Hub router should be the DR no BDRs

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Cisco CCIE Prep v1.0—Module 7-27

When the network type is set to non-broadcast, which is the default on NBMA networks, OSPF operates as if it were running in a broadcast multi-access environment, such as Ethernet. Therefore, a DR and BDR are elected for the NBMA network, and the DR originates the LinkState Advertisements (LSAs) for the network. If you are operating in a full mesh environment and the broadcast keyword is specified in your Frame Relay map statements, no other configuration is needed. However, in a hub and spoke topology, OSPF neighbors must be statically configured using the neighbor command.

The ip ospf priority command must be set to 0 on all spoke routers as well to ensure that the hub router becomes the DR and there is no BDR elected. The hub router is required to become the DR, since it is the only router that has full connectivity to all other routers in the network. This restriction also requires that no BDR be elected.

For example, if one of the spokes is acting as a BDR and the hub router, which is performing the DR function, goes down, one of the spoke routers will take over as the DR. The spoke router will not have connectivity to all other routers in the network and communication will fail. Also, when the hub router comes back online it will not resume the role of the DR, since the spoke router has already attained that role and is still functioning.

Table 6-9: < ip ospf network non-broadcast > Commands

Command

Description

ip ospf network non-broadcast

The default network type for NBMA networks. Requires statically defined neighbors and the use of a DR/BDR.

Command

Description

neighbor ip address priority <0-255> poll-interval <sec> cost <l-65535>

Router configuration command that statically defines the router's OSPF neighbors.

ip address - IP address of the neighbor.

priority - (Optional) 8-bit number that indicates the priority value (used during the DR/BDR election) of the non-broadcast neighbor. The default is 0. Neighbors with no specific priority configured will assume the priority assigned to their interface that connects to the NBMA network.

poll-interval - (Optional) If a neighboring router has become inactive (hello packets have not been seen and the dead interval has elapsed), it may still be necessary to send hello packets to the dead neighbor. These hello packets will be sent at a reduced rate called the poll interval. RFC1247 recommends that this value be much larger than the hello interval. The default is 120 seconds (2 minutes).

cost - (Optional) Assigns a cost (1-65535) to the neighbor. Neighbors with no specific cost configured will assume the cost of their interface, based on the bandwidth or the ip ospf cost command.

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