the job of a router is to evaluate incoming traffic determine what the destination might be for that traffic and then send it out the appropriate interface the router is effectively determining what direction a packet might go to depending on the knowledge it has about the rest of the network to be able to make this decision about where traffic goes we need a routing table and fortunately most devices have a routing table the workstation that you're using has a routing table table the servers that you're accessing have routing tables and of course the routers themselves maintain their own routing tables as well for the router to be able to make the right decision on what the best route would be to a destination it refers to this routing table so everything that we do when we're troubleshooting a router will begin and end with the information contained within that routing table one thing you might even find with a router is that there may be multiple ways to get to a location and there will be a tie as to which route a router might choose fortunately there are ways to break the tie within the router and in this video we'll look at a number of ways that that tiebreaking process might occur let's look at a routing table this is a routing table I pulled from a Cisco router but it's almost identical in Form and Function to routing tables that you would find in a workstation in a server or in any other manufacturer's router in this particular example this routing table table was built based on a number of networks that were directly connected and routes that were received using the rip Dynamic routing protocol specifically rip version 2 if we look at the routing table the top part of this routing table is a legend that gives us more information about the codes that are used within the routing table itself in the bottom is the routing table and you can see that the first set of letters on each one of these lines corresponds back to a different set of codes that are shown in that legend for example you can see the first line of this routing table starts with the letter c which means that that particular route is directly connected to this router if we want to see the route that was added using rip version 2 then we need to find a line that starts with an r and indeed there is a line that has an R which means that everything on this line was created because of information that was received by this router using the rip version 2 Dynamic routing protocol and you can see there is a lot of information contained within this line we can make out some IP addresses there are different values such as 1201 and it does appear there are even counters here along with information about specific interfaces let's break out just this line of information to determine what is contained within this line of a routing table let's break out this line of information to see which each one of these provides we know that that first r at the beginning of the line is the route code code and if we refer back to the r in the list of codes it does say that this was received using the rip protocol the next one is 101030 do0 sl24 this is the destination subnet that has been added into the routing table along with a prefix length of sl24 we also have the 120 SL1 this is actually two different values the 120 is an administrative distance we'll learn more about administrative distance in a moment and the next is a metric we'll also talk about metrics in this video you can also see that it's via 101050 two this would be the next hop or the destination that we would be sending this traffic if we needed to go to this particular location we also have this time value this is a time stamp that tells us how long this route has been active inside of this routing table and in this case this route has only been active for 14 seconds and lastly you can see the outgoing interface that would be used sometimes this interface is included sometimes it's optional depending on the routing protocol or the router that you're using but it's sometimes nice to see that this particular next hop is one that we would reach by going out a specific physical interface in that router so when the router is making its routing decision on where traffic should go it's evaluating everything in this routing table to determine if this would be the best possible route to use to forward this traffic along to the next hop one of the first things that a router is going to evaluate is where this particular traffic is destined we need to be able to look at the destination IP address and compare that to the subnet IDs and prefix links that are contained within the routing table if there's a match then we'll know where to forward this traffic this means that this IP address range and prefix length becomes an important consideration when we begin to forward traffic we need need to look at both of those together to determine if this is the best possible route for this traffic and if you were to look at a routing table you may find that there are multiple routes listed to a particular subnet but there might be different prefix lengths in each one of those lines so you might need to evaluate each individual line of a routing table to determine which one is the most specific for this particular route for example let's say that you need to communicate with a device that's located at one 192.168.1 6 and your routing table has three different routes that would match that particular IP address one of them is 192.168.0.0 20116 the other is 192.168.1.0 24 and the third is 192.168.1 632 all of these would be valid routes to that particular destination but only one is the most specific we need to evaluate not just the subnet ID but also the prefix link to determine which one of these is the most specific for that particular destination and if we were to look at 192.168.1.0 we would know that 192.168.1 632 would be the most specific route in fact that is the most specific route because a sl32 is specific to an individual IP address in this case 192.168.1 6 now if this route was not in our routing table we would have to choose between 192.168.0.0 2016 and 192.168.1.0 sl24 the sl24 is the more specific route than the sl16 so we would choose the middle route if we had a choice between those two locations this can get even more complicated if your routing table happens to have identical routes to a location that go to different next tops how do you know which Next Top would be the correct one if everything else was identical and the one way that you would be able to make that determination is by examining the administrative distance different routing Protocols are assigned different administrative distances within the router itself this allows the router to pick the best route based on the type of protocols or the type of information that it is received for example if this is a local connection it is phys physically connected to the router this is the best possible way to get to that subnet so it has an administrative distance of zero as you can see here the lower the administrative distance the better the route might be if you have manually configured a static route the router assumes that you must be the most knowledgeable person for that particular route so static routes have an administrative distance of one if this route was added to the routing table using eigrp the administrative distance is 90 and and if you receive this route via OPF the administrative distance is 110 you can see that other Dynamic routing protocols and methods are listed in this table with the appropriate administrative distance for a Cisco router in some cases the routing protocol itself may be in a position where there might be duplicate routes to a particular location and the routing protocol has to make a decision on where the best route might be in that case we will want to look at the routing metric to be able to break that tie routing metrics are an internal value that are used by the routing protocol itself so bgp has its own set of routing metrics OPF has a completely different set of routing metrics and eigrp uses its own set of routing metrics as well this means that you can't compare routing metrics across different routing protocols the routing metrics used for bgp are very different than the routing metrics used for errp and there's no way to compare or contrast those routing metrics across routing protocols but similar to an administrative distance bgp might create its own set of routing metrics to a particular site and then it can determine what the best route might be based on its own internal set of routing metrics for example the routing metrics for bgp might be one or two it will choose the route for one because that is the lowest routing metric let's look at our routing table for rip version 2 again we have that one line in the routing table to tend 030.0 sl24 it has an administrative distance of 120 because it is Rip version 2 and it has a routing metric of one rip uses the number of hops to a location as its routing metric so we know that this particular destination is one hop away to be able to reach that particular Network we would go to 101050 do2 and we would get to that particular location by sending this traffic out serial 0/3 SL1 on this router if you were to use a different routing protocol for example eigrp you would have a very different routing metric here is the same routing table to the same location but we've enabled eigrp instead of rip 2 you can see this line in the routing table was created with the code D and if we refer back to our Legend we can see that D does refer to eigrp everything in this line is very similar to what we saw with rip version 2 although you'll notice the administrative distance is different it's a 90 because eigrp has a higher priority in its administrative distance than rip version 2 but you'll notice that the routing metric that is determined by eigrp is very different than the routing metric that we saw for rip version 2 rip version 2 uses the number of hops whereas eigrp has a completely different calculation that it uses to determine what the best route might be as you work more with routing t and begin evaluating these routes on the Fly you'll become much more comfortable with determining where the best next hop might be for a particular packet one of the challenges we have when working with routers is that there is only one best route to a different location if you were to look at the IP configuration of your device you'll notice that you have a default gateway that default gateway is your local router that allows you to communicate outside of your local IP subnet but you'll notice that there is only one IP address available to list as the default gateway you can't list multiple gateways in that list and this brings up a challenge especially when you'd like to have redundant routers on a single Network one way to provide redundancy even though you only have places for a single default gateway is to create a virtual IP address for the router that's in use we refer to this virtual IP as a VIP and the idea is that if the primary router disappears we can move that virtual IP to another router on the subnet so that you don't have to change everyone's workstation to maintain the uptime and availability of the network this means you as the in user may have no idea how many different redundant routers might be on your local subnet and if the primary router on your network was to fail and the virtual IP address was to move you would seamlessly continue to have connectivity using that redundant router here's how this would work you as the in user would communicate to a router or a default gateway on your network and that default gateway gives you access to other subnets or allows you to communicate out to the internet in this particular example we're going to use the first hop redundancy protocol or fhrp in conjunction with a virtual IP address that we've associated with router 1 this is our active router also on the same subnet is a standby or backup router that our router 2 our default gateway is going to be associated with the virtual IP address that we've got associated with router 1 and that means that all of our traffic will flow out of our Network through router 1 if router 1 was to fail there would need to be some type of method to fail over to router 2 but router 2 has a completely different IP address in this configuration router 1 and router 2 are always in communication with each other but if router 2 suddenly realizes that it's not able ble to communicate with the active router it then removes that as the active router and becomes the active router itself then it takes control of that virtual IP address so that everyone on the network is now able to communicate out to the internet through router to using that original virtual IP address all of the traffic will flow just as it normally did and then user has no idea that it's now using a completely different router to be able to perform that communication another interesting characteristic of switches and routers is that you can assign multiple interfaces to a single physical interface we refer to these as sub interfaces so even though there might be a single ethernet interface on a router we can take that single ethernet interface and separate it into multiple virtual interfaces for example we might have a trunk connection that has multiple IP subnets connecting to a single physical connection but we may need to reference each individual VLAN in a trunk with a separate IP address on our router we do that by assigning that physical interface with an additional parameter called a sub interface so although the physical interface on that router is referred to as ethernet 1/1 we would have separate sub interfaces within that physical interface that might be called ethernet 1/110 ethernet 1/120 and ethernet 1 sl1100 this means that we can set different configurations for each of these sub interfaces so that we can reference them with the appropriate IP address for that VLAN here's how this would look on a network configuration you can see there are three devices all three of these devices are in separate vlans there's the red VLAN the green VLAN and the blue VLAN and of course they're connecting to separate physical interfaces on a switch the switch then has a single cable between the switch and the router and it's trunking each individual VLAN within that single gigabit connection on the router side we've configured three sub interfaces for that ethernet connection sub interface G 0.12 and do3 and then we can assign IP addresses subnet masks and have completely different routing tables for each of these individual sub interfaces as if they were physical interfaces on that router