CCNP BSCI 15

Most of the CCIE R+S examination focuses on Route Redistribution. Now, performing Route Redistribution already means that you're moving routes across different routing protocols (well, with the exception of static routes). Why would people use multiple routing protocols?
In the real world, most of the time it's due to political reasons. As companies get larger, more and more policies and politics would surface. Some divisions of organizations forbid the use of proprietary protocols. On the other hand, the rest of the division is alright with proprietary protocols. This leads to a split in network politics where different parts of a large organization use different protocols.

Other reasons include upgrading of routing protocols where parts of organizations convert in phases. Also, implementation of different vendors would lead to this, such as EIGRP for Cisco equipment and OSPF for the others.

Redistribution from a bird's eye view is very simple. You simply have to have a router running two protocols at once. That router then mixes the routes between the protocols (can be one or two ways).

However, it can become complicated when we talk about the characteristics of different protocols. What happens when we want to redistribute OSPF into RIP? What happens to the metric? In this case, we would have to translate the "cost" type of metric into a "hop count" type of metric. Typically people create a "seed" metric for all routes from the OSPF domain, which is done manually by estimation and proves to be unreliable.

Another problem we can get from this is routing loops. Refer to the following topology:


If we perform full redistribution at both links, the RIP routes would be sent to the OSPF domain then redistributed back into RIP.

We'll talk about fixing this in a future article. Right now, we'll deal with a very simple redistribution exercise:



R1 has 10.0.0-3.0/24 networks connected to it. R1 uses RIP for the link to R2, and the networks it's connected to. R2 uses RIP as well as OSPF on the link to R1 and R3. R3 and R4 are full OSPF routers.

The point-to-point links between routers use 172.16.0.0/30 networks from left to right.

Here's the configuration on R1:
router rip
no auto
ver 2
network 10.0.0.0

On R2, we have:
router rip
no auto
ver 2
network 172.16.0.0
router ospf 1
network 172.16.0.5 0.0.0.0 area 0

On R3, we have:
router ospf 1
network 172.16.0.6 0.0.0.0 area 0
network 172.16.0.9 0.0.0.0 area 0

Finally, on R4, we have:
router ospf 1
network 172.16.0.10 0.0.0.0 area 0

Now, R1 knows how to go up till the link between R2 and R3. R2 has full connectivity. R3 has only routes within OSPF.

Now, to redistribute OSPF into RIP, go to R2 and type:
router rip
redistribute ospf 1 metric 5

Now R1 would have all routes in both domains. It would appear as RIP routes. The direct RIP route from the link between R2 and R3 would be preferred to the redistributed one because of the metric. Pay attention to this as I would demonstrate how this would cause routing loops in future articles.

Now we'll redistribute RIP into OSPF from R2:
router ospf 1
redistribute rip subnets metric 1000

When redistributing into OSPF, we can also change the Metric Type to E1 by using the "metric-type" keyword. Recall that E1 routes will have the cost incremented through the OSPF domain, while E2 routes will not.

You can also tag routes using the "tag" keyword. This marks the route so that you would be able to check where it came from later on. This tag also allows you to manipulate the route later on using route filtering. This is what we'll use to prevent the routing loop described above.

When redistributing, you can also set a default-metric for all redistributed routes. This is done by typing:
default-metric 1800

This can be used in all protocols. The default-metric is overridden if you manually specify it in the redistribute statement.