CCIE Topic: 1.2a Administrative Distance

hey everyone i'm charles judd and welcome to the first video under the 1.2 section of the ccie blueprint dedicated to routing concepts the first item 1.2a is the topic of administrative distance administrative distance is a way that we can rate the trustworthiness of a routing source such as another router or a completely separate network so let's talk a bit more about what that means the main job of our routers is to choose the best path among all of the available options for our traffic and that's done by building and maintaining a routing table our network can use all sorts of different routing protocols be that ospf eigrp bgp and so on our routers will build a routing table by collecting information from those routing processes this can include directly connected networks or remote networks the packet forwarding process on our router will request information from this routing table in order to make packet forwarding decisions if we learn about a route from multiple routing protocols let's say we have the same route available via ospf and internal eigrp then the lowest administrative distance will be given preference so in that case eigrp with an ad of 90 would be preferred over the ad of 110 that we see with ospf here's a table that you definitely want to memorize it's just one of those many pieces of information that you're going to need to commit to memory these are the default administrative distance values and they start with an administrative distance of zero for a directly connected interface an administrative distance of one we can see is a statically configured route and then of course we start to see our dynamic protocol administrative distances and at the bottom we see an ad of 255 indicating that the route source is unknown and that this route will not be installed in the routing table again these are the default values but we can modify those administrative distances and why would we want to do that well for one if you're performing route redistribution which we'll take a look at in a future video this will let you select a routing protocol to take precedence regardless of the default a.d let's say you have both ospf and rip learned routes out to the same destination now by default ospf would be used because it has a lower administrative distance however by either lowering the ad of rip or by raising the ad of ospf you can force the router to use those rip learned routes instead of the ospf learned routes another common reason to do that is to use a static route as a backup to a dynamically learned route a static route as you can see has an administrative distance of one which is going to take precedence over any dynamic routing protocol by default however let's say we have a static route that we want to use as a backup for an ospf learned route we can raise the static routes ad to a value that is higher than ospf maybe we would raise that to 115 and then it would only be used if the ospf route is unavailable we call that a floating static route which is a static route that has a higher administrative distance than the value of one and we use those to back up another type of route let's look at this in action and see how we can change the ad values for some of our routing protocols here you can see the topology that we're going to use i have four routers and i have a switch acting as an access switch out to a host subnet of 192.168.0.0.24. r1 has its gig 0 0 interface running eigrp and that connects out to router 2 which is also running eigrp the gig zero slash one interface on router one is running ospf and that's connecting out to router 3 which is also running ospf so essentially what we have here is we have two different paths that we can take from r1 in order to get to r4 and eventually out to the 192.168.0.0.24 network so here on router 1 if we say show ip eigrp topology we can see that with eigrp the 192.168.0.0.24 subnet that's going to use the next hop address of 10.0.0.2 which is router 2. if we say show ip ospf rib then we can see our 192.168.0.0.24 network can be reached via 10.0.0.6 which is the next hop address on router 3. if we say show ip route on router 1 we can see our routing table and here at the very bottom we see our 192.168.0.0.24 network and at the beginning we see the code d listed if we look at our code table at the top this tells us that this route was learned via eigrp and additionally if we look again at the bottom this is telling us that we get to that network via 10.0.0.2 which is of course router 2. why is that well that's because the administrative distance for eigrp is lower than that of ospf and therefore it is the preferred path we can actually see that here as well on our two routes that we have listed via eigrp notice the administrative distance is listed as 90. for our ospf learned route the administrative distance is 110. those are both the default ad values for those particular protocols so this is behaving exactly as we would expect now what will happen if we install a static route since a static route has an administrative distance of one by default this should be the preferred route so let's actually try that and see let's go under global configuration mode and let's say i p route and we want to save the route to 192.168.0.0 with a 24-bit subnet mask we want to say the next hop address let's use router 3 which will be the next hop address 10.0.0.6 we'll hit enter let's break out and now let's say show ip route this time we'll notice the s code at the beginning letting us know that this is a static route and we see that this network is going to be reached now via 10.0.0.6 which is of course r3 we can also see the administrative distance value of one listed here as well