the most simple and conserves the most bandwidth and memory on the spoke routers. Note
Although EIGRP is a classless routing protocol, it has classful behavior by default, such as having automatic summarization on by default. When you configure the hub router to send a default route to the remote router, ensure that the ip classless command is issued on the remote router. By default, the ip classless command is enabled in all Cisco IOS images that support the EIGRP stub routing feature. The EIGRP stub routing feature prevents sending queries to the spoke router under any condition. It is highly recommended that you use both EIGRP route summarization and EIGRP stub features to provide the best scalability. Without the stub feature, a hub router sends a query to the spoke routers if a route is lost somewhere in the network. If there is a communication problem over the WAN link between the hub router and the spoke router, replies may not be received for all queries (this is known as being SIA), and the network may become unstable. Figure illustrates how using the EIGRP stub feature affects the network shown earlier. Each of the remote routers is configured as a stub. Queries for network 10.1.8.0/24 are not sent to routers C, D, or E, thus reducing the bandwidth used and the chance of the routes being SIA. Using the EIGRP stub feature at the remote sites allows the hub (regional offices) sites to immediately answer queries without propagating the queries to the remote sites, saving CPU cycles and bandwidth and lessening convergence time, even when the remote sites are dual-homed to two or more hub sites.
Content 2.6 Using EIGRP in the Enterprise 2.6.7 Configuring EIGRP Stubs Example Figure through illustrates network and configuration examples for router B when the various options of the eigrp stub command are used. In Figure , with the eigrp stub connected command, router B advertises only 10.1.2.0/24. Notice that although 10.1.3.0/24 is also a connected network, it is not advertised to router A because it is not advertised in a network command, and connected routes are not redistributed. In Figure , with the eigrp stub summary command, router B advertises only 10.1.2.0/23, the summary route that is configured on the router. In Figure , with the eigrp stub static command, router B advertises only 10.1.4.0/24, the static route that is configured on the router. In Figure , with the eigrp stub receive-only command, router B does not advertise anything.
Content 2.6 Using EIGRP in the Enterprise 2.6.8 Preventing SIA Connections A route becomes active when it goes down, or its metric worsens and there are no feasible successors. SIA-Query and SIA-Reply are two new additions to the Type, Length, Value (TLV) triplets in the EIGRP packet header. These packets are generated automatically with no configuration required, using Cisco IOS Software Release 12.1(5) and later with the active process enhancement feature. This feature enables an EIGRP router to monitor the progression of the search for a successor route and ensure that the neighbor is still reachable. Reducing the unintended termination of the neighbor adjacency results in improved network reliability. Figure illustrates what happens without the active process enhancement feature. Router A sends a query for network 10.1.1.0/24 to router B. Router B has no entry for this network, so it queries router C. If problems exist between router B and C, the reply packet from router C to router B may be delayed or lost. Router A has no visibility of downstream progress and assumes that the lack of response indicates problems with router B. After router A’s three-minute active timer expires, the neighbor relationship with router B is reset, along with all known routes from router B. By contrast, Figure illustrates what happens with the active process enhancement feature. Router A queries router B downstream (with an SIA-Query) at the midway point of the active timer (1.5 minutes by default) about the status of the route. Router B responds (with an SIA-Reply) that it is searching for a replacement route. Upon receiving this SIA-Reply response packet, router A validates the status of router B and does not terminate the neighbor relationship. Meanwhile, router B sends up to three SIA-Queries to router C. If they go unanswered, router B terminates the neighbor relationship with router C. Router B then updates router A with an SIA-Reply indicating that the network 10.1.1.0/24 is unreachable. Routers A and B remove the active route from their topology tables. The neighbor relationship between routers A and B remains intact.
Content 2.6 Using EIGRP in the Enterprise 2.6.9 Graceful Shutdown Graceful shutdown, implemented with the goodbye message feature, is designed to improve EIGRP network convergence. A graceful shutdown prevents loss of packets when routers go down. In Figure , router A is using router B as the successor for a number of routes. Router C is the feasible successor for the same routes. Router B normally would not tell router A if the EIGRP process on router B was going down; for example, if router B was being reconfigured. Router A would have to wait for its hold timer to expire before it would discover the change and react to it. Packets sent during this time would be lost. With graceful shutdown, the goodbye message is broadcast when an EIGRP routing process is shut down to inform adjacent peers about the impending topology change. This feature allows supporting EIGRP peers to synchronize and recalculate neighbor relationships more efficiently than would occur if the peers discovered the topology change after the hold timer expired. The goodbye message is supported in Cisco IOS Software Release 12.3(2), 12.3(3)B, and 12.3(2)T and later. Goodbye messages are sent in hello packets. EIGRP sends an interface goodbye message with all K values set to 255 when taking down all peers on an interface. The message in Figure is displayed when a goodbye message is received by routers that support goodbye messages. A Cisco router that does not support goodbye messages misinterprets the message as a K-value mismatch and displays the message in Figure . Note
The receipt of a goodbye message by a peer that does not support this feature does not disrupt normal network operation. The peer terminates the session when the hold timer expires. The sending and receiving routers reconverge normally after the sender reloads.
Content 2.7 EIGRP Lab Exercises 2.7.1 Lab 2-1 EIGRP Configuration, Bandwidth, and Adjacencies Lab Activity

Lab Exercise: Lab 2-1 EIGRP Configuration, Bandwidth, and Adjacencies You are responsible for configuring the new network to connect your company’s Engineering, Marketing, and Accounting departments, represented by the loopback interfaces on each of the three routers. The physical devices have just been installed and are connected by Fast Ethernet and serial cables. Your task is to configure EIGRP to enable full connectivity between all departments.
Note
It is required that the student study the commands covered in the module using the labs and the Command Reference. Not all required commands are covered in sufficient detail in the text alone. Successful completion of this course requires a thorough knowledge of command syntax and application. The Command Reference can be found on the Cisco.com website at the following URL:
http://www.cisco.com/en/US/products/
ps6441/prod_command_reference_list.html
Content 2.7 EIGRP Lab Exercises 2.7.2 Lab