 redirect will bring the same path selection, The difference between two types of external routes in this environment is only the metric seen on the routing table, and there is no difference. This is different, however, if there are multiple ASBR in an OSPF domain that can lead to the same external route (commonly known as a single-point multi-directional redistribution). This is the question to be added today. Look at a topology first.

The topology is that there are two asbr in an OSPF domain leading to the same eigrp as, both of which can reach the same external route. And for the path within the OSPF domain, the above path is a 100M link, so the cost is much lower than the 10M link below, which for the E1 type of external routing, it is obvious that the R6 will choose the path above to the EIGRP as, which we all understand. But for E2 external routes, things can change a bit.

Let's look at the first case:

Two redirects are made on the R2 and R3, and the EIGRP route is redirected to the internal OSPF. The two commands are exactly the same, and I use the following command:

Redistribute EIGRP metric Subnets

After using this command on two asbr, the information that appears in the routing table in R6 is as follows:

O E2 211.1.12.0 [110/100] via 211.1.46.4, 00:01:22, fastethernet0/0

O E2 211.1.13.0 [110/100] via 211.1.46.4, 00:01:22, fastethernet0/0

O E2 192.168.1.1 [110/100] via 211.1.46.4, 00:01:23, fastethernet0/0

All external routes are routed through the links above, including the network segment between R1 and R3. The reason is the same as what was said yesterday, because the metric used by the EIGRP redirect to the OSPF domain on the two asbr is the same, so the metric contained in the LSA from the above link or the following link is the same, This choice of path depends primarily on the forward metric decision within the OSPF.

R6#sh IP Route 192.168.1.1

Routing entry for 192.168.1.1/32

Known via "OSPF 1", distance, metric, type extern 2, forward metric 2

Last update from 211.1.46.4 on fastethernet0/0, 00:05:00 ago

Routing Descriptor Blocks:

* 211.1.46.4, from 192.168.2.2, 00:05:00 ago, via fastethernet0/0

Route metric is m, traffic share count is 1

The above command can see that the forward metric on the link above is 2, while the forward metric of the same LSA delivered from the link below should be 20, or two 10M links. This is obviously the same result as routing with the E1 type. However, if the metric on the two ASBR is different, the result will change.