Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett
config t Router(config)# hostname SF
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config t Router(config)# hostname SF SF(config)# enable secret GlobalNet SF(config)# no ip domain-lookup SF(config)# int s0/0/0 SF(config-if)# desc WAN Connection to Corp SF(config-if)# ip address 172.16.10.2 255.255.255.252 SF(config-if)# no shut SF(config-if)# clock rate 1000000 SF(config-if)# int f0/0 SF(config-if)# desc SF LAN SF(config-if)# ip address 192.168.10.1 255.255.255.0 SF(config-if)# no shut SF(config-if)# line con 0 SF(config-line)# password console SF(config-line)# login SF(config-line)# logging sync SF(config-line)# exit SF(config)#line vty 0 ? <1-1180> Last Line number SF(config)# line vty 0 1180 SF(config-line)# password telnet SF(config-line)# login SF(config-line)# banner motd #This is the SF Branch router# SF(config)# exit SF#
copy run start Destination filename [startup-config]? Building configuration... [OK]
Let’s take a look at our configuration of the interfaces with the following two commands: SF#
interface FastEthernet0/0 description SF LAN ip address 192.168.10.1 255.255.255.0 duplex auto speed auto ! interface FastEthernet0/1 no ip address shutdown
duplex auto speed auto ! interface Serial0/0/0 description WAN Connection to Corp ip address 172.16.10.2 255.255.255.252 clock rate 1000000 ! SF# sh ip int brief Interface IP-Address OK? Method Status Protocol FastEthernet0/0 192.168.10.1 YES manual up up FastEthernet0/1 unassigned YES unset administratively down down Serial0/0/0 172.16.10.2 YES manual up up Serial0/0/1 unassigned YES unset administratively down down SF#
Now that both ends of the serial link are configured, the link comes up. Remember, the up/up status for the interfaces are Physical/Data Link layer status indicators that don’t reflect the layer 3 status! I ask students in my classes, “If the link shows up/up, can you ping the directly connected network?” And they say, “Yes!” The correct answer is, “I don’t know,” because we can’t see the layer 3 status with this command. We only see layers 1 and 2 and verify that the IP addresses don’t have a typo. This is really important to understand! The show ip route command for the SF router reveals the following: SF#
sh ip route C 192.168.10.0/24 is directly connected, FastEthernet0/0 L 192.168.10.1/32 is directly connected, FastEthernet0/0 172.16.0.0/30 is subnetted, 1 subnets C 172.16.10.0 is directly connected, Serial0/0/0 L 172.16.10.2/32 is directly connected, Serial0/0/0 Notice that router SF knows how to get to networks 172.16.10.0/30 and 192.168.10.0/24; we can now ping to the Corp router from SF: SF#
Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 172.16.10.1, timeout is 2 seconds:
!!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms Now let’s head back to the Corp router and check out the routing table: Corp> sh ip route 172.16.0.0/30 is subnetted, 1 subnets C 172.16.10.0 is directly connected, Serial0/0 L 172.16.10.1/32 is directly connected, Serial0/0 10.0.0.0/24 is subnetted, 1 subnets C 10.10.10.0 is directly connected, FastEthernet0/0 L 10.10.10.1/32 is directly connected, FastEthernet0/0 On the SF router’s serial interface 0/0/0 is a DCE connection, which means a clock rate needs to be set on the interface. Remember that you don’t need to use the clock rate command in production. While true, it’s still imperative that you know how/when you can use it and that you understand it really well when studying for your CCNA exam! We can see our clocking with the show controllers command: SF#
sh controllers s0/0/0 Interface Serial0/0/0 Hardware is GT96K
Corp>
sh controllers s0/0 Interface Serial0/0 Hardware is PowerQUICC MPC860
Since the SF router has a DCE cable connection, I needed to add clock rate to this interface because DTE receives clock. Keep in mind that the new ISR routers will autodetect this and set the clock rate to 2000000. And you still need to make sure you’re able to find an interface that is DCE and set clocking to meet the objectives. Since the serial links are showing up, we can now see both networks in the Corp routing table. And once we configure LA, we’ll see one more network in the routing table of the Corp router. The Corp router can’t see the 192.168.10.0 network because we don’t have any routing configured yet—routers see only directly connected networks by default. LA Configuration To configure LA, we’re going to do pretty much the same thing we did with the other two routers. There are two interfaces to deal with, Serial 0/0/1 and FastEthernet 0/0, and again, we’ll be sure to add the hostname, passwords, interface descriptions, and a banner to the router configuration: Router(config)#
