2. Troubleshooting Admin Distance (AD) - Cisco ENARSI (300-410)

troubleshooting administrative distance well what in the world could go wrong with them find out next right here on itu pro tv you're watching it all right we are actually back and we are taking a look at troubleshooting administrative distance as i actually think about it i wonder what in the world could actually go wrong with them here to help us out it's gonna be anthony sakura all right anthony i kind of hinted that something could go wrong with administrative distance is that actually true yeah it is it is true and and you know before we dive into troubleshooting scenarios that might involve administrative distance which i will say tend to be rare let's just back up a moment here and let's just remind ourselves of what really is going on when we talk about administrative distance first of all i'm gonna put ronnie on the spot here i'm gonna ask ronnie for how he likes to define in you know plain english how he likes to define administrative distance for his ccna students ronnie how do you like to describe this all right so anthony the way that i've always described the idea of administrative distance is as a way to rank the routing protocols that the router's actually going to end up choosing so that's how i like to define it the lower the ranking the better it's actually going to to use that particular routing protocol wow okay i like it and i i knew this would happen it seems like every time i ask an instructor uh this they have a slightly different way it's described i really really like ronnie's it's a ranking yeah it's a it's a it's a ranking assigned by engineers of cisco and it's the believability of the routing protocol that's a another descriptor i hear often that i love the believability ranking of the routing protocol now i said that it's not all that common that we run into this and you're probably understanding one of the reasons why right away and that is in a lot of designs today we have an area of the enterprise network that is running a single routing protocol so its chances of learning about a prefix from two different dynamic routing protocols are absolutely zero since it is running a single routing protocol there is one huge exception and we'll be taking a look at it in this episode as a matter of fact and that is when you're running like an igp and bgp well that's certainly a case where you have two dynamic riding protocols running yeah now anthony i was about to ask that question like when do we actually see those scenarios but that's a great scenario what about another possibility here and you just tell me whether i'm right or wrong with this what if i'm actually transitioning from let's say the idea here of actually moving from uh eigrp transitioning over into another interior gateway protocol like ospf is that when i'd also see mixed uh administrative distances as well you better believe it ronnie that is by far the most common scenario where we have a couple of things that we are doing that we don't normally do not only are we tending to run multiple routing protocols as we're doing some kind of technology migration when it comes to the layer 3 routing protocol but also we tend to do redistribution in those scenarios and administrative distance can come into play big time when we are doing redistribution now i know redistribution actually scares a lot of students they don't really understand it it gets complicated real quickly for them and they're afraid of it because they have heard horror stories of layer 3 infrastructures being brought to their knees because of a routing loop in redistribution one of the episodes here in nrc that you can definitely look forward to is our episode where we'll tackle redistribution and i think you'll come away from that experience loving it well what we've put in the show notes for you is a a nice look at the administrative distances of the various routing protocols from cisco systems and by the way you will notice that static routes are part of this too we haven't mentioned that and it's no surprise that a static route gets the second best administrative distance of all the ad values and that is a value of one so when you have a routing protocol like ospf and it's got an ad of 110 and you have a static route with an i uh a d of one it overrides and the static route would go into the routing table and be used instead of the ospf prefix and when you think about it that's exactly how we would want it to work right you as the god-like administrator you walk up to that device and you say i don't care what routing protocol you're using fancy pants and then you go ahead and you configure a static route with the static route having an ad of one it's certainly going to win out over the dynamic routing protocol so again we've given you the list of the default ad values in the show resources and you really need to make flash cards on these ad values if you have not already and pretty much commit them to memory i know all the big ones like ospf and eigrp and eigrp external and ibgp and ebgp do i remember that rip is 120 uh i did just then but i definitely have gone through periods of my life where someone had to remind me what the heck it was and the reason why of course the ad for rip starts to slip from my memory banks is who the heck uses rip anymore so as protocols are no longer seen that much of course you can start to safely forget the ad value but ronnie let's do a little fun here with this let's go to my command line because i have set up for us i this is really fun i just went into viral and i just like spun up five routers real quick and i said viral go ahead and pre-configure for us all of the igp and egp all of the interior gateway protocol stuff and automatically configure the exterior gateway protocol stuff with bgp and knock yourself out i just want full connectivity well when it was done it created kind of an interesting situation let's look at the bgp table on this r1 device and look at this we actually saw this when we were all working together in the encore show here at it pro tv but when we look at this we see that there's these rib failures these routing information based failures what this means is these prefixes are in the bgp topology table but four of them did not make it into the routing table something beat them or some reason for some reason the prefixes aren't in the routing table and let's just confirm this with show iprout and do a filter for bgp we can see there is no bgp prefix information in the routing table and you might say well wait a minute anthony there was one of the prefixes that was considered valid and the best path where is it in the routing table well look at that address that's the loopback on this machine so if i do a show iprout without the bgp filter i'm going to see the 19216801 and it's in the routing table as a connected prefix and guess what the ad is of connected prefixes zero it's zero you got it roddy yeah zero you talk about believability when something's directly attached to you it's very believable imagine that so let's go ahead and have some fun with this by the way first of all is the rib failures is that a problem well i mean in some designs it wouldn't be in fact you would prefer it this way what's happening right now well those prefixes that we saw had the rib failure those are learned on this device using ibgp peerings so those have a really crappy and that's the technical term crappy administrative distance of 200. so ospf for those prefixes with its ad of 