Parallel Countersignature Routing Made Easy

Parallel countersignature routing

hey YouTube welcome to another video from practical networking net this video is going to explain Route precedents and exactly how a router chooses its best path if multiple paths exist here's the topology we'll be using will be doing most of our work on r1 and will be mostly focused on the 9.99 at 128 / 25 network behind r6 r6 is going to advertise the ninth network via OSPF to r4 and r5 via edgy repeated r3 and via rip to r2 we're then going to progressively for those advertisements to r1 to show you exactly what our 1 is doing at each step to determine its preferred path to the ninth network to begin we're gonna have our 4 and r5 summarize the / 25 network that's a slash 24 to r1 now why we're summarizing it as a slash 24 will make more sense later on in this video that said this video is not about route summarization so we're not going to go into the details of how that is being done in fact the configuration of every router has already been completed except for r1 so let's go ahead an able oh s BF let r1 on the links between r1 and r4 and between R 1 and R 5 now before I do that take a look at our ones routing table you'll notice that at the current time there are no entries for the 9s Network at this moment our one has zero paths to get to the nines network we're gonna go ahead and enable OSPF on r1 on the link between router 1 and router for and we'll do the same for adder 1 and writer 5 you'll notice as soon as I do that the neighbor ship for router for and router 5 completes and before we had 0 routes to the nines Network now if I do my show IP route command again you'll see we currently have two routes to get to the ninth Network okay so there are three attributes that are compared when determining route precedence and right now they are all tied before we show you those three things I want to show you what a router does when all three of those attributes are identical first I'm going to show you a traceroute to a network that the router only has one path to I'm going to trace route to the IP address 10.5 dot 6.6 which is router 6 as IP address on the link between router 500 or 6 this is what a normal trace route looks like when we have one path you'll see the first hop on the first line and you'll see that 3 packets were sent each taking a different amount of time take him to come back now I want to show you a trace route to a path that the router has multiple identical paths to and we'll show you how it's different notice the router still sent three packets but something pretty cool is happening each of those packets had a different first hop the first one went through router for the next one we went through outer v and the next one went through writer for what's going on is when the router has multiple identical paths to get to a target network the router is actually load balancing across those two paths right now r1 has two paths to get to the nines network one v r4 + 1 VAR v what r1 is doing is it's sending one packet this way via r4 in the next packet this way via r5 and the next packet this way via r4 and another packet this way through r5 you'll see that it's actually load balancing across both of those paths this is known as equal cost multi path and this is what happens when the router has multiple paths with identical attributes to determine route precedence now this video is not about equal cost multi path so we're going to do is tweak some of the values that are involved in determining route precedence first let's take a look at our show IP route again and you'll see this long output is the output of show IP route throughout this video we're primarily gonna be concern about those two lines so we're actually using a different command than this one we're going to do show IP route but we're going to tell the router to only show us whatever routes match the 9.9 at 0 / 24 network or longer prefixes you'll notice these two lines show up right here but this way we can use this command to show us simpler output for we're looking for okay so with that said let's talk about the three attributes for route precedence we're going to talk through them backwards so the last thing that router looks at when determining route precedence is the metric the metric is actually visible in the show IP route command that's this second number in the brackets over here both of these routes learn by OSPF have a metric of 30 now the way OSPF calculates the metric is as an additive value based upon each link speed to the target network in our topology all the links are the same speed and have an OSPF cost of 10 that 30 is coming from 10 plus 10 plus 10 for the path to r4 or 10 plus 10 plus 10 through the pass through r5 you can see this more clearly if you look at the route in detail in the US PF database I can use the command show IP ospf database summary notice we have two paths to the ninth Network learn the OSPF one from our four and one from our five both of them have a metric of 20 that's because for r4 and r5 to get to the ninth network it'll cost them 20 our one then adds the local cost of 10 for its local interface to get a total of 30 in our routing table so let's go ahead and tweak the cost on the link between r5 and r6 to show you this attributes effect on route precedence will jump on router 5 and we'll jump on the link between router 5 and router 6 and we'll change the IP ospf cost to 15 instead of the default of 10 before we had two paths the nines network both costing 31 via r5 and one OVR for if I do my show IP route command again you'll see that now we only have one path to get to the nines work it's this path right here which remains unchanged what we did was changed this path to make it cost 35 and between a cost of 35 and a cost of 30 our one preferred the cost of 30 and therefore this is now the only path that r1 will use to get to the ninth Network you'll notice now a first hop for all three paths were router for our one still knows of the second path if I do my show IP