Senior Acquisitions Editor: Kenyon Brown Development Editor: Kim Wimpsett
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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#config SF(config)#
SF(config-router)# network 192.168.10.0 SF(config-router)# network 172.16.0.0 SF(config-router)# version 2 SF(config-router)# no auto-summary SF(config-router)# do show 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, 3 subnets R 172.16.10.4 [120/1] via 172.16.10.1, 00:00:08, Serial0/0/0 C 172.16.10.0 is directly connected, Serial0/0/0 L 172.16.10.2/32 is directly connected, Serial0/0 S 192.168.20.0/24 [150/0] via 172.16.10.1 10.0.0.0/24 is subnetted, 1 subnets R 10.10.10.0 [120/1] via 172.16.10.1, 00:00:08, Serial0/0/0 That was pretty straightforward. Let’s talk about this routing table. Since we have one RIP buddy out there with whom we are exchanging routing tables, we can see the RIP networks coming from the Corp router. All the other routes still show up as static and local. RIP also found both connections through the Corp router to networks 10.10.10.0 and 172.16.10.4. But we’re not done yet! LA Let’s configure our LA router with RIP, only I’m going to remove the default route first, even though I don’t have to. You’ll see why soon: LA#
config t LA(config)# no ip route 0.0.0.0 0.0.0.0 LA(config)# router rip LA(config-router)# network 192.168.20.0 LA(config-router)# network 172.16.0.0 LA(config-router)# no auto LA(config-router)# vers 2 LA(config-router)# do show ip route R 192.168.10.0/24 [120/2] via 172.16.10.5, 00:00:10, Serial0/0/1 172.16.0.0/30 is subnetted, 3 subnets C 172.16.10.4 is directly connected, Serial0/0/1 L 172.16.10.6/32 is directly connected, Serial0/0/1 R 172.16.10.0 [120/1] via 172.16.10.5, 00:00:10, 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 10.0.0.0/24 is subnetted, 1 subnets R 10.10.10.0 [120/1] via 172.16.10.5, 00:00:10, Serial0/0/1 The routing table is sprouting new R ’s as we add RIP buddies! We can still see that all routes are in the routing table. This output shows us basically the same routing table and the same entries that it had when we were using static routes—except for those R ’s. An R indicates that the networks were added dynamically using the RIP routing protocol. The [120/1]
is the administrative distance of the route (120) along with the metric, which for RIP is the number of hops to that remote network (1). From the Corp router, all networks are one hop away. So, while yes, it’s true that RIP has worked in our little internetwork, it’s just not a great solution for most enterprises. Its maximum hop count of only 15 is a highly limiting factor. And it performs full routing-table updates every 30 seconds, which would bring a larger internetwork to a painful crawl in no time! There’s still one more thing I want to show you about RIP routing tables and the parameters used to advertise remote networks. Using a different router on a different network as an example for a second, look into the following output. Can you spot where the following routing table shows [120/15] in the 10.1.3.0 network metric? This means that the administrative distance is 120, the default for RIP, but the hop count is 15. Remember that each time a router sends out an update to a neighbor router, the hop count goes up by one incrementally for each route! Here’s that output now: Router#
10.0.0.0/24 is subnetted, 12 subnets C 10.1.11.0 is directly connected, FastEthernet0/1 L 10.1.11.1/32 is directly connected, FastEthernet0/1 C 10.1.10.0 is directly connected, FastEthernet0/0 L 10.1.10.1/32 is directly connected, FastEthernet/0/0 R 10.1.9.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.8.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.12.0 [120/1] via 10.1.11.2, 00:00:00, FastEthernet0/1
R 10.1.3.0 [120/15] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.2.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.1.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.7.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1 R 10.1.6.0 [120/2] via 10.1.5.1, 00:00:15, Serial0/0/1 C 10.1.5.0 is directly connected, Serial0/0/1 L 10.1.5.1/32 is directly connected, Serial0/0/1 R 10.1.4.0 [120/1] via 10.1.5.1, 00:00:15, Serial0/0/1 So this [120/15]
