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Configuring OSPF Routing

Introduction to OSPF

MultiVoice Gateway for the MAX— User’s Guide Preliminary November 23, 1998 9-5

dedicate the MultiVoice Gateway to WAN processing. The administrator chooses a DR and

BDR on the basis of the device’s processing power and reliability.

To reduce the number of adjacencies each router must form, OSPF calls one of the routers the

designated router. A designated router is elected as routers are forming adjacencies, and then

all other routers establish adjacencies only with the designated router. This simplifies the

routing table update procedure and reduces the number of link-state records in the database.

The designated router plays other important roles as well, to reduce the overhead of OSPF

link-state procedures. For example, other routers send link-state advertisements to the

designated router only by using the all-designated-routers multicast address of 224.0.0.6.

To prevent the designated router from becoming a serious liability to the network if it fails,

OSPF also elects a backup designated router at the same time. Other routers maintain

adjacencies with both the designated router and its backup router, but the backup router leaves

as many of the processing tasks as possible to the designated router. If the designated router

fails, the backup immediately becomes the designated router and a new backup is elected.

The administrator chooses which router is to be the designated router on the basis of the

processing power, speed, and memory of the system, and then assigns priorities to other routers

on the network in case the backup designated router is also down at the same time.

Configurable metrics

The administrator assigns a cost to the output side of each router interface. The lower the cost,

the more likely the interface is to be used to forward data traffic. Costs can also be associated

with the externally derived routing data.

The OSPF cost can also be used for preferred path selection. If two paths to a destination have

equal costs, you can assign a higher cost to one of the paths, to configure it as a backup to be

used only when the primary path is not available.

Figure 9-4 shows how costs are used to direct traffic over high-speed links. For example, if

Router-2 in Figure 9-4 receives packets destined for Host B, it routes them through Router-1

across two T1 links (Cost=20) rather than across one 56Kbps B channel to Router-3

(Cost=240).

Figure 9-4. OSPF costs for different types of links

The MultiVoice Gateway has a default cost of 1 for a connected route (Ethernet) and 10 for a

WAN link. If you have two paths to the same destination, the one with the lower cost will be

used. You might want to account for the bandwidth of a connection when assigning costs. For

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Ascend MultiVoice Gateway Guide de l'utilisateur Page 167