607 BGP path selection process

[Music] pgp path selection process now in this video we'll see what is the path selection process in bgp like if you remember we we have seen some of the igp protocols like ospf ehrp and rip every routing protocol is having its own best path manipulation like ospf always consider the bandwidth whichever the root is having the least band with that root will be considered as the best route whereas ehrp is going to see bandwidth delay load md reliability whereas rap will count the number of hop counts and whichever the route is having the least half count that's true is considered as the best and in case if they come across a scenario where both the routes are having the equal metric or equal cost then it is automatically going to do load balancing but when we compare the bgp bgp is totally different when we compare with your igp protocols now bgp is the protocol it's a forwarding bgp forwarding table usually when it is having multiple paths it is going to choose any one anyone as the best part now the best part is based on the policy or a step by step procedure of shaking it's not at all based on the bandwidth like we have in igb so this is a default pass legendary so let's try to understand what is the default pass selection process but before that uh you need to have a very good understanding of different attributes now likewise discuss that bgp is going to consider the attributes which is going to define the policy which route should be preferred over the other route now in that we have seen lot of attributes like weight attribute and local preference attribute so these two attributes will define how to exit the is so whichever the root is having the higher weight or higher local preference that route will be considered as a pistol now let's try to understand what is the default path selection process in bgp when you have multiple attributes or multiple values are applied to ones any any specific parts now when when it comes to selection process in bgp ensuring that all the routes are synchronized with no loops and all the routes are having with a valid next hop valid next stop means the next stop is reachable now we are going to consider by default it is going to prefer the highest rate now it will consider the weight local preference yes but anyway we have many options we have which will decide the best route but when when it comes to preference the weight will be highest preferred than any other route so which means if we if we say let's take an example i have a router and it is having two exit paths outside our a's and one root is having the weight of 1000 and the other route is having the weight of just 200 now when it comes to pass region process our router will always prefer this route this route is much better than this route because of higher weight so even if you apply the local preference let's say if i apply the local preference of 500 here and the local preference of this route is just 100 now even though we have two or two things applied like we have a local preference applied as well as we have a weight and according to weight this is the best route the first one and according to local preference this is the best route but the router or the bgp has to listen to anyone so by default it is going to listen to weight so it is local to the router and it is the highest preferred than any other attribute so that's what when you come across this type of scenarios where you have two two or more than two attributes applied it's going to choose any one of the attribute based on this path selection process so it is highest prefer than any other attribute but let's say in case if you don't have a weight let's say i don't have a weight and i got two multiple parts again now this time this route is having a local preference value of 200 and i have a local preference value of 100 on this side and on both the sides there's no weight applied so when there is no weight applied means the default weight for any network which is not directly connected will be the default weight will be zero so in case if there is a tie in the weight or if there is no weight is applied so when there is no it is applied means it will be by default zero in that case then only it is going to prefer the local preference now as per this example this will be considered as your best route and the reason is because the weight is same and it is going to decide the best route based on the local preference values now let's take an example in case if you don't have a weight or local preference in case if weight is same which is zero by default for any non directly kind of network and the default local preference value is 100 and if it is same on both the sides now in both the cases the weight and local preference are same on both the sides it is same let's say this is 0 and the local reference default is 100 and both are same now in that case it is going to see prefer the route originated by the local router which means it will see if this route is originated by the local router let's say there is a network called one dot network so this is my one dot network which is connecting between our router one and router two and to reach this one dot network the other end this is the default best rule now even you can go from alternate route which is not directly connected now by default it will prefer the directly connected route so now you can see here even weight and local preference are more preferred than directly connected in case if i apply the weight of 40 000 here and the weight of directly connected network will be 32768 now the router will decide the best route based on the weight rather than going directly connected so you can see that's the reason that's the impact of a weight and the local preference so they are more preferred than directly get a network now if if there are two multiple routes one is rightly connected and the other one is not coming directly and if both the sides weight is same let's say the weight is same and also the local preference is also same then it is going to prefer the directly connected route so the next thing it is going to consider when it is when it comes to the best pass selection process now in case if there is no weight applied or if the weight is same and the local preference is also same and both the sides the route is received but they are not directly connected in that case it is going to prefer the next condition is autonomous system path now again i am going to take the similar example i got two roots in both the sides i got the weight is same local preference is also same on both the sides and also both the sides there is one network called ten dot network which is learning from different autonomous system members and this network is also coming from this side now first it will decide based on the weight now in our example there is no weight applied or if it is applied it can be same local preference is also same and this standard network is not directly connected network of router 1 it's somewhere advertised by other router into bgp now in that case it is going to decide the best route based on the autonomous system path information which means let's say the route is