is really bad. We’re basically doomed because the next router that receives the table from this router will just discard the route to network 10.1.3.0 since the hop count would rise to 16, which is invalid! If a router receives a routing update that contains a higher- cost path to a network that's already in its routing table, the update will be ignored. Holding Down RIP Propagations You probably don’t want your RIP network advertised everywhere on your LAN and WAN. There’s enough stress in networking already and not a whole lot to be gained by advertising your RIP network to the Internet! There are a few different ways to stop unwanted RIP updates from propagating across your LANs and WANs, and the easiest one is through the passive-interface command. This command prevents RIP update broadcasts from being sent out of a specified interface but still allows that same interface to receive RIP updates. Here’s an example of how to configure a passive-interface on the Corp router’s Fa0/1 interface, which we will pretend is connected to a LAN that we don’t want RIP on (and the interface isn’t shown in the figure): Corp#
Corp(config)# router rip Corp(config-router)# passive-interface FastEthernet 0/1 This command will stop RIP updates from being propagated out of FastEthernet interface 0/1, but it can still receive RIP updates.
Should We Really Use RIP in an Internetwork? You have been hired as a consultant to install a couple of Cisco routers into a growing network. They have a couple of old Unix routers that they want to keep in the network. These routers do not support any routing protocol except RIP. I guess this means you just have to run RIP on the entire network. If you were balding before, your head now shines like chrome. No need for hairs abandoning ship though—you can run RIP on a router connecting that old network, but you certainly don't need to run RIP throughout the whole internetwork! You can do what is called redistribution, which is basically translating from one type of routing protocol to another. This means that you can support those old routers using RIP but use something much better like Enhanced IGRP on the rest of your network. This will prevent RIP routes from being sent all over the internetwork gobbling up all that precious bandwidth! Advertising a Default Route Using RIP Now I’m going to guide you through how to advertise a way out of your autonomous system to other routers, and you’ll see this is completed the same way with OSPF. Imagine that our Corp router’s Fa0/0 interface is connected to some type of Metro-Ethernet as a connection to the Internet. This is a pretty common configuration today that uses a LAN interface to connect to the ISP instead of a serial interface. If we do add an Internet connection to Corp, all routers in our AS (SF and LA) must know where to send packets destined for networks on the Internet or they’ll just drop the packets when they get a remote request. One solution to this little hitch would be to place a default route on every router and funnel the information to Corp, which in turn would have a default route to the ISP. Most people do this type of configuration in small- to medium-size networks because it actually works pretty well! But since I’m running RIPv2 on all routers, I’ll just add a default route on the Corp router to our ISP, as I would normally. I’ll then add another command to advertise my network to the other routers in the AS as the default route to show them where to send packets destined for the Internet. Here’s my new Corp configuration: Corp(config)#
Corp(config)# router rip Corp(config-router)# default-information originate Now, let’s take a look at the last entry found in the Corp routing table: S* 0.0.0.0/0 is directly connected, FastEthernet0/0 Let’s see if the LA router can see this same entry: LA#
Gateway of last resort is 172.16.10.5 to network 0.0.0.0 R 192.168.10.0/24 [120/2] via 172.16.10.5, 00:00:04, Serial0/0/1 172.16.0.0/30 is subnetted, 2 subnets C 172.16.10.4 is directly connected, Serial0/0/1 L 172.16.10.5/32 is directly connected, Serial0/0/1 R 172.16.10.0 [120/1] via 172.16.10.5, 00:00:04, 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 10.0.0.0/24 is subnetted, 1 subnets R 10.10.10.0 [120/1] via 172.16.10.5, 00:00:04, Serial0/0/1 R 192.168.218.0/24 [120/3] via 172.16.10.5, 00:00:04, Serial0/0/1 R 192.168.118.0/24 [120/2] via 172.16.10.5, 00:00:05, Serial0/0/1 R* 0.0.0.0/0 [120/1] via 172.16.10.5, 00:00:05, Serial0/0/1 Can you see that last entry? It screams that it’s an RIP injected route, but it’s also a default route, so our default-information originate command
is working! Last, notice that the gateway of last resort is now set as well. If all of what you’ve learned is clear and understood, congratulations— you’re ready to move on to the next chapter right after you go through the written and hands-on labs, and while you’re at it, don’t forget the review questions!
