Adding a Network to the Topology Table

Imagine the router (Router A) that hears a new network. The administrator has plugged in another Ethernet cable to service a department that has moved into the building.

• As soon as Router A becomes aware of the new network, it starts to send Hello packets out the new interface. No one answers because this is an access router giving connectivity to the workstations and other end devices.

• There are no new entries in the neighbor table because no neighbors have responded to the Hello protocol. There is a new entry in the topology table, however, because this is a new network.

• EIGRP, sensing a change, is obliged to send an update to all its neighbors, informing them of the new network. The sent updates are tracked in the topology table and the neighbor table because the updates are connection-oriented and the acknowledgments from the neighbors must be received within a set time frame.

Router A, having added the network to the topology table, adds the network to the routing table. The network will be marked as passive because it is operational.

• Router A's work is done. Router D's work has just begun. Router D is the backbone router in the basement of the building acting at the distribution layer. Its neighbors are routers on each floor and the routers in the other buildings.

• On hearing the update from Router A, Router D updates the sequence number in the neighbor table and adds the network to the topology table. It calculates the FD and the successor to place in the routing table. It is then in a position to send an update to all of its neighbors, except Router A. It is obeying the split horizon rule here.

In this manner, the new network is propagated to the affected routers. Figure 7-6 shows this propagation. The initial bit is set in the EIGRP header to indicate that the routes in the update represent a new neighbor relationship.

Figure 7-6 EIGRP—Updating the Topology Table with a New Router

Routing update initial bitset

Routing update initial bitset

Hello r"

Build topology table


Routing update initial bitset


Hello if

ABC Hello not connection-oriented Routing updates connection-oriented

Topology table







Destination mask


Feasible distance

Via (next logical hop)

Metric = Feasible distance/

advertised distance Outgoing interface

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