For this objective we need to summarise 172.30.0.0 172.30.7.255 into a single network, we cant include 172.30.8.0/24 as the summarisation will not be as efficient
1 1 1 1 1 5 bits needed
128 64 32 16 8 4 2 1
8 8 5 = bits
11111111.11111111.11111000.0------- = 248
mask = 255.255.248.0 /21 in CIDR notation
Routing table BEFORE
R2>show ip route
Gateway of last resort is 10.1.2.3 to network 192.168.1.0
172.30.0.0/24 is subnetted, 9 subnets
D 172.30.2.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.3.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.0.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.1.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.6.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.7.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.4.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.5.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
D 172.30.8.0 [90/10665472] via 10.1.2.3, 01:08:09, Ethernet0/0
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
C 10.1.2.0/24 is directly connected, Ethernet0/0
C 10.1.24.1/32 is directly connected, Serial0/0
C 10.1.25.0/24 is directly connected, Loopback2
C 10.1.24.0/30 is directly connected, Serial0/0
D