This chapter covered IP routing in detail. Again, it’s extremely important to fully understand the basics we covered in this chapter because everything that’s done on a Cisco router will typically have some kind of IP routing configured and running. You learned how IP routing uses frames to transport packets between routers and to the destination host. From there, we configured static routing on our routers and discussed the administrative distance used by IP to determine the best route to a destination network. You found out that if you have a stub network, you can configure default routing, which sets the gateway of last resort on a router. We then discussed dynamic routing, specifically RIPv2 and how it works on an internetwork, which is not very well!
the frame changes at each hop but that the packet is never changed or manipulated in any way until it reaches the destination device (the TTL field in the IP header is decremented for each hop, but that’s it!). List the information required by a router to successfully route packets. To be able to route packets, a router must know, at a minimum, the destination address, the location of neighboring routers through which it can reach remote networks, possible routes to all remote networks, the best route to each remote network, and how to maintain and verify routing information.
will never pass a router’s interface. A frame uses MAC (hardware) addresses to send a packet on a LAN. The frame will take the packet to either a host on the LAN or a router’s interface (if the packet is destined for a remote network). As packets move from one router to another, the MAC addresses used will change, but normally the original source and destination IP addresses within the packet will not.
show ip
route command to view the routing table. Each route will be listed along with the source of the routing information. A C to the left of the route will indicate directly connected routes, and other letters next to the route can also indicate a particular routing protocol that provided the information, such as, for example, R for RIP. Differentiate the three types of routing. The three types of routing are static (in which routes are manually configured at the CLI), dynamic (in which the routers share routing information via a routing protocol), and default routing (in which a special route is configured for all traffic without a more specific destination network found in the table).
creates no routing update traffic and creates less overhead on the router and network links, but it must be configured manually and does not have the ability to react to link outages. Dynamic routing creates routing update traffic and uses more overhead on the router and network links.
route is ip route [destination_network] [mask] [next-hop_address or
.
syntax ip route 0.0.0.0 0.0.0.0 ip-address or
exit interface type and number
. Understand administrative distance and its role in the selection of the best route. Administrative distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router. Administrative distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route. All routing protocols are assigned a default AD, but it can be changed at the CLI. Differentiate distance-vector, link-state, and hybrid routing protocols. Distance-vector routing protocols make routing decisions based on hop count (think RIP), while link-state routing protocols are able to consider multiple factors such as bandwidth available and building a topology table. Hybrid routing protocols exhibit characteristics of both types.
in global configuration mode and then you type the command router rip . Then you add all directly connected networks, making sure to use the classful address and the version 2 command and to disable auto- summarization with the no auto-summary command.
Written Lab 9 In this section, you’ll complete the following lab to make sure you’ve got the information and concepts contained within them fully dialed in: Lab 9.1: IP Routing You can find the answers to this lab in Appendix A, “Answers to Written Labs.” Write the answers to the following questions: 1. At the appropriate command prompt, create a static route to network 172.16.10.0/24 with a next-hop gateway of 172.16.20.1 and an administrative distance of 150. 2. When a PC sends a packet to another PC in a remote network, what destination addresses will be in the frame that it sends to its default gateway? 3. At the appropriate command prompt, create a default route to 172.16.40.1. 4. On which type of network is a default route most beneficial? 5. At the appropriate command prompt, display the routing table on your router. 6. When creating a static or default route, you don’t have to use the next- hop IP address; you can use the______________. 7. True/False: To reach a remote host, you must know the MAC address of the remote host. 8. True/False: To reach a remote host, you must know the IP address of the remote host. 9. At the appropriate command prompt(s), prevent a router from propagating RIP information out serial 1. 10. True/False: RIPv2 is considered classless.
