Case Study Adding a New OSPF Router to a Network
This case study provides a scenario that covers most of the information presented in this chapter. Suppose that a new OSPF router is added to a network. With this scenario, follow the case study to understand the ramifications of how adding a new OSPF router would affect an operating network. Refer to Figures 2-12 through 2-15, which detail each step of the process as it occurs in the following sequence:
1 A new OSPF router is added to the network.
2 This new router immediately transmits a multicast Hello packet by using the all-OSPF routers multicast address of 224.0.0.5. At this point, the router does not know if there is a DR (see Figure 2-12).
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Figure 2-12 Adding a New Router
- New Router
TIP If a DR already exists on a network, a new router, even one with a better priority, does not take over as the DR.
3 The DR and BDR respond to the new router with a unicast Hello packet that is specifically addressed to it. This begins the process of building an adjacency (see Figure 2-13).
Figure 2-13 DR and BDR Respond
Designated Router Backup Designated Router DR BDR
Unicast Hello
Designated Router Backup Designated Router DR BDR
Unicast Hello i f t
New Router
New Router
4 After the adjacency has been established with the DR, the new router sends a router LSA (Type 1) to the DR describing the specific links it has available and the status of these links. During this time, the BDR is also listening to see if the DR responds, thus proving that the BDR is operational (see Figure 2-14).
Figure 2-14 Router LSA Is Sent
DR BDR
DR BDR
5 The BDR continues to listen, thereby ensuring that the DR is operating. The DR sends out a multicast (224.0.0.5) network LSA to the entire network, informing them of the new routes available as a result of the new router. All routers must then respond with an acknowledgment so that the DR knows that they have received the new information (see Figure 2-15). If the DR fails, the BDR becomes the DR, and another election takes place to determine the new BDR.
Figure 2-15 Network LSA Is Sent
DR BDR
DR BDR
New Router
Network
LSA and acks back to DR
New Router
Network
LSA and acks back to DR
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At this point, the new router has been fully identified to all other routers within the network. This information also includes any networks the new router can reach.
In certain OSPF networks, you want to force the determination of which router is to be the OSPF DR and BDR. For example, perhaps you have some routers that have larger processors and more memory, in which case you would want to make them the DRs. In addition, perhaps you are preparing for a certification lab test. In these cases, you use a command, applied on a per-interface basis, known as ip ospf priority.
The default value is 1, so the higher the router priority, the more likely it is to become the DR or BDR. This command can also be used to guarantee that a router does not become the DR by setting the priority to zero.
NOTE Router priority is configured only for interfaces to multiaccess networks (in other words, not for point-to-point networks).
For example, to set the router priority value to 5, you enter the following commands:
interface ethernet 0 ip ospf priority 5
It is important to understand that the order in which routers join an OSPF area can have an effect on which routers are elected as the DR and BDR. Specifically, an election is necessary only when a DR or BDR does not exist in the network. As a router starts its OSPF process, it checks the network for an active DR and BDR. If they exist, the new router becomes a DR other; in other words, it does not become the DR or the BDR, regardless of its priority or router ID. Remember, the roles of DR and BDR were created for efficiency; new routers in the network should not force an election when roles are already optimized.
Continue reading here: Case Study Developing the Link State Database
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