OSPF Network Topologies
How an OSPF protocol communicates via the Hello protocol to its neighbors depends on the physical medium being used. OSPF identiﬁes ﬁve distinct network types or technologies:
■ Broadcast multiaccess
■ Nonbroadcast multiaccess (NBMA)
■ Virtual links
The next sections describe each in more detail.
Broadcast Multiaccess Network
Broadcast multiaccess is any LAN network, such as Ethernet, Token Ring, or FDDI. In this environment, OSPF sends out multicast trafﬁc. A DR and a BDR will be elected. Figure 6-5 illustrates a broadcast multiaccess network and the designated and BDRs.
Figure 6-5 A Broadcast Multiaccess Network
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Point-to-point technology is used where there is only one other router directly connected to the transmitting or receiving router. A typical example of this is a serial line. OSPF has no need for a DR or BDR in this scenario. OSPF messaging is sent using the multicast address for AllSPFRouters, 184.108.40.206. Figure 6-6 illustrates a point-to-point network.
Point-to-multipoint is a single interface that connects to multiple destinations. The underlying network treats the network as a series of point-to-point circuits. It replicates LSA packets for each circuit. OSPF trafﬁc is sent as multicast. There is no DR or BDR election. This technology uses one IP subnet for all endpoints on the network.
Figure 6-7 illustrates a point-to-multipoint network.
Figure 6-7 Point-to-Multipoint Network
Nonbroadcast Multiaccess Network
Physically, some point-to-multipoint networks cannot support multicast or broadcast trafﬁc. In an NBMA topology, special conﬁguration is required. NBMA physically resembles a point-to-point line, but in fact, many destinations are possible. WAN clouds, including X.25 and Frame Relay, are examples of this technology. NBMA uses a fully meshed or partially meshed network. OSPF sees it as a broadcast network, and it will be represented by one IP subnet.
This technology requires manual conﬁguration of the neighbors and the DR and BDR selection. The conﬁguration options have increased with the different versions of Cisco IOS software.
DR and BDR routers are elected, and the DR will generate an LSA for the network. The DR and BDR must be directly connected to their neighbors. All network trafﬁc sent between neighbors will be replicated for each physical circuit using unicast addresses, because multicast and broadcast addresses are not understood. Figure 6-8 illustrates an NBMA network.
Figure 6-8 An NBMA Network
A virtual link is a virtual connection to a remote area that does not have any connections to the backbone (Area 0). Typically, this is because the network has become segmented. Although OSPF treats this link as a direct, single-hop connection to the backbone area, it is a virtual connection that tunnels through the network. The OSPF network trafﬁc is sent in unicast datagrams across these links.
Having discussed OSPF network topologies, including WAN technologies, the next section, “OSPF Across NBMA Networks,” discusses the NBMA topologies in more detail. Remember that the method by which the routers in an OSPF network ﬁnd one another and exchange information depends on the physical characteristics of the network.
OSPF Across NBMA Networks
An NBMA network has certain characteristics. The main ones are identiﬁed in the name of the technology: It is a network that cannot carry broadcast trafﬁc but has multiple destinations. Examples of NBMA networks include Frame Relay, X.25, and ATM.
The crux of the problem is how OSPF operates using multicast trafﬁc to exchange network information and to create adjacencies in order to synchronize databases across this WAN cloud without using the multicast addresses.
The solution to the problem varies, depending on the technology involved and the network design. The modes available fall into two technologies, within which there are additional options. The two technologies are point-to-point and NBMA.
The NBMA technology is then subdivided into two categories, under which different conﬁguration options are available. These two categories are the RFC-compliant solution and the Cisco proprietary solution, as follows:
■ RFC-compliant —The RFC-compliant category offers a standard solution, which is independent of the vendor platform. The conﬁguration options are:
■ Cisco-specific —These conﬁguration options are proprietary to Cisco and include:
— Point-to-multipoint nonbroadcast
The option you select depends on the network topology that is in use. The OSPF technology is separate from the physical conﬁguration, and the choice of implementation is based on the design topology.
The Frame Relay topologies include:
■ Full mesh —Every router is connected to every other router. This solution provides redundancy, and it might allow load sharing. This is the most expensive solution.
■ Partial mesh —Some routers are connected directly; others are accessed through another router.
■ Star, or hub and spoke —One router acts as the connection to every other router. This is the least expensive solution because it requires the fewest number of permanent virtual circuits (PVCs). A single interface is used to connect to multiple destinations.
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Choosing a Topology
Some of the considerations in choosing the OSPF topology depend on its method of updating the network and its effect on network overhead. These considerations are mentioned in RFC 1586, which suggests that the different virtual circuits have different functions, as follows:
■ A point-to-point circuit —Although no DR or BDR is required, each circuit will have an adjacency, which will create many more adjacencies on the network and will increase the need for network resources.
■ An NBMA environment —This might require a DR and a BDR, unless the underlying technology is viewed as point-to point. This is economical for most routers, requiring only two adjacencies, except for the DR and BDR. However, it might require more administration in terms of conﬁguration.