LA(config)# enable secret GlobalNet LA(config)# no ip domain-lookup LA(config)# int s0/0/1 LA(config-if)# ip address 172.16.10.6 255.255.255.252 LA(config-if)# no shut LA(config-if)# clock rate 1000000 LA(config-if)# description WAN To Corporate LA(config-if)# int f0/0 LA(config-if)# ip address 192.168.20.1 255.255.255.0 LA(config-if)# no shut LA(config-if)# description LA LAN LA(config-if)# line con 0 LA(config-line)# password console LA(config-line)# login LA(config-line)# logging sync LA(config-line)# exit LA(config)# line vty 0 ? <1-1180> Last Line number LA(config)# line vty 0 1180 LA(config-line)# password telnet LA(config-line)# login LA(config-line)# exit LA(config)# banner motd #This is my LA Router# LA(config)# exit LA#
copy run start Destination filename [startup-config]? Building configuration... [OK]
Nice—everything was pretty straightforward. The following output, which I gained via the show ip route command, displays the directly connected networks of 192.168.20.0 and 172.16.10.0: LA#
sh ip route 172.16.0.0/30 is subnetted, 1 subnets C 172.16.10.4 is directly connected, Serial0/0/1 L 172.16.10.6/32 is directly connected, Serial0/0/1 C 192.168.20.0/24 is directly connected, FastEthernet0/0 L 192.168.20.1/32 is directly connected, FastEthernet0/0 So now that we’ve configured all three routers with IP addresses and administrative functions, we can move on to deal with routing. But I want to do one more thing on the SF and LA routers—since this is a very small network, let’s build a DHCP server on the Corp router for each LAN. Configuring DHCP on Our Corp Router While it’s true that I could approach this task by going to each remote router and creating a pool, why bother with all that when I can easily create two pools on the Corp router and have the remote routers forward requests to the Corp router? Of course, you remember how to do this from Chapter 7! Let’s give it a shot: Corp#
config t Corp(config)# ip dhcp excluded-address 192.168.10.1 Corp(config)# ip dhcp excluded-address 192.168.20.1 Corp(config)# ip dhcp pool SF_LAN Corp(dhcp-config)# network 192.168.10.0 255.255.255.0 Corp(dhcp-config)# default-router 192.168.10.1 Corp(dhcp-config)# dns-server 4.4.4.4 Corp(dhcp-config)# exit Corp(config)# ip dhcp pool LA_LAN Corp(dhcp-config)# network 192.168.20.0 255.255.255.0 Corp(dhcp-config)# default-router 192.168.20.1 Corp(dhcp-config)# dns-server 4.4.4.4 Corp(dhcp-config)# exit Corp(config)# exit Corp#
copy run start Destination filename [startup-config]? Building configuration... Creating DHCP pools on a router is actually a simple process, and you would go about the configuration the same way on any router you wish to add a DHCP pool to. To designate a router as a DHCP server, you just create the pool name, add the network/subnet and the default gateway, and then exclude any addresses that you don’t want handed out. You definitely want to make sure you’ve excluded the default gateway address, and you’d usually add a DNS server as well. I always add any exclusions first, and remember that you can conveniently exclude a range of addresses on a single line. Soon, I’ll demonstrate those verification commands I promised I’d show you back in Chapter 7, but first, we need to figure out why the Corp router still can’t get to the remote networks by default! Now I’m pretty sure I configured DHCP correctly, but I just have this nagging feeling I forgot something important. What could that be? Well, the hosts are remote across a router, so what would I need to do that would allow them to get an address from a DHCP server? If you concluded that I’ve got to configure the SF and LA F0/0 interfaces to forward the DHCP client requests to the server, you got it! Here’s how we’d go about doing that: LA#
LA(config)# int f0/0 LA(config-if)# ip helper-address 172.16.10.5 SF#
config t SF(config)# int f0/0 SF(config-if)# ip helper-address 172.16.10.1 I’m pretty sure I did this correctly, but we won’t know until I have some type of routing configured and working. So let’s get to that next!
