Episode 24 - Understanding BGP MED Attribute - Journey to CCIE

there are a couple of different bgp attributes that are really confusing to me and i want to show you what i learned about well bgp med join me next [Music] before we jump right into it make sure of course you subscribe to our youtube channel as well as hit the bell for those notifications so you can see more of these videos now as we get started here in terms of learning about med this was something that absolutely confused me as i'm going back now and reviewing everything for bgp i'm coming across of course the asp path attribute there as well as the idea of med to help us to decide what is the best path selection that we can have so as i configure them both i realized that they do just about the same thing in terms of the results but the only thing that really seemed different is that i could actually see that the med was limited in terms of what it would actually influence which was the next neighboring autonomous system but the autonomous system path itself could actually go beyond that next autonomous system as well otherwise it could influence more than that too so i was very confused about which one to actually choose in what particular situation till it finally clicked on me as i continued to actually run over both of them and tried to actually distinguish what the difference it was between them now don't get me wrong there's a lot more differences but this is the key for me when it finally came down to it let's take a look at my diagram and this will kind of help us out so when it comes down to the way that the as path ended up working for us in terms of how we'd actually do pre-pending and to be able to do that well the key here is when we're actually trying to influence that we're actually working with multiple autonomous systems so we take as65123 if i'm actually choosing which path i want to influence to get access into my autonomous system here i can go from router 2 or router 3 but notice that what they're connected to is going to be as65004 or as65005 well that is a perfect situation if we're going to use the idea here of the as path itself and prepending in what we're going to do so i finally figured that out after a couple of rounds of actually trying to do both them and seeing what the real difference was now what does that mean that the bgp med or multi-exit discriminator does is this instead over on the right hand side i have router 3 and router 5 and both of them are actually connected to the same exiting as number which is 65 right so when we actually take a look there's only two autonomous systems involved there so if i'm actually saying that from as65005 i want it to actually take well the path on the left which is the dot 35.0 or i can say i want to take the path on the right which is the 53.0 well i'm actually going to use the med value to help the inside of 65005 to choose which of those two that i want to come in to my own autonomous system so that's the real big difference in making that work when it comes down to it so make sure you understand that as you actually get ready and as you continue to prepare for your lab i know that those of you that have been working with it on a daily basis you understand that but if you're like me and it's been a while since you've actually gone over this this was kind of one of those eye-opening aha moments that really kind of at least you know made me realize that this is what's going on now let's take a look at how we're actually going to configure this to make this understand or to help us to understand this a little bit better so if i go over to router 5 we can take a look here and we can see for example here is a route inside of autonomous system or 65123 so that's actually perfectly fine for us and the one that we're really focusing on is going to be right here notice where the path actually only goes to 65 123 whereas that last one it actually hops around the other way instead but you can see in terms of the next hop one got chosen 60 uh the uh 1 192 168 35.3 and then the other one which is right beside it is 192 168 53.3 that one did not get chosen as valid and best well what if we want to use the 53.3 as being the valid and best route well the med value of course is actually going to be one that is based on the lowest med value that we can have as well so you can do this in a couple of different ways i went ahead and i configured some route maps to help us out do show route maps oops oh not right there it's on router 3. so that doesn't help show route map all right so here's the two route maps that i actually ended up making i made one and i called it med b i made another one i called med a and i went ahead and set a metric for everything 1 of course to 350 and the other one to 700 and we're actually going to get the lowest one if i remember correctly here as we do so so now that i've got those created i now need to assign it to neighbor statements that will of course make that work for us so let's go ahead and do that as well so we're going to go to our global configuration and then into our router configuration and this is going to be 65123 for our bgp then for our neighbor 192.168. this is going to be 35.55 i'm going to do route map and this one i'm going to set one to the med a because this is the one that's already set to accept it so i'm actually going to do med a at this particular point then i'm telling it to advertise it out of my own autonomous system number here then we're going to actually kind of change this up we're going to do 53 and this one we're going to change to what med b so notice that should assign the different metric or yeah med values that we actually have to those neighborships and that should be perfectly fine for us if i actually set it up correctly the one that i wanted should be fine and then we're going to clear ipbgp and i'm going to do a soft and that should kind of do everything that we need it to so at this particular point if everything is actually kind of working the way that we hope that means i should be able to go to router 5 and router 5 is the one that should show me that this has actually taken place and is different since that's what the med is doing is saying hey here's how you actually get access into me as well so let's go over router five and so we see the preview before so this is the original one that i showed you and notice that the 35.5 that is the best and valid path at this point with nothing else actually kind of being set now if we run the same command again let me just do a show pbgp and now you can see the difference here so as i highlight this notice now that the 53.3 has now been selected as the valid and best path and we can see the metric that i set right there as well and that helps us to be able to understand this so even though this might not be the most exciting video that you've seen it is one that was an aha moment for me and hopefully it will also stick for you so make sure you continue to stay tuned for this ccie journey and i'll see you on the very next video

Channel: ITProTV

Views: 571

Keywords: bgp med, cisco bgp med configuration example, cisco bgp med, cisco ccie med, ccie med explained, cisco med attribute, ccie med attribute, understanding bgp med attribute, what is mgp med, Multi Exit Discriminator, cisco multi exit, ccie multi exit process, ccie multi exit discriminator, cisco multi exit discriminator, multi exit discriminator bgp

Id: VQjPQZt_wBU

Length: 8min 40sec (520 seconds)

Published: Sun Jun 13 2021