110 won that battle but these prefixes are just waiting in the wings if you will and if there was a problem with ospf they would be injected into the routing table and used so this isn't necessarily a bad thing and by the way not to get us too sidetracked here on an exploration of bgp because that's for later episodes here in nrc but i did want to point this out there is a show ip bgp rib failure command that will tell you whether or not it's administrative distance that's causing your rib failure the rib failure could technically be something else okay so what if we wanted to manipulate a d well the first tip i have for you is don't forget it's a router by router type configuration so whatever you're going to do on this router you'd want to do to all of the other routers in your autonomous system but what we'll do is just have some fun with this we'll go into bgp router bgp and since viral spun up this topology for me with the defaults they're not very creative on as numbers so it's as1 for bgp here and i'm just gonna say distance uh let's see why isn't it taking that command uh router bgp1 oh i know why it's not taking the command this is hilarious uh viral likes to do the bgp configuration using the address family configuration mode so no problem let's do this let's do a show run and let's begin with router bgp so we can see the layout of that address family configuration mode and notice that we have our bgp just the straight up router bgp section with our neighbors and then we have to go into the address family for ipv4 in order to activate those neighbors and make configuration changes so i'll do router bgp1 address family ipv4 and now i'll say distance and we'll use context sensitive help when we're manipulating ad because that's going to vary the exact syntax based on the protocol you're working with i'll say bgp and we will make the distance for eb gp learn prefixes the exact same we'll keep that at 20. we don't want to mess with that what we want to mess with is the distance for routes internal to the as and i'll make it one click better than ospf so it wins that battle and then there's the distance for local routes that you would be generating for instance with the network command in bgp and i'll set those to 109 as well we'll end the configuration and we'll do a show ipbgp and we'll see uh oh nothing worked well be careful here when we are working with border gateway protocol as ronnie is going to actually explain to us beautifully in a primer that he did for us to get us ready for an rc if we haven't worked with bgp in a while bgp just isn't great at fast convergence it's not what it's built for so remember if you're going to look for your command to take effect right away with bgp you might be in for some disappointment there is some convergence delay here now anthony you've actually manipulated the administrative distance now as far back as i can remember i was always remembering that we don't really want to manipulate administrative distance if we don't have to but where is a context that we would choose to do something like this or is this just for a lab demonstration this is uh this is just kind of an artificial scenario where we're saying you know what we want the ibgp prefixes to win out and we'll have the ospf prefixes be backups i mean that could be something we want to do though that i mean i say it's artificial but it is legitimate potentially um but ronnie i'll tell you where we really see ad used is in complex redistribution scenarios and those aren't all that common right so very complex redistribution scenarios though for large enterprises that are doing some kind of complex migration or maybe the complex integration with another network so they're going to be maintaining their own asses but they need to share prefix information ad really could enter into the equation and i know you ronnie you're such a and it's why one of the reasons you're so uh so esteemed in this industry of networking you are so logical when it comes to approaching problems i know one of the things you look at this and you go hey wait a minute if you don't want the ospf prefixes don't advertise those loopbacks at ospf and absolutely i mean there are common sense approaches to this but let's see by the way ronnie we've been having a conversation here that's lasted a couple of minutes so let me turn back to the console and nothing up my sleeve we're going to run show ipbgp again and look at that we still have not had any recalculation of the ad so i can definitely speed things along i will say let's refresh the routing table information on this device i'm thinking that having the router just take all of the prefixes and i wouldn't do this in production for obvious reasons but right here we don't have time to wait um if i do a clear ip route asterisk and have this device purge that routing table it's certainly going to need to go through all of its all of its intelligence to rebuild it i'm thinking that will do the trick so let's take a look now at the show ipb show ip bgp command once again and yeah look at that it did work so now we have all of these prefixes marked as valid and best and that should have impacted the routing table if i do a show ip route i see yeah it's bringing in as bgp prefixes those loopbacks where it can reach the next hop where it has you know all of the parameters that are uh necessary for the prefix to actually be injected into the routing table we can see in this example it's just the 19216802 but you get the idea this was not a bgp learned prefix before it now most definitely is and that's a result of our manipulation of the ad and if you don't believe me ronnie we can i know you believe me ronnie but maybe one of our students doesn't and um just look at that right there look at that bizarre a.d that's jumping out at us of 109 associated with that prefix so ronnie manipulating the default administrative distances it is not difficult it's just going to vary the syntax from machine to machine so be ready to use your context-sensitive help and the other thing i would say is with the ad is always remind yourself of what it's there for it is going to break ties when your router knows how to get to a specific prefix from multiple different sources now think about what i just said it's the specific prefix right so when that router your router is like oh wow i have a static route telling me to get to this destination an ospf route a bgp route maybe even a fourth source right maybe we're even learning it via isis what is the router going to do to break that tie well administrative distance comes to the rescue all right anthony well thank you again for helping us to review and understand more about the nature of the administrative distance that we may have to troubleshoot and also showing us how to manipulate it if we actually need to now anthony also did mention that we'll include in of course the show resources are our learner resources here a table that will actually help you to be able to memorize all these if you haven't done it already well that's going to be enough for this picker episode so signing off for it pro tv i'm your host ronnie wong and i'm anthony sakura stay tuned right here for more of your cisco and narcissi show thank you for watching i.t pro tv

Channel: Coral Cert

Views: 35

Keywords: Networking, Help Desk, System Administrator, Cisco, Cybrary, CyberCourses, Coral Cert, CCNP, ENARSI, Admin Distance, Cisco ENARSI (300-410)

Id: umWyAa8ouuQ

Length: 19min 15sec (1155 seconds)

Published: Mon May 17 2021