So is our network really good to go? After all, I’ve configured it with IP addressing, administrative functions, and even clocking that will automatically occur with the ISR routers. But how will our routers send packets to remote networks when they get their destination information by looking into their tables that only include directions about directly connected networks? And you know routers promptly discard packets they receive with addresses for networks that aren’t listed in their routing table! So we’re not exactly ready to rock after all. But we will be soon because there are several ways to configure the routing tables to include all the networks in our little internetwork so that packets will be properly forwarded. As usual, one size fits all rarely fits at all, and what’s best for one network isn’t necessarily what’s best for another. That’s why understanding the different types of routing will be really helpful when choosing the best solution for your specific environment and business requirements. These are the three routing methods I’m going to cover with you: Static routing Default routing Dynamic routing We’re going to start with the first way and implement static routing on our network, because if you can implement static routing and make it work, you’ve demonstrated that you definitely have a solid understanding of the internetwork. So let’s get started.
Static routing is the process that ensues when you manually add routes in each router’s routing table. Predictably, there are pros and cons to static routing, but that’s true for all routing approaches. Here are the pros: There is no overhead on the router CPU, which means you could probably make do with a cheaper router than you would need for dynamic routing. There is no bandwidth usage between routers, saving you money on WAN links as well as minimizing overhead on the router since you’re not using a routing protocol. It adds security because you, the administrator, can be very exclusive and choose to allow routing access to certain networks only. And here are the cons: Whoever the administrator is must have a vault-tight knowledge of the internetwork and how each router is connected in order to configure routes correctly. If you don’t have a good, accurate map of your internetwork, things will get very messy quickly! If you add a network to the internetwork, you have to tediously add a route to it on all routers by hand, which only gets increasingly insane as the network grows.
Due to the last point, it’s just not feasible to use it in most large networks because maintaining it would be a full-time job in itself. But that list of cons doesn’t mean you get to skip learning all about it mainly because of that first disadvantage I listed—the fact that you must have such a solid understanding of a network to configure it properly and that your administrative knowledge has to practically verge on the supernatural! So let’s dive in and develop those skills. Starting at the beginning, here’s the command syntax you use to add a static route to a routing table from global config: ip route [destination_network] [mask] [next-hop_address or exitinterface] [administrative_distance] [permanent] This list describes each command in the string: ip route The command used to create the static route. destination_network The network you’re placing in the routing table. mask The subnet mask being used on the network. next-hop_address This is the IP address of the next-hop router that will receive packets and forward them to the remote network, which must signify a router interface that’s on a directly connected network. You must be able to successfully ping the router interface before you can add the route. Important note to self is that if you type in the wrong next-hop address or the interface to the correct router is down, the static route will show up in the router’s configuration but not in the routing table. exitinterface Used in place of the next-hop address if you want, and shows up as a directly connected route.
