network 172.16.1.0 0.0.0.255 command starts EIGRP on the Fast Ethernet 0/0 interface and allows router R1 to advertise this network. With the wildcard mask used, this command specifies that only interfaces on the 172.16.1.0/24 subnet participate in EIGRP. Note, however, that the full Class B network 172.16.0.0 will be advertised, because, by default, EIGRP automatically summarizes routes on the major network boundary. The network 192.168.1.0 command starts EIGRP on the Serial 0/0/1 interface, and allows router R1 to advertise this network. The configuration of the R2 router is shown in Figure . EIGRP is enabled in AS 100. The network 172.17.2.0 0.0.0.255 command starts EIGRP on the Fast Ethernet 0/0 interface and allows router R2 to advertise this network. With the wildcard mask used, this command specifies that only interfaces on the 172.17.2.0/24 subnet participate in EIGRP. Note, however, that the full Class B network 172.17.0.0 will be advertised, because, by default, EIGRP automatically summarizes routes on the major network boundary. The network 192.168.1.0 command starts EIGRP on the serial 0/0/1 interface and allows router R2 to advertise this network.

Content 2.3 Implementing and Verifying EIGRP 2.3.5 The show ip eigrp neighbors Command You can display the EIGRP IP neighbor table with the show ip eigrp neighbors command, as shown in Figure . This table includes the following key elements:

H (handle): A number used internally by the Cisco IOS software to track a neighbor. It records the order in which the neighbors were learned.
Address: Network-layer address of the neighbor router.
Interface: Interface on this router through which the neighbor can be reached.
Hold (hold time): Maximum time, in seconds, that the router waits to hear from the neighbor before considering the link unavailable. Originally, the expected packet was a hello packet, but in current Cisco IOS software releases, any EIGRP packets received after the first hello from that neighbor resets the timer.
Uptime: Elapsed time, in hours, minutes, and seconds, since the neighbor was added to the table.
SRTT (smoothed round-trip time): Average number of milliseconds it takes for an EIGRP packet to be sent to this neighbor and for the local router to receive an acknowledgment of that packet. This timer determines the retransmit interval, also known as the retransmission timeout (RTO).
RTO (retransmission timeout): Amount of time, in milliseconds, that the router waits for an acknowledgment before retransmitting a reliable packet from the retransmission queue to a neighbor. If an EIGRP update, query, or reply is sent, a copy of the packet is queued. If the RTO expires before an acknowledgment is received, another copy of the queued packet is sent.
Q Cnt (queue count): Number of packets waiting in the queue to be sent out. If this value is constantly higher than 0, a congestion problem might exist. A value of 0 indicates that no EIGRP packets are in the queue.
Seq Num: Sequence number of the last update, query, or reply packet that was received from this neighbor. Used to detect out-of-order packets.

Content 2.3 Implementing and Verifying EIGRP 2.3.6 The show ip route eigrp Command Figure shows the show ip route eigrp command, which displays only the EIGRP routes in the IP routing table. EIGRP supports the following route types: internal, external, and summary. EIGRP routes are identified with a D in the left column; any external EIGRP routes (from outside of this autonomous system) are identified with a D EX. After the network number, the number in brackets ([ ]) is the EIGRP metric. (The default EIGRP metric is the least cost bandwidth plus the accumulated delays.) The EIGRP metric for a network is the same as its feasible distance in the EIGRP topology table. The first number, 90 in this case, is the administrative distance, which is used to select the best path when a router learns two or more routes from different routing sources. For example, this router also uses RIP, and RIP has a route to network 172.17.0.0 that is three hops away. The router, without administrative distance, cannot compare the three hops of RIP to an EIGRP metric of 40514560. The router does not know the bandwidth associated with hops, and EIGRP does not use hop count as a metric. To correct this problem, an administrative distance is used for each routing protocol. The lower the value, the more strongly preferred the route is. By default, EIGRP internal routes have an administrative distance of 90, EIGRP external routes have an administrative distance of 170, and RIP has an administrative distance of 120. Because EIGRP has a metric based upon bandwidth and delays, it is preferred over the RIP hop count. As a result, in this example, the EIGRP route is installed in the routing table. The next field, “via 192.168.1.102” in this example, identifies the address of the next-hop router to which this router passes the packets for the destination network 172.17.0.0/16. The next-hop address in the routing table is the same as the successor in the EIGRP topology table. Each route also has the length of time, perhaps days or months, since EIGRP last advertised this network to this router. EIGRP does not refresh routes periodically. It resends the routing information only when neighbor adjacencies change. The next field in the output is the interface, serial 0/0/1 in this case, from which packets for 172.17.0.0 are sent. The routing table includes routes to null0 for the advertised routes. These routes are called summary routes and are automatically placed in the table when automatic summarization is enabled. Null0 is a directly connected, software-only interface. The null0 interface prevents the router from trying to forward traffic to other routers in search of a more precise, longer match. For example, if the router in the figure receives a packet to an unknown subnet that is part of the summarized range—172.16.3.5, for example—the packet matches the summary route based on the longest match. The packet is forwarded to the null0 interface (in other words, it is dropped or sent to the bit bucket), which prevents the router from forwarding the packet to a default route and possibly creating a routing loop.

Content 2.3 Implementing and Verifying EIGRP 2.3.7 The show ip protocols Command The show ip protocols command gives information about all dynamic routing protocols running on the router, as shown in Figure . The output displays any routing filtering occurring on EIGRP outbound or inbound updates. It also identifies whether EIGRP is generating a default network or receiving a default network in EIGRP updates. The command output provides information about additional default settings for EIGRP, such as K values, hop count, and variance. Note
Because the routers must have identical K values for EIGRP to establish an adjacency, the show ip protocols command helps to determine the current K value setting before an adjacency is attempted. This sample output also indicates that automatic summarization is enabled (the default) and that the router is allowed to load-balance over a maximum of four paths. (Up to six paths for equal-cost load balancing can be configured with the maximum-path command.) The networks for which the router is routing are also displayed. The format of the output varies, depending on the use of the wildcard mask in the network command. If a wildcard mask is used, the network address is displayed with a prefix length. If a wildcard mask is not used, the Class A, B, or C major network is displayed. The Routing Information Sources section identifies all other routers that have an EIGRP neighbor relationship with this router. The command output lists two administrative distances. The internal distance (administrative distance 90) applies to