CCNP BSCI route-redistribution (why use it?)

Answered Question
Jan 14th, 2008

Hi,

I'm reviewing material for the BSCI exam and a question arises that I thought someone here might be able to answer.

Why is route redistribution so complex? Ie. Why wouldnt you simply connect to ASes together by having the border routers run both routing protocols? Wotn that automatically populate the route table with all the routes?

I understand it means more memory and CPU used up on storing networks and path calculation but is that the only reason?

TIA

Scott

Correct Answer by bwgray about 9 years 1 month ago

To consider your example as well, having the border router run both sets of protocols you have extra traffic going between the two routers as well, due to two protocols running. Both would run across the link if you have made them in similar AS's, but the catch in the end would be that it would only use the protocol with the lower metric for the routing anyway, assuming you don't get routing loops from what you've created. That said you would likely ditch one of the two routing protocols, but then would need to reconfigure one set of routers, to maklto make it all 1 protocol.

So that long winded example hopefully showed that yes it's more complex, but if done right, can save you from having to reconfigure a group of routers.

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fabio.mendes Tue, 01/15/2008 - 04:34

There are a number of answers for this question.

But the main one is that different routing protocols use different route metrics.

Hop counts are ok for RIP () but wouldn't for EIGRP (bandwidth and delay, by default). You need to balance these differencies between the protocols when you need to redistribute one into another.

In the end that's why redistribution is so complex.

bwgray Tue, 01/15/2008 - 08:05

Well there could be many reasons to use route redistribution. Say you had to connect 2 sites, that had 2 seperate networks that you wanted to connect together. Would you want to reconfigure 1 networks routing protocol - on every router - or on the ASBR just configure route redistribution to accoplish the same goal?

There are other considerations, but I think it's just another tool in the bag - quite a kewl one at that... =)

Correct Answer
bwgray Tue, 01/15/2008 - 11:42

To consider your example as well, having the border router run both sets of protocols you have extra traffic going between the two routers as well, due to two protocols running. Both would run across the link if you have made them in similar AS's, but the catch in the end would be that it would only use the protocol with the lower metric for the routing anyway, assuming you don't get routing loops from what you've created. That said you would likely ditch one of the two routing protocols, but then would need to reconfigure one set of routers, to maklto make it all 1 protocol.

So that long winded example hopefully showed that yes it's more complex, but if done right, can save you from having to reconfigure a group of routers.

bwgray Tue, 01/15/2008 - 11:44

To consider your example as well, having the border router run both sets of protocols you would have extra traffic going between the two routers as well, due to two protocols running. The catch in the end would be that it would only use the protocol with the lower metric for the routing anyway, assuming you don't get routing loops from what you've created. That said you would likely ditch one of the two routing protocols, but then would need to reconfigure one set of routers, to make it all 1 protocol.

So that long winded example hopefully showed that yes it's more complex, but if done right, can save you from having to reconfigure a group of routers.

Scott Cannon Tue, 01/15/2008 - 16:26

Thanks for the prompt responses guys.

I think my misunderstanding was that running to protocols on a border router will not propogate routes into the other routing process.

For some reason I had it in my head that if you have 2 protocols on a router connecting 2 networks each using different routing proctocols the border router could simply run both (which would enable it to add routes to its routing table) but it wouldnt pass them on to neighbors in the other network.

A moment of weakness for me brain I think.

A good point of clarification for the uses and potential pitfalls of rotue redistribution is http://classroom.internetworkexpert.com/p42549585

Thanks again and goodluck to anyone sitting this exam.

Regards

Scott

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