On a Cisco router, it is possible to conﬁgure a physical interface to be many logical interfaces. You can conﬁgure these subinterfaces to be point-to-point or point-to-multipoint. One of the main determining factors is the number of subnets to be used. A point-to-point interface requires its own subnet to identify it.
If you select the point-to-point option, managing the network is a little easier because the routers at each end create the adjacencies. The point-to-point option does require more network overhead and restricts some communication, in particular, the capability to indirectly connect through a hub router.
In a point-to-point network, the concept of a broadcast is not relevant because the communication is direct to another router. In a point-to-multipoint network, although OSPF simulates a broadcast environment, the network trafﬁc is replicated and sent to each neighbor.
Table 6-4 indicates the characteristics and options for each case.
Table 6-4 OSPF over NBMA
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Table 6-4 OSPF over NBMA (Continued)
All the differing characteristics of the various network topologies can be very confusing, because it is not clear which type of network corresponds to a particular physical conﬁguration. The following list clariﬁes the characteristics of the various network topologies:
■ For serial interfaces with HDLC encapsulation, the default network type is point-to-point. Timers: hello 10, dead 40.
■ For serial interfaces with Frame Relay encapsulation, the default network type is nonbroadcast. Timers: hello 30, dead 120.
■ For serial interfaces with Frame Relay encapsulation and using point-to-point subinterfaces, the default network type is point-to point. Timers: hello 10, dead 40.
■ For serial interfaces with Frame Relay encapsulation and using point-to-multipoint subinterfaces, the default network type is nonbroadcast. Timers: hello 30, dead 120.
Now that you understand the mechanism of the OSPF routing protocol, this information will be useful in understanding how to conﬁgure the protocol on a Cisco router.
TIP If OSPF is used in an environment across different vendor equipment, it should be researched and tested to ensure interoperability.
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The “Foundation Summary” section of each chapter lists the most important facts from the chapter.
Although this section does not list every fact from the chapter that will be on your exam, a wellprepared
candidate should, at a minimum, know all the details in each “Foundation Summary”
before going to take the exam.
Table 6-5 explains common OSPF terms.
Table 6-5 OSPF Terms
Foundation Summary 209
Table 6-5 OSPF Terms (Continued)
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Table 6-5 OSPF Terms (Continued)
Foundation Summary 211
Table 6-6 The Hello Packet (Continued)
Five packets are used to build the routing table for the ﬁrst time:
■ Hello protocol
■ Database descriptor
■ Link-state request
■ Link-state update
■ Link-state acknowledgement
Figure 6-9 is a ﬂowchart illustrating the updating of the topological database
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Figure 6-9 Updating the Topological Database
Foundation Summary 213
Table 6-7 indicates the characteristics and options for OSPF over NBMA.
Table 6-7 OSPF over NBMA
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As mentioned in the introduction, “All About the CCNP, CCDP, and CCIP Certiﬁcations,” you have two choices for review questions. The questions that follow next give you a bigger challenge than the exam itself by using an open-ended question format. By reviewing now with this more difﬁcult question format, you can exercise your memory better and prove your conceptual and factualknowledge of this chapter. The answers to these questions are found in Appendix A.
For more practice with examlike question formats, including questions using a router simulator and
multichoice questions, use the exam engine on the CD.
1. What information is held in the topology table?
2. What command is used to determine manually which router on a LAN will become the DR?
3. How many subnets are required in an OSPF conﬁguration over a point-to-point network that has multiple connections?
4. State the different types of packets used to build a routing table for the ﬁrst time.
5. In creating an adjacency, what is the exstart state?
6. What is the database descriptor and when is it used?
7. Explain the difference between an LSR and an LSA.
8. What packet is used to maintain the neighbor table?
9. What is the metric used by OSPF standards? Is this the same metric that Cisco uses?
10. Explain the meaning of the letters BDR.
11. What is used to elect the DR when the election is dynamic?
12. When a new router joins the OSPF network, will it learn about the rest of the OSPF network through the ﬂooding method or the exchange method?
13. If an LSA is received that is present in the OSPF database, and the receiving LSA is older than the one currently held by the router, what action is taken?
14. A router has made a neighbor relationship with another router and exchanged DDP. Having compared the routing information from its neighbor, the router realizes that its topology database is incomplete. Name the different stages or states that a router goes through to update its topology database.
15. How many equal-cost paths will Cisco enter into the routing table?
16. An LSA is received by a router, and when checked against the topology database, it ﬁnds the LSA is new or a change in the status of an existing route that has been received. What action will the receiving router take?
17. Which NBMA conﬁguration options are Cisco-speciﬁc?
18. What is the difference between a point-to-point interface and a point-to-multipoint interface?
19. What is the default network type for serial interfaces with HDLC encapsulation, and how often is the hello packet sent?
20. On a multiaccess link, what role does the BDR play?