Alright, so let's carry on with our introduction to Redistribution. So we've talked about the reasons you'd want to use Redistribution. Now let's just talk about some of the specifics of Redistribution. Redistribution normally goes both ways. Now that's one of those, well, well doh statements, of course, if I have OSPF and EIGRP, I want EIGRP routes in my OSPF and I want my OSPF in my EIGRP. That's you know logic and common sense, however, I do have to point it out because a lot of people will go into configure Redistribution which we'll do in our basic lab here as soon as we're done with the slide deck and they'll go into OSPF and they'll say Redistribute EIGRP with this Metric and there you go, I'm Redistributing EIGRP and then they'll wonder well why are my routes not going the other way? You have to configure your Redistribution to go both ways. Again, like I said, it normally goes both ways, there's no point in giving them routes so that they can get to you if you can't get back to them. If you can't see what networks they're advertising back to you, you know, the, the traffic will just go one way. Remember how that happened in our OSPF Lab. I could get to that router over there on the East but it couldn't get back to me and so therefore no traffic went through. Normally you'll Redistribute between two routing protocols, like in this little example I've got here, you'll Redistribute between EIGRP and OSPF. However, if you have the business need to, you can Redistribute between two EIGRP routing domains that happen to have different Autonomous System numbers. Going back to the example of the merger that I talked about on the last slide, Bank A and Bank B both run EIGRP because they're both an all Cisco Network, they're just running different Autonomous System numbers, so there's no easy way to merge those together, at least not quickly and so you'll set up Redistribution and in that case, since it is EIGRP, you'd probably just dump all the routes from you know one side of the network into the other and, and vice versa. Now on the subject of controlling what routes go into which routing domain, you can control Redistribution with Route Maps and Distribution Lists and we'll do that in our advanced lab. That's where it really gets cool in my opinion, where you can go in and say I want you to match just the slash 24 networks and I want you to Redistribute those networks into OSPF with this Tag and then further down the path, you can say oh and if you see networks coming from you with this Tag, then don't Redistribute them back into EIGRP. Again, I've got a really cool lab set up for the advanced section of this and, and so we'll, we'll get into that in a little bit. And as you may have figured out from my going on about Route Lists and what not, you have to use Route Maps and Distribution Lists and Tagging if you have more than one Redistribution Point or else you'll end up with a Routing Loop. And again I'll point out what I'm talking about when we get to our advanced lab, but basically if you have two routers that are doing Redistribution for Redundancy, you know, if routes come in from OSPF to EIGRP on one router, they go through EIGRP and the end up on the other router, well how is that router going to know to not Redistribute those same routes back into OSPF. It won't unless you specifically Tag those routes and tell these routers, don't Redistribute these routes if they've been Tagged. Let's talk a little bit about Administrative Distance. Now Administrative Distance is that first number that's listed in the cost portion in the Routing Table. You may have noticed when we've talked about EIGRP and OSPF or I've pointed out the Metric and the Administrative Distance, there's been another number in front of that cost or Administrative Distance, usually 110 or 90 or 170. That is the Administrative Distance and that is how you can manipulate what routes are preferred inside routing domains. It's a locally significant number meaning you can have one router say well I want RIP routes to be a lower Administrative Distance and Router B say I want those RIP routes to have a higher Administrative Distance and the long and short of it is, the Administrative Distance is how believable a particular routes is. Obviously the lower the Administrative Distance the more believable that route is, because it's closer to you, it's not as distant, that's the way I, I always remember that. And of course, it goes without saying, that different interior gateway protocols have different Administrative Distance values. For example, locally connected networks have an Administrative Distance of one, I'm always going to believe locally connected networks over any other routing protocol. If I have it directly connected to me and you're advertising it to me using any type of routing protocol, I'm just going to ignore you because your Administrative Distance is higher. Most routers will believe EIGRP over RIP because, you know, RIP is a protocol that's been around forever and if you set it up wrong you can advertise an entire Class A or Class B blocks of networks, where as with EIGRP it, it's kind of hard to do that and you've really got to not configure EIGRP correctly. Whereas with RIP, especially RIP version 1, by default it advertises using the Classful Network Boundaries, so you may not want to believe these RIP routes just because they're coming from a less reliable routing protocol. And again, you can manipulate these Administrative Distances to prefer specific routes over others and we'll see that in our advanced Redistribution lab. I've got a objective in this lab that says, you need to prefer the route from this particular router into this other router and, and again it makes more sense when you see it written out. I don't have the diagram in this slide deck and that is our basic introduction to Redistribution. Again, a lot of this will make more sense when we get into our basic lab where we're just setting up basic Redistribution, but at least now you can refer back to this slide deck for some of the terms that we'll be using.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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