advertised from this side with air's hop of 500 600 and reaching to my s and then it is advertised from the other end with with some other differences let's say 700 800 900 and i now the router one is receiving this standard network from both the ends and there is no weight applied there is no local preference and this network is not directly kind of network it is going to decide the factor based on autonomous system path attribute now it's going to see which route is having the less number of autonomous systems now in this case it is 2 and in this case it is 3 now the default best path will be this one because of least autonomous system path so that's the reason when we say bgp is going to calculate the best route based on the autonomous system path information why because the default weight will be the same or whatever zero for any down directly kind of network and the default local preference will be always 100 and maybe the route is not directly connected so definitely it will go in most cases the best truth will be calculated based on as path information now you may come across a scenario where you may have the same autonomous system paths let's take an example i'm going to consider the same network if i'm receiving the same route from both the ends with the same as hop let's say this side i'm receiving with 700 800 and i from this side and i'm also receiving the same network from this side with two asops now in this case the weight is same logo preference is same and it's not directly kind of network so probably the next part will see as path is also same then it is going to decide the best route based on based on the origin code now based on the origin means now in some cases you may you may oh you may see the routes now this standard network is going to advise from this side and it is doing redistribution let's say so from this side i'm receiving the route to redistribution and then from this side we are going to receive that out from the other end as i so if in case if weight is same logo preference and as path is same then it is going to see the origin code now by default it is going to prefer i which is always the best when you say i means it is the network which is advised in bgp more prefer than e more preferred than question mark so when the route is coming with bgp network command and also coming through the distribution from the other end it always prefer the route which is learned with this origin code i okay so that is the next step which it will see when it decides the best rule after that in case if the original code is also same so weight is same logo preference is same and the route is not directly connected and both the routes having the same autonomous system paths and let's say both the routes are learned through igp or learn advertised in igp so in that case in both the sites if you say both are coming as i i and also same as hops then it is going to see something called med value md now md is one of the attribute which is used in other words we call it as metric now it is little bit opposite to your local preference or weight loco preference and weight are going to decide how to exit the is whereas metric value is used to define how to enter the is so it's going to impact on the remote as in general so the default made value will be always zero and we are we are just assuming that we have the default made value of zero so which is a multi-exit discriminator exchange between autonomous system numbers now in case if the mid value is also same the metric value is also same which is default zero then it is going to see the nearest neighbor type now assuming that all the above six steps are same then it will decide the best route based on administrative distance or the neighbor type we can say now by default it is going to prefer the route coming from ebgp when compare over ibgp so which means in our scenario let's take an example i'm getting a route so my router is receiving route from this side this router which is part of the same is and also i'm receiving that out from another router which is in different days this is my es500 and this is my a600 so now the same network whatever the network that is standard network is getting advertised from this site as well as getting advertised from this side now my router is going to receive the router one receiving from the same nintendo network from both the ends now assuming that all the six above steps are same like the weight look preference as path origin code memory value now it is going to consider the best route the best route will be calculated based on now this is your ebgpp and this is your ipgp so now it is going to install the route coming from epgp because of administered distance 20 and from administrator distance 200 so ebgp will be more preferred route now the router 1 is going to use this route to reach dental network because of eb gpp so it's going to prefer the route coming from ebgp rather than preferring the route coming from ibgp so that is the next step it is it is going to see when it decides the best route now in case if both the routes are coming from the same type of neighbor let's say both the routes are coming from either ebgp or either ibgp so if both the routes are coming from ebgp let's say both are in different days then again there is a tie in this condition also now in that scenario it is going to see the nearest igp metric the smaller value is preferred to the next hop and if both are both are having the same metric in case which is very rare generally then it's going to prefer the oldest route for ibgp ebgp paths whichever is the oldest route in the bgp paths and if both are equally old then it is going to prefer the lowest neighbor id router id and if both routes are coming from the same neighbor then it is going to prefer the lowest neighbor ip address so this is the default route selection process but bgp will not do load balancing by default and it is going to use any one specific route based on this default pass selection process now at least you should you should understand the six to seven steps at least you have to memorize the default translation process in order to figure out which path is used by the bgp now to remember we can use some simple shortcuts from cisco dock we can say here now generally this is the shortcut which i which are generally used to memorize the pass rating process and and n says that it says valid next hop then we say wlla w is weight local preference the local draft and autonomous system path the four the four values and in case if all these four values are same then it's going to use something called omni origin met neighbor type ebgp preferred over ib gp and then the nearest igp metric now you can you can use any other keywords as well to memorize these things but it's it's really good to know and understand at the same time you have to memorize these things so whenever you see bgp is bgp table when you say show ipbgpr show ip route bgp you need to figure out which route is calculated as a best route and based on these values we can decide you

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Id: gu6Wb3NXzoI

Length: 16min 10sec (970 seconds)

Published: Sat Aug 08 2020