Route Redistribution Question

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Aug 28th, 2007
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I noticed something interesting as I'm going over all of my protocols for redistribution for the BSCI exam.

In a scenario where I'm redistributing OSPF into RIP, I configured the following:

1) One way distribution of OSPF into RIP

2) On the ASBR, I configured a default route of pointing back to the RIP AS, on the boundry router.

The thing I found interesting was this route is advertised in OSPF as E2 for external. Why is this? Why does OSPF pick up the route and run with it, and not rip as well? *I know the rip command is different, but just trying to make sure I'm clear on all of this.

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thomas.anthony Wed, 08/29/2007 - 00:25
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Route redistribution is the process of taking routes learned via one routing protocol and injecting those routes into another routing domain. (Static and connected routes can also be redistributed.) When a router running OSPF takes routes learned by another routing protocol and makes them available to the other OSPF-enabled routers it's communicating with, that router becomes an Autonomous System Border Router (ASBR).

as per you example where R1 is running both OSPF and RIP. R4 is in the same OSPF domain as R1, and we want R4 to learn the routes that R1 is learning via RIP. This means we have to perform route redistribution on the ASBR. The routes that are being redistributed from RIP into OSPF will appear as E2 routes on R4:

E2 is the default route type for routes learned via redistribution. The key with E2 routes is that the cost of these routes reflects only the cost of the path from the ASBR to the final destination the cost of the path from R4 to R1 is not reflected in this cost. (Remember that OSPF's metric for a path is referred to as "cost".)

In below example, we want the cost of the routes to reflect the entire path, not just the path between the ASBR and the destination network. To do so, the routes must be redistributed into OSPF as E1 routes on the ASBR, as shown here.

R1#conf t

Enter configuration commands, one per line. End with CNTL/Z.

R1(config)#router ospf 1

R1(config-router)#redistribute rip subnets metric-type 1

Now on R4, the routes appear as E1 routes and have a larger metric, since the entire path cost is now reflected in the routing table.

bwgray Wed, 08/29/2007 - 04:37
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Though I appreciate the response, it really was not my question.

What I was wondering why a static route configured on the ASBR, was being advertised as an E2 route in OSPF, since both OSPF & RIP have been configured on the ASBR.

Someone in another forum pointed to the fact that it's being advertised by OSPF as OSPF has a lower Administrative distance. But I'm still unclear why it would show as an E2 route in OSPF. I'll likely do the config over again to make sure I had everything set right...

mohammedmahmoud Wed, 08/29/2007 - 05:04
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When the static route is redistributed into OSPF it is redistributed as E2 by default (as any other redistribution into OSPF), if this wasn't the answer you are looking for please elaborate more.


Mohammed Mahmoud.

cyphur353 Wed, 08/29/2007 - 05:16
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Can you post up the appropriate protocol configurations from running-config, as well as the routing tables?

I think I know what you're asking, but want to be sure. This could clear up some ambiguity.

bwgray Wed, 08/29/2007 - 07:03
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I would but I can't seem to replicate the issue.....

At least I thought it did'nt make sense, so I must have had something extra configured in there....



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