ospf database summary command again you'll see that our four still has still advertised the path to the ninth network with a cost of 20 and r5 still advertised the path this time with a cost of 25 because of how we tweak the metric on the link between r5 and r6 when router one got this route it added 10 its local interface cost and win router 1 got this route it also added 10 giving us some interest a total metric of 30 through our 4 or 35 to our 5 and between those the one that it preferred is the 1 through r4 with a metric of 30 so r1 currently has two paths to get to the ninth Network one costing 30 through our 4 and the other one costing 35 through our 5 and between the two of them it preferred the path through r4 because that is a lower metric the next thing that the router looks at for route precedence is was known as the administrative distance the administrative distance is an arbitrary value associated with each method of learning routes the administrative distance is actually visible in the routing table that's the other number in this bracket you'll notice that the OSPF has an admin distance of 110 there are many ways to learn routes in the routing table and each of them has a different administrative distance that is used to compare the different methods of learning routes all OSPF routes will always have an admin distance of 110 now to show you the effect of administrative distance on route precedence we're gonna have to learn of the ninth network from another path so we're gonna have our three forward a route via EIGRP 29.99 dot 0 / 24 once again the configuration on our three has already been done so I'm gonna jump on our one I'm gonna enable yeah JP for the network between router 1 and router 3 and we'll take a look at what changes and Lotus as soon as I enable the network a neighbor adjacency forms with our three and before I had one route learned via OSPF would had it which had an atom distance of 110 and now I have still one route but this one was learned via EIGRP that's what that D means and the e edger peer out has an admin distance of 90 yes your P has an admin a sense of 90 which is better than OS BFS 110 now the router still knows of the OSPF routes i can still do my show IP ospf database summary command and you'll see we still have a route through our 4 or the cost of 20 and a route through our 5 with a cost of 25 which means at the moment our one knows of 3 ways to get to the 9s network it can go via our 5 which was learned via OSPF with the cost of 35 the our 4 which was learned via OSPF with a cost of 30 and between those two this was the better route because it had a better cost router 1 also learned of the path through the 9s network from ERP through our 3 and between these 2 it preferred the path through router 3 because it had a better admin distance and you can see that if we do a trace route to the 9s Network you'll see in all cases the first hop was router 3 now we're going to learn about yet another path the ninth network this time from rip rip has an administrative distance of 120 which is worse than EIGRP is 90 but notice something crucial the HRP and OSPF routes summarising the slash 25 as a slash 24 whereas rip is sending the route as a slash 25 so let's go ahead and take a look and see how that changes the rap precedence prior to me an abling rip this is the route that our one knows about it was learned via a jarppi I'll go ahead and enable rip and before I had this route learned via eigrp now if I do my show IP route command you'll see I currently still have that route that route was unchanged it still exists just like before still learned via a jarppi but I now have an additional route the slash 25 from RIP notice rips administrative distance is 120 compared to EIGRP is 90 that is worse so you would think the erp route would take precedence but it doesn't because the number one thing the router looks at for route precedence is the most specific route a slash 25 is more specific than a slash 24 so this rip route with the worst administrative distance is chosen over the ARP route and you can see this in a trace route you'll notice the first hop in each case was router - even though packets sent 29.99 dot 129 technically match both of these routes the most specific route always takes precedence so there you have it that covers everything a router uses to decide which path to take when multiple paths exist the number one thing that breaks all ties is route specificity the most specific route always takes precedence if there are two routes with the same specificity the next thing that is looked at is the administrative distance and the lower administer distance is better if there are two routes with the same size and the same admin distance the next thing that breaks ties is the metric and again a lower metric is better and finally if all three of these are identical the router will simply load balanced across any available path now before I let you go I have a couple bonus questions for you to test whether you actually understood all this let's say I add a static route that looks like this for the 9.99 0/24 Network pointing via router five this static route is telling r1 that to get to the nines network go via r5 I should say that a static route has an administrative distance of one with that said I have two questions for you four will happen if I add the static route the first question is if I add the static route what will happen to the routing table what entries will be added and what entries will be taken away the second question is what path will r1 take if I do a trace route to the 9.99 dot 129 IP address go and post your answers in the comments section I'll answer the question officially with another video that I'll release next week which is also a good reminder to say that if you enjoyed this content please subscribe and share to encourage me to keep publishing videos that's it for now I look forward to reading your answers in the comments below otherwise thank you for watching and we'll see you in the next video you