By default, static routes have an administrative distance of 1 or 0 if you use an exit interface instead of a next-hop address. You can change the default value by adding an administrative weight at the end of the command. I’ll talk a lot more about this later in the chapter when we get to the section on dynamic routing. permanent If the interface is shut down or the router can’t communicate to the next-hop router, the route will automatically be discarded from the routing table by default. Choosing the permanent option keeps the entry in the routing table no matter what happens. Before I guide you through configuring static routes, let’s take a look at a sample static route to see what we can find out about it: Router(config)# ip route 172.16.3.0 255.255.255.0 192.168.2.4 The
ip route command tells us simply that it’s a static route. 172.16.3.0 is the remote network we want to send packets to. 255.255.255.0 is the mask of the remote network. 192.168.2.4 is the next hop, or router, that packets will be sent to. But what if the static route looked like this instead? Router(config)#
That 150 at the end changes the default administrative distance (AD) of 1 to 150. As I said, I’ll talk much more about AD when we get into dynamic routing, but for now, just remember that the AD is the trustworthiness of a route, where 0 is best and 255 is worst. One more example, then we’ll start configuring: Router(config)#
Instead of using a next-hop address, we can use an exit interface that will make the route show up as a directly connected network. Functionally, the next hop and exit interface work exactly the same. To help you understand how static routes work, I’ll demonstrate the configuration on the internetwork shown previously in Figure 9.8 . Here it is again in Figure 9.9 to save you the trouble of having to go back and forth to view the same figure. FIGURE 9.9 Our internetwork Corp Each routing table automatically includes directly connected networks. To be able to route to all indirectly connected networks within the internetwork, the routing table must include information that describes where these other networks are located and how to get to them. The Corp router is connected to three networks. For the Corp router to be able to route to all networks, the following networks have to be configured into its routing table: 192.168.10.0
192.168.20.0 The following router output shows the static routes on the Corp router and the routing table after the configuration. For the Corp router to find the remote networks, I had to place an entry into the routing table describing the remote network, the remote mask, and where to send the packets. I am going to add a 150 at the end of each line to raise the administrative distance. You’ll see why soon when we get to dynamic routing. Many times this is also referred to as a floating static route because the static route has a higher administrative distance than any routing protocol and will only be used if the routes found with the routing protocols go down. Here’s the output: Corp#
config t Corp(config)# ip route 192.168.10.0 255.255.255.0 172.16.10.2 150 Corp(config)# ip route 192.168.20.0 255.255.255.0 s0/1 150 Corp(config)# do show run | begin ip route ip route 192.168.10.0 255.255.255.0 172.16.10.2 150 ip route 192.168.20.0 255.255.255.0 Serial0/1 150 I needed to use different paths for networks 192.168.10.0 and 192.168.20.0, so I used a next-hop address for the SF router and an exit interface for the LA router. After the router has been configured, you can just type
to see the static routes: Corp(config)#
S 192.168.10.0/24 [150/0] via 172.16.10.2 172.16.0.0/30 is subnetted, 2 subnets C 172.16.10.4 is directly connected, Serial0/1 L 172.16.10.5/32 is directly connected, Serial0/1 C 172.16.10.0 is directly connected, Serial0/0 L 172.16.10.1/32 is directly connected, Serial0/0 S 192.168.20.0/24 is directly connected, Serial0/1 10.0.0.0/24 is subnetted, 1 subnets C 10.10.10.0 is directly connected, FastEthernet0/0 L 10.10.10.1/32 is directly connected, FastEthernet0/0 The Corp router is configured to route and know all routes to all networks. But can you see a difference in the routing table for the routes to SF and LA? That’s right! The next-hop configuration showed up as via, and the route configured with an exit interface configuration shows up as static but also as directly connected! This demonstrates how they are functionally the same but will display differently in the routing table. Understand that if the routes don’t appear in the routing table, it’s because the router can’t communicate with the next-hop address you’ve configured. But you can still use the permanent parameter to keep the route in the routing table even if the next-hop device can’t be contacted. The
S in the first routing table entry means that the route is a static entry. The [150/0]
stands for the administrative distance and metric to the remote network, respectively. Okay—we’re good. The Corp router now has all the information it needs to communicate with the other remote networks. Still, keep in mind that if the SF and LA routers aren’t configured with all the same information, the packets will be discarded. We can fix this by configuring static routes. Don’t stress about the 150 at the end of the static route configuration at all, because I promise to get to it really soon in this chapter, not a later one! You really don't need to worry about it at this point.
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