This scenario presents some of the design recommendations found in common literature. This is not a practical scenario but rather a presentation of some design guidelines to help you in real-life network situations you might come across in designing today's complex IP networks.
By no means are these rules standard, and they are provided here for reference so you can easily refer to a sample network design and the common rules experts adhere to in large OSPF networks.
When architecting a network, you should try to accomplish five basic goals with dynamic routing protocols:
• Functionality— The network works; that is, no matter what failure or scenario, the network must always be functioning.
• Scalability— As the network grows in size, your initial topology or design must be able to cope with growth and new challenges, such as new
acquisitions.
• Adaptability— With ever-increasing new technologies, such as Voice over IP, your network should cope with and embrace new features.
• Manageability— This point refers to proactive management. Any large network should be able to foresee new challenges before the network grinds to a halt.
• Cost effectiveness— In reality, cost drives most network designers. The bigger your budget, the better able you are to provide users the ability to work around network failures.
You can manage and configure OSPF so that the preceding five criteria are fully supported.
Implementing a hierarchical IP addressing scheme and performing summarization wherever possible are two key points in any large OSPF network.
The following are some general guidelines when designing a large OSPF network.
Determine the number of routers in each area. Anything between 40–50 is an
acceptable number. The number of calculations any given router must perform given m LSAs is mlogm. For example, 100 routers require 100log 100 or 100 × 2 = 200.
Keeping these calculations to a minimum means the CPU/memory requirements are also kept low. The IETF standards committee provides the following sample design guidelines:
• The minimum number of routers per domain is 20; the maximum is 1000.
• The minimum routers per single area is 20; the maximum is 350.
• The number of areas per domain is 1; the maximum is 60.
TIP
OSPF is such a large topic that many books have been written about it. For a concise guide to OSPF and a more detailed guide, you cannot invest in anything better than the following two quality Cisco Press titles: Routing TCP/IP by Jeff Doyle and OSPF Network Design Solutions by Tom Thomas.
Practical Exercise: OSPF and RIP Redistribution
NOTE
Practical Exercises are designed to test your knowledge of the topics covered in this chapter. The Practical Exercise begins by giving you some information about a situation and then asks you to work through the solution on your own. The solution can be found at the end.
Configure the network in Figure 4-8 for OSPF between the three routers named SanFran, Mel, and Simon. Configure the edge router named Sydney for RIP and ensure IP connectivity among all four routers. You must use only RIPv1 and OSPF as your IP routing protocols. Ensure that a default route appears on all routers so users
can connect to the Internet. Configure summarization wherever possible to minimize IP routing tables.
Figure 4-8. RIP-to-OSPF Redistribution
Practical Exercise Solution
The router named Simon is configured in the OSPF area 0 (backbone) and the RIP domain and needs to run redistribution between OSPF and OSPF. Also, because you are using RIPv1, you must also provide summary addresses for all networks, but not /24 because RIPv1 does not carry subnet mask information in routing updates.
(RIPv2 does).
Router SanFran is connected to the Internet, so you need to configure SanFran to provide a default route to the rest of the internal network by using the OSPF command default-information originate always. This IOS command injects a default route into the OSPF domain and Router Simon because redistribution also injects a default route into the RIP domain.
The following are the full working configurations of all four routers with the shaded portions highlighting critical configuration commands. Example 4-60 displays the full working configuration of Router Sydney. Sydney is running RIP only.
Example 4-60 Full Working Configuration of Router Sydney
hostname Sydney
!
logging buffered 64000 debugging enable password cisco
!
ip subnet-zero no ip domain-lookup interface Ethernet0/0
ip address 141.108.1.1 255.255.255.0 no ip directed-broadcast
!
interface Serial0/0 shutdown
!
interface Serial0/1 shutdown
!
router rip
network 141.108.0.0
!
line con 0 line aux 0 line vty 0 4
! end
Example 4-61 displays the full working configuration of Router Simon. Simon is running OSPF and RIP. You must always be careful when redistributing information from one routing domain into another. Simon advertises the non /24 subnets as Class C networks so the RIP domain (Sydney router) can inject them into the routing table. Because RIPv1 is classless and the subnet 141.108.1.0/24 is configured
locally, all interfaces in this Class B network (141.108.0.0) are assumed to be Class C.
Example 4-61 Full Working Configuration of Router Simon
Building configuration...
Current configuration:
!
version 12.0
service timestamps debug uptime service timestamps log uptime no service password-encryption
!
hostname Simon
!
enable password cisco
!
ip subnet-zero no ip domain-lookup
!
cns event-service server
!
interface Ethernet0
ip address 141.108.2.1 255.255.255.128
!
interface Ethernet1
ip address 141.108.1.4 255.255.255.0
!
interface Serial0 shutdown
!
interface Serial1 shutdown
!
interface Serial2
ip address 141.108.255.5 255.255.255.252 clockrate 128000
!
interface Serial3
ip address 141.108.255.1 255.255.255.252 clockrate 128000
!
router ospf 1
summary-address 141.108.2.0 255.255.255.0 summary-address 141.108.255.0 255.255.255.0 summary-address 141.108.3.0 255.255.255.0 summary-address 141.108.4.0 255.255.255.0 redistribute connected subnets
redistribute rip metric 10 subnets network 141.108.2.0 0.0.0.127 area 0 network 141.108.255.0 0.0.0.255 area 0
!
router rip
redistribute ospf 1 metric 2
passive-interface Ethernet0 -> Stops RIP updates on OSPF interfaces passive-interface Serial2
passive-interface Serial3 network 141.108.0.0
!