that is the default value let's go back let's remove this static route entry by prepending the keyword no at the end of that if we break out and again say show iprout notice that our route to this network is back as it was we're back to normal again we're back to using eigrp via router 2 to get to our 192.168.0.0.24 network now i mentioned the concept also of a floating static route where we change the administrative distance so that our route will act as a backup route so let's take a brief look at that let's go under global configuration mode and let's arrow up to our original iprout command that we used to install a static route but this time if we look at contextual help you'll see that we're able to set a distance metric or in other words an administrative distance for this route and we can set that to a number between 1 and 255. one of course is the default and if we assign that the number 255 then it's not going to be installed in the routing table if we look at our output again notice that eigrp again has an ad of 90. ospf has an ad of 110. so what if we wanted to make this static route a backup route that takes over if eigrp goes down rather than using the ospf learned route then we could set our administrative distance to a value between 90 and 110 so in this case i'll just set that to 100 and i'm going to hit enter now let's break out of here let's say show run pipe to the section including ip route so that we can verify that this is in our configuration we do see that here we see our ip route our static route our floating static route in other words because we've changed the ad we've given that an administrative distance of 100. now if we say show iprout we're not going to see this route in our routing table and that's normal this route will not be installed into the routing table until our route with the lower administrative distance which is of course eigrp until that route is unavailable so let's jump over to router 2. let's go under interface gig 0 0 which is connecting to router 1 using eigrp now i'm just going to say shut to shut down this interface our eigrp neighborship is going to go down over to r1 and we're gonna see that message pop into the console and there we actually see that happen the neighbor is down at the moment so now if we say show iprout you'll notice that we're now using that static route and we're going to go over router 3 to reach our in device subnet we can see our administrative distance of 100 so this has taken over rather than allowing ospf to take over if we jump back to router 2 and say no shut to bring that interface back up we're going to see our eigrp neighborship reform and once we do that we'll again say show iprout if we say show iprout now things are again back to normal and we are using eigrp to reach that network and our floating static route has been removed from the routing table you should also know how to change the administrative distance of the protocols found within the ccie blueprint which are eigrp ospf and bgp now this is something we'll dive into more when we look at route redistribution route redistribution is where we would commonly see a case where we would want to change the default administrative distance values but i do want to briefly just show you how we do that from the command line if we go under global configuration mode we can go under our eigrp autonomous system number in my case i would say router eigrp one and once we're under there i would simply say distance eigrp and we follow that with two values and by the way this will change the administrative distance globally for eigrp the first number as you can see here from contextual help is the internal administrative distance which by default is 90. we would follow that with the external administrative distance which of course is the value 170 by default if we go back we can also do this with our ospf as well by calling out our ospf process id so in my case i would say router ospf 1 and again i want to say the distance keyword followed by ospf and if we look at contextual help notice we can split this out into external into enter area or intra area again ospf is a concept we'll examine more in depth later but just for now know that the default for all of these areas in ospf is 110 but you can change each of those individually here if you want to do that and finally let's just briefly look at bgp if we say router bgp one one of course being the autonomous system number and we can say distance bgp and if we look at contextual help you'll see the first number that we're able to change is the administrative distance for routes external to the autonomous system number the default value being 20 and we follow that with the administrative distance for routes internal to the autonomous system or in other words ibgp and the default for that is 200 and of course we've already looked at how we can alter the a d of a static route now there are also ways to change the bgp administrative distance for specific neighbors and we'll look at that closer at a later time so that's a look at administrative distance and how that's used in our routing table to determine the best path to a particular destination i hope you found this content useful and i want to thank you sincerely for watching

Channel: Charles Judd

Views: 1,146

Keywords: cisco, ccie, cisco enarsi, ccie enterprise infrastructure, cisco enarsi 300-410, cisco encor 350-401, ccie lab, my ccie journey, ccie training, ccie blueprint, section 1.2, routing concepts, 1.2a administrative distance, administrative distance, AD, static route, floating static route, 1.2 routing concepts, cisco router, cisco routing, routing table

Id: f9nvknJ56LU

Length: 13min 33sec (813 seconds)

Published: Fri Aug 21 2020