Step 2: Configure eigrp.

1. Configure R1 and R2 to run EIGRP in autonomous system 1. On R1, add in all connected interfaces either with classful network commands or with wildcard masks. Use a classful network statement on R2 and disable automatic summarization.

R1(config)# router eigrp 1

R1(config-router)# no auto-summary

R1(config-router)# network 172.16.0.0

R1(config-router)# network 192.168.48.0

R1(config-router)# network 192.168.49.0

R1(config-router)# network 192.168.50.0

R1(config-router)# network 192.168.51.0

R1(config-router)# network 192.168.70.0

or

R1(config)# router eigrp 1

R1(config-router)# no auto-summary

R1(config-router)# network 172.16.0.0

R1(config-router)# network 192.168.0.0 0.0.255.255

R2(config)# router eigrp 1

R2(config-router)# no auto-summary

R2(config-router)# network 172.16.0.0

2. Verify the EIGRP configuration using the show ip eigrp neighbors and show ip route eigrp commands onR1.

R1# show ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(1)

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 172.16.12.2 Se0/0/0 10 00:00:22 42 2340 0 3

R1#

R1# show ip route eigrp

<Output omitted>

172.16.0.0/16 is variably subnetted, 7 subnets, 2 masks

D 172.16.2.0/24 [90/40640000] via 172.16.12.2, 00:00:31, Serial0/0/0

D 172.16.23.0/24 [90/41024000] via 172.16.12.2, 00:00:31, Serial0/0/0

D 172.16.100.0/24 [90/40640000] via 172.16.12.2, 00:00:31, Serial0/0/0

R1#

3. Verify the EIGRP configuration on R2.

R2# show ip eigrp neighbors

EIGRP-IPv4 Neighbors for AS(1)

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 172.16.12.1 Se0/0/0 11 00:04:14 35 2340 0 3

R2#

R2# show ip route eigrp

<Output omitted>

172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks

D 172.16.1.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

D 192.168.48.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

D 192.168.49.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

D 192.168.50.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

D 192.168.51.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

D 192.168.70.0/24 [90/40640000] via 172.16.12.1, 00:01:40, Serial0/0/0

R2#

4. Verify that R1 and R2 can reach all of the networks in the EIGRP routing domain using the following Tcl script.

R1# tclsh

foreach address {

172.16.1.1

192.168.48.1

192.168.49.1

192.168.50.1

192.168.51.1

192.168.70.1

172.16.12.1

172.16.12.2

172.16.2.1

} { ping $address }

All pings should be successful. Troubleshoot if necessary.

Step 3: Manually summarize with eigrp.

To make routing updates more efficient and ultimately reduce the size of routing tables, contiguous EIGRP routes can be summarized out an interface by using the ip summary-address eigrp as network maskinterface configuration command.

1. On R1, advertise one supernet route summarizing the networks of loopback 48 and 49 to R2.

R1(config)# interface Serial0/0/0

R1(config-if)# ip summary-address eigrp 1 192.168.48.0 255.255.254.0

R1(config-if)#

*Oct 26 15:46:36.839: %DUAL-5-NBRCHANGE: EIGRP-IPv4 1: Neighbor 172.16.12.2 (Serial0/0/0) is resync: summary configured

R1(config-if)# exit

R1#

2. Verify the routing table of R1 using the show ip route eigrp command.

R1# show ip route eigrp

<Output omitted>

172.16.0.0/24 is subnetted, 6 subnets

D 172.16.23.0 [90/41024000] via 172.16.12.2, 00:45:21, Serial0/0/0

D 172.16.2.0 [90/40640000] via 172.16.12.2, 00:45:21, Serial0/0/0

D 172.16.100.0 [90/40640000] via 172.16.12.2, 00:08:12, Serial0/0/0

D 192.168.48.0/23 is a summary, 04:27:07, Null0

R1#

Notice how EIGRP now has a route to the summarized address going to the Null 0 interface in the routing table.

3. Verify the specifics for the summarized routes using the show ip route 192.168.48.0 255.255.254.0command on R1.

R1# show ip route 192.168.48.0 255.255.254.0

Routing entry for 192.168.48.0/23, supernet

Known via "eigrp 1", distance 5, metric 128256, type internal

Redistributing via eigrp 1

Routing Descriptor Blocks:

* directly connected, via Null0

Route metric is 128256, traffic share count is 1

Total delay is 5000 microseconds, minimum bandwidth is 10000000 Kbit

Reliability 255/255, minimum MTU 1514 bytes

Loading 1/255, Hops 0

Notice the low administrative distance (AD) for this route. Why does EIGRP add the summarized route pointing to the Null 0 interface with a low AD?

EIGRP automatically create a summary route to the Null0 interface because it is the source of the summary. It has knowledge of all existing subnets advertised by the summary route. If it receives a packet destined to the summary route, but the route does exist, then the packet will be dropped (sent to Null0) rather than being forwarded out to another default gateway.

The Null0 interface is assigned and advertised to neighbors with an AD of 5. The low AD ensures the summary route is preferred over other routing protocols (e.g., BGP, EIGRP, and OSPF). However, routes with an AD of 4 or less (e.g., connected interfaces, static routes) would be preferred over the Null0 route.

4. Verify the routing table of R2 using the show ip route eigrp command.

R2# show ip route eigrp

<Output omitted>

172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks

D 172.16.1.0/24 [90/40640000] via 172.16.12.1, 00:09:49, Serial0/0/0

D 192.168.48.0/23 [90/40640000] via 172.16.12.1, 00:09:49, Serial0/0/0

D 192.168.50.0/24 [90/40640000] via 172.16.12.1, 00:09:49, Serial0/0/0

D 192.168.51.0/24 [90/40640000] via 172.16.12.1, 00:09:49, Serial0/0/0

D 192.168.70.0/24 [90/40640000] via 172.16.12.1, 00:09:49, Serial0/0/0

R2#

Notice how the routing table is slightly smaller as the entry to 192.168.49.0/24 is now missing. However, 192.168.49.1 is still reachable due to the summarized route to 192.168.48.0/23. Verify by pinging the loopback 49 interface from R2.

R2# ping 192.168.49.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 192.168.49.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 28/28/28 ms

R2#