=
ip route 141.108.3.0 255.255.255.0 Null0 ip route 141.108.4.0 255.255.255.0 Null0
!
line con 0 line aux 0 line vty 0 4
! end
Example 4-62 displays the full working configuration of Router Mel. Mel is running OSPF only.
Example 4-62 Full Working Configuration of Router Mel
hostname Mel
enable password cisco
!
ip subnet-zero no ip domain-lookup
!
interface Ethernet0
ip address 141.108.3.1 255.255.255.248
!
interface Serial0
ip address 141.108.255.6 255.255.255.252
!
interface Serial1 shutdown
!
router ospf 1
network 141.108.0.0 0.0.255.255 area 0
!
line con 0 line 1 8 line aux 0 line vty 0 4
! end
Example 4-63 displays the full working configuration of Router SanFran. SanFran has a default static route pointing to Serial 1, which is the Internet connection. Under the routing OSPF process, this default route is injected by using the default-
information originate always command.
Example 4-63 Full Working Configuration of Router SanFran
hostname SanFran
!
no ip domain-lookup
!
interface Ethernet0
ip address 141.108.4.1 255.255.255.240
!
interface Serial0
ip address 141.108.255.2 255.255.255.252
!
interface Serial1 shutdown
!
router ospf 1
network 141.108.0.0 0.0.255.255 area 0 default-information originate always
!
ip route 0.0.0.0 0.0.0.0 Serial1
!
line con 0 line aux 0 line vty 0 4
! end
Review Questions
Based on the following IP routing table, answer the following questions relating to the preceding Practical Exercise on OSPF/RIP routing. Example 4-64 displays the IP routing table of Router Sydney.
Example 4-64 Sydney IP Routing Table
Sydney#show ip route
Gateway of last resort is 141.108.1.4 to network 0.0.0.0
141.108.0.0/24 is subnetted, 5 subnets
R 141.108.255.0 [120/2] via 141.108.1.4, 00:00:05, Ethernet0/0 C 141.108.1.0 is directly connected, Ethernet0/0
R 141.108.3.0 [120/1] via 141.108.1.4, 00:00:05, Ethernet0/0 R 141.108.2.0 [120/2] via 141.108.1.4, 00:00:05, Ethernet0/0 R 141.108.4.0 [120/1] via 141.108.1.4, 00:00:05, Ethernet0/0 R* 0.0.0.0/0 [120/2] via 141.108.1.4, 00:00:05, Ethernet0/0 The answers to these question can be found in Appendix C, “Answers to Review Questions.”
1: What does the routing entry shaded in Example 4-64 display?
2: In Example 4-64, what is the hop count or metric to the remote network 141.108.2.0/24?
3: What path does the packet sent to the IP subnet 171.108.255.0/24 take?
4: What type of OSPF routers are the Routers Simon, Mel, and SanFran.
5: Why are static routes injected into the router named Simon?
6: How many OSPF neighbor adjacencies do you expect to see on the router named Simon?
7: Two methods are used in OSPF to summarize IP networks. What are they and what IOS command is used to provide summarization?
8: Why does creating areas reduce the size of the OSPF database?
Summary
OSPF and integrated IS-IS have the advantage of being an industry-wide standard and have a long-term success rate of routing IP in large IP networks. The capabilities of link-state routing protocols are demonstrated in this chapter along with some challenging scenarios. Although only one solution per scenario is presented, there
are many alternative ways to enable OSPF and Integrated IS-IS to meet the needs of any network in today's large networking environments.
Table 4-8 summarizes the OSPF commands used in this chapter.
Table 4-8. Summary of IOS Commands Used in This Chapter
Command Purpose
show ip route Displays IP routing tables.
router ospf process id
Enables OSPF routing. The process ID is local to the router.
You can have more than one OSPF running.
network mask Enables network advertisements from a partic ular interface and also the routing of the same interface through OSPF.
show ip ospf Displays the OSPF process and details, such as OSPF process ID and router ID.
show ip ospf
database Displays a router's topological database.
show ip ospf
neighbor Displays OSPF neighbors.
show ip ospf virtual-links
Displays OSPF virtual links, if any.
show ip ospf
interface Displays information about how OSPF is configured for a given interface.
interface loopback
number Creates a loopback interface.
ip ospf network
point-to-point Enables a more specific route on loopback interfaces.
interface ethernet
mod/num In configuration mode, enables you modify the Ethernet, for example, interface Ethernet0/0.
interface serial
mod/num In configuration mode, enables you to modify serial interface parameters by module and interface number, for example, interface S0/0.
no ip domain-
lookup Disables automatic DNS lookup.
ip subnet-zero Enables you to use subnet zero on a Cisco router.
ip ospf name-
lookup Enables OSPF DNS lookup.
hostname name Configures a name on a router.
summary network mask
Enables summarization of external routes in OSPF.
area area id range
mask Enables interarea summarization in OSPF.
redistribute Redistributes from one IP routing protocol to another.
[no] shutdown Enables or disables an interface. All hardware interfaces are shut down by default.