Static route configuration | Network Fundamentals Part 18

moving traffic through the network is something we should all be interested in routing is arguably one of the most important considerations for a network engineer in this and the following two videos we're going to get an understanding of how routers forward traffic and how to configure routing the whole point of routers is to route or forward traffic from one network to another sometimes packets have to be forwarded across many networks before they reach their destination I say routers but these days many switches are also capable of routing packets they are called layer 3 switches or multi-layer switches and as the name suggests they're good for both routing and switching layer 3 switches are especially good for routing traffic between VLANs while routers are often used when connecting to the internet or for LAN connections in these videos I'm just going to continue using the term router but be aware that the principles can apply to layer 3 switches as well each router in the network will need to make its own decisions about how a packet should be forwarded but before they can do this they need to learn about paths through the network they don't just magically know where everything is as an example if we open router r5 and try to ping an IP in a different Network we can see that it fails to start with they will know about any network they're directly connected to these networks will be added to the routing table which we can see with show IP route all layer 3 devices will have a routing table although they may look different depending on the vendor here we can see a list of networks with a code on the left anything with a C code is a directly connected network if you forget what a particular code means you can always look at the handy reference at the top next to the code is the network itself the first part is the prefix and the second part is the subnet mask shown inside a notation the information just to the right of that tells us now router is directly connected to this network and it tells us which interface is used the other type of route we can see here is called a local route which has an ell code these represent the routers IP addresses within the connected network that means that there is one local route per connected route can you also see that each local route has a slash thirty-two mask subnet masks are 32 bits long so a slash 32 mask has every bit turned on so this refers to a single host in the routing table this is called a host route as it is pointing to a single host rather than an entire network connected and local routes will appear in the routing table automatically when an active interface is configured with an IP address you might also notice that the routing table has sections that are listed as either submitted or variably submitted this goes back to the classful networking days all this means is that the submitted network is part of a larger classful network to be honest I don't really think about these things that much as I've said in some other videos there's not a lot of reason to think in terms of classes anymore that's quite a bit to cover in a very short time so if you want test whether this is making sense by trying out these questions if we want a router to reach other networks that is networks that aren't connected we can configure a static route to point the way he is the anatomy of a static route it includes the destination network that we want to reach this may be a nearby network or it could be some distance away the subnet mask of the remote network the next hop IP that we forward packets to the next hop IP in most cases will be the IP address of another router in a network that our router is directly connected to for example router r5 may use the IP address of r3 as a next hop as they both have interfaces in the 170 to 1600 Network maybe it's easier to see this in action let's configure r5 with a route so we can reach the one or two one six eight 3.0 Network we use the IP route command add in one or two one six eight 3.0 give it the subnet mask of 255.255.255.0 1603 we're not limited to just one static route of course we can create many more for example a route to 172 1636 zero now let's take a look at that routing table again see how there are now two new routes listed they have an S code which refers to a static route a static route is one that we can forget manually the entry for a static route looks different to the connected and local routes next to the network and mask we have the numbers 1 / 0 in square brackets and I'll explain that one a little bit later further to the right we have the IP address of the next hop this is the IP that the router will forward packets to if it needs to send them to this network I have a few good labs at the end of this video too that you can try this on if you want to what do you think will happen to a static route if a link fails we're going to simulate this in the background by breaking our 5s interface to switch 1 if a router no longer has an interface in a network that contains the next hop then the static route will be removed from the routing table if we want we could bypass this behavior enforce the route to stay in the routing table no matter what by adding the permanent keyword to the end of the IP route command this won't magically make the route work though it will only force the route to stay in the routing table if we now fix that interface the route appears back in the routing table once again let's take a look at something that you might not have thought of first let's ping one onto 1 6 8 3.3 and that works just as we would expect that's using one of the routes we configured a few moments ago when we send a ping the router select an IP e that it thinks is suitable as the source of this packet so the ping will be sent from 170 to 1605 when r3 needs to respond it will send the response message back to 172 1605 we can change this behavior by adding the source key word this changes the source IP that our router users when sending the ping message and this fails do you know why do you remember earlier that I said that each router needs to make its own routing decisions when r3 gets the ping it will need to look at its routing table to decide how to send a response back to one seven two sixteen 205 however right now our three does not have a route back to 170 216 205 the key takeaway here is that when we think about routing we need to think about how to get traffic to its destination as well as how the