Different Network Types and OSPF
There are three distinct classifications of physical network media that OSPF can differentiate between. Each of these network types requires a slightly different configuration to optimize the performance of OSPF. Configuring your OSPF network type is one of the most prominent features of OSPF. The strength of OSPF lies in its flexibility to meet certain network design requirements. The following sections show you how to customize OSPF to your network's design.
Cisco IOS Software allows five main network types to be configured, as displayed in Table 5-5. Each network type has predefined n timers, such as Hello/dead.
Table 5-5 OSPF over NBMA Using Cisco IOS Software
|
Method |
Description |
|
Point-to-point nonbroadcast |
Typically used for Frame Relay interfaces |
|
Point-to-point |
Default mode for subinterfaces |
|
Point-to-multipoint |
Used for multiple destinations |
|
Nonbroadcast |
NBMA mode |
|
Broadcast |
Used in Ethernet and broadcast environments where the election of DR/BDR takes place |
This section covers the methods and procedures needed to configure OSPF over different physical networks. In this case, consider the following four network types:
• Broadcast networks (Ethernet, Token Ring, FDDI)
• Nonbroadcast multiaccess networks (Switched Multimegabit Data Service [SMDS], Frame Relay, X.25, ATM)
• Point-to-multipoint networks (ATM and Frame Relay)
• Point-to-point networks (HDLC, PPP)
Configuring the Network Type
OSPF is highly adaptable. Consider its capability to accommodate either broadcast or nonbroadcast multiaccess (NBMA) network types. OSPF responds accordingly by altering its operation to reflect the configuration that you have given it.
OSPF attempts to sense the physical media and defaults to the appropriate behavior for that media. In the event that you do not want this default behavior, you can change it by using the following command:
HAL9000(config-if)#ip ospf network ?
broadcast Specify OSPF broadcast multi-access network non-broadcast Specify OSPF NBMA network point-to-multipoint Specify OSPF point-to-multipoint network point-to-point Specify OSPF point-to-point network
HAL9000(config-if)#ip ospf network ?
broadcast Specify OSPF broadcast multi-access network non-broadcast Specify OSPF NBMA network point-to-multipoint Specify OSPF point-to-multipoint network point-to-point Specify OSPF point-to-point network
This command alters the behavior of OSPF and how adjacencies are built because the underlying requirement is based on the network type.
Using this feature, you can configure broadcast networks as NBMA networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure NBMA networks to simulate a broadcast network. This feature saves you from having to configure neighbors.
Why would it be beneficial not to have a neighbor? Assume, for example, that you have a point-to-point network. By not using neighbors, you can reduce router memory and processor usage because there is only one other router to talk with. Having a simple adjacency in this scenario works well.
Keep in mind that this is a book on OSPF that details the granular level of control that OSPF affords you and how OSPF's adaptability lends itself to disparate media types. Specifically consider the following as examples:
• You have a pair of directly connected routers via their physical interfaces (with no subinterfaces). Using the frame-relay map commands you could make the link broadcast-capable; however, use the ip ospf network point-to-point command in this case. OSPF would view this as an NBMA network and form an adjacency with the opposite router.
• You can use a point-to-point subinterface and configure Frame Relay via the interface dlci command. OSPF understands this is a point-to-point link because of the subinterface type and forms an adjacency with the opposite router.
The value of having this capability becomes clear when you examine the relationship made between configuring OSPF in a myriad of environments and how those configuration choices affect OSPF's behavior as well as its capability to function when forming adjacencies. The ip ospf network type command allows you to force OSPF to behave in a manner that ignores all other concerns.
Broadcast Networks
Set an interface to operate in broadcast network mode when the network is a fully meshed. For this command to be properly applied, all routers must have virtual circuits (links) to all other routers in the network. When this command is applied to an interface, OSPF elects a DR/BDR, builds an adjacency with the DR, and builds neighbor relationships with all other OSPF routers in the network. Ethernet interfaces are considered to be broadcast-mode network interfaces by OSPF's default operation. Consider the network example shown in Figure 5-4.
278 Chapter 5: Routing Concepts and Configuration
|
DR Adjacency Database |
||
|
Router |
Neighbor |
Adjacency |
|
A |
B |
Yes |
|
A |
C |
Yes |
|
A |
□ |
Yes |
|
BDR Adjacency Database |
||
|
Router |
Neighbor |
Adjacency |
|
D |
A |
Yes |
|
D |
B |
Yes |
|
D |
C |
Yes |
Continue reading here: Area Configuration
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