In the following hands-on labs, you will configure a network with three routers. These exercises assume all the same setup requirements as the
labs found in earlier chapters. You can use real routers, the LammleSim IOS version found at www.lammle.com/ccna , or the Cisco Packet Tracer program to run these labs. This chapter includes the following labs: Lab 9.1: Creating Static Routes Lab 9.2: Configuring RIP Routing The internetwork shown in the following graphic will be used to configure all routers. Table 9.2 shows our IP addresses for each router (each interface uses a /24 mask).
Our IP addresses Router Interface IP Address Lab_A Fa0/0 172.16.10.1 Lab_A S0/0 172.16.20.1 Lab_B
S0/0 172.16.20.2 Lab_B S0/1
172.16.30.1 Lab_C
S0/0 172.16.30.2 Lab_C Fa0/0
172.16.40.1 These labs were written without using the LAN interface on the Lab_B router. You can choose to add that LAN into the labs if necessary. Also, if you have enough LAN interfaces, then you don’t need to add the serial interfaces into this lab. Using all LAN interfaces is fine.
Hands-on Lab 9.1: Creating Static Routes In this lab, you will create a static route in all three routers so that the routers see all networks. Verify with the Ping program when complete. 1. The Lab_A router is connected to two networks, 172.16.10.0 and 172.16.20.0. You need to add routes to networks 172.16.30.0 and 172.16.40.0. Use the following commands to add the static routes: Lab_A#
Lab_A(config)# ip route 172.16.30.0 255.255.255.0 172.16.20.2 Lab_A(config)# ip route 172.16.40.0 255.255.255.0 172.16.20.2 2. Save the current configuration for the Lab_A router by going to privileged mode, typing
, and pressing Enter. 3. On the Lab_B router, you have direct connections to networks 172.16.20.0 and 172.16.30.0. You need to add routes to networks 172.16.10.0 and 172.16.40.0. Use the following commands to add the static routes: Lab_B#
Lab_B(config)# ip route 172.16.10.0 255.255.255.0 172.16.20.1 Lab_B(config)# ip route 172.16.40.0 255.255.255.0 172.16.30.2 4. Save the current configuration for router Lab_B by going to the enabled mode, typing copy run start , and pressing Enter. 5. On router Lab_C, create a static route to networks 172.16.10.0 and 172.16.20.0, which are not directly connected. Create static routes so that router Lab_C can see all networks, using the commands shown here: Lab_C#
config t Lab_C(config)# ip route 172.16.10.0 255.255.255.0 172.16.30.1 Lab_C(config)# ip route 172.16.20.0 255.255.255.0 172.16.30.1 6. Save the current configuration for router Lab_C by going to the enable mode, typing
, and pressing Enter. 7. Check your routing tables to make sure all four networks show up by executing the show ip route command.
8. Now ping from each router to your hosts and from each router to each router. If it is set up correctly, it will work. Hands-on Lab 9.2: Configuring RIP Routing In this lab, we will use the dynamic routing protocol RIP instead of static routing. 1. Remove any static routes or default routes configured on your routers by using the no ip route command. For example, here is how you would remove the static routes on the Lab_A router: Lab_A#
Lab_A(config)# no ip route 172.16.30.0 255.255.255.0 172.16.20.2 Lab_A(config)# no ip route 172.16.40.0 255.255.255.0 172.16.20.2 Do the same thing for routers Lab_B and Lab_C. Verify that only your directly connected networks are in the routing tables. 2. After your static and default routes are clear, go into configuration mode on router Lab_A by typing
. 3. Tell your router to use RIP routing by typing router rip and pressing Enter, as shown here:
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