destination router can send traffic back so how do we fix this we can add a new route to r3 this route goes to the 172 16 200 zero network using R 5 as the next hop if we go back to our fire and try that ping again we can see that it is now working let's consider another scenario we've seen that a static route will be removed from the routing table if a critical interface fails what happens if something else along the path breaks but the interface stays up let's simulate this by shutting down an interface on our three as you would expect the static route on r3 is removed from our three Z routing table what do you think will happen on r5 r5 is not physically connected to r3 it's connected to a switch so while our threes interface is shut down r5 will stay up as our 5s interface is still up the static route will remain in the routing table they won't be able to reach our three though so traffic using this route will flow into a black hole this is a key limitation of static routes by default they are not very aware of the state of the network while we're still talking about static routes there is an alternative way that we can use them if we want we can configure an outgoing interface rather than a next hop IP in cases like this the router will use an ARP message to find the IP address of the next hop this might be used in a case where we have a small network with only two routers in it for example like the small network between r1 and r2 personally I don't really like using these types of static routes very often I much prefer using a next hop IP address but you need to be aware that sometimes you will see them out there I've already mentioned a few times that each router will make its own decisions about how to handle and forward packets let's take a little time to think about how we'll make these decisions and how packets flow through the network to start with a host connected to the network will create a packet if this needs to go to any network that is not a part of it we'll send the packet to its default gateway once the packet arrives at the router the router needs to decide what to do technically at this point it is a frame not a packet as it will still have a layer 2 header probably Ethernet so the first router will check to see if the frame is valid or corrupt if it is corrupt it will be discarded if it's fine it will D capsulate the frame leaving a packet it can now retrieve the destination IP address in the IP header and compare it to routes in the routing table if there is a route the router will prepare to or the packet to the next hop this includes getting the MAC address of the next hop and encapsulating the packet with new Ethernet headers and trailers if there is no suitable route in the routing table the router will drop the packet keep in mind though that the router will not check if the next hop is up if the next hop does not exist the packet will be dropped and if you want to test yourself out here are a few more quiz questions you can try so hosts have a default gateway guess what routers do too if a router does not have a route to a particular destination it can use its default route it's kind of a catch-all route that's used if nothing better can be found the best example of when this is used is for your internet connection in most networks you can't reasonably be expected to have a route for every possible destination on the Internet so for this you will have a default route we can still configure this like any other route though the difference is that the destination network is 0.0.0.0 with a subnet mask of 0.0.0.0 this will match everything assuming that there's nothing more specific of course when we look at the routing table we see this in two places for one it will show up as a regular route the only difference is the star symbol the star symbol means candidate default you can have more than one default route configured if you want to but the router will only use one at a time so the candidate default is the one that the router is using right now we also see this listed as the Gateway of last resort this is also the default route that is currently in use either default route only for internet access no we can use this in other cases to look at our six in this topology there is only one way in and one way out and that is the path through r3 when configuring routing on r6 we could create separate routes for all the networks in our topology but that will take a fair bit of effort so instead why not just configure r6 with a default route using our three is the next hop it catches all traffic anyway which we can see with the ping this is a type of summary route we're making life easier by combining several routes into one simplified route the best way to lock all this in your brain is to practice so I have two challenges for you firstly start by building this topology for the networks around the edges I recommend using loopback interfaces our members have it easier as you can download the starting topology from the website next configure static routing on all routers so every router can reach every other network you can test it out with a ping remembering to change your source IP on occasion if you are willing to download labs from the site I have an additional challenge for you I have configured the topology but it's broken you need to use the skills that you've learned in this video to try to repair the network so all the routers can reach all the other networks once again this lab is the one that will probably give you the most benefit if you can try to practice the skills you've learned here there really is no substitute for practice also consider going back to videos 12 and 13 where we talked about VLANs and router on a stick see if you can figure out how the router on a stick configuration works and how packets are forwarded between VLANs in the next video we begin discussing dynamic routing so I hope to see you there

Channel: Network Direction

Views: 38,168

Rating: 4.9292731 out of 5

Keywords: Network direction, Routing, Switching, Ip address, Subnet, Static route, Ip route, Routing table, Show ip route, Ping, Connected route, Local route, Next hop, CIDR, Mask, Distance, Ospf, Rip, Eigrp, Arp, Lab, Gateway, Gateway of last resort, Default gateway, Ethernet, Frame rewrite, Summary route, Candidate default, best training, free training, ccna routing and switching, ip routing, free ccna network fundamentals, online training, static routing

Id: cupc6Helc6Y

Length: 14min 17sec (857 seconds)

Published: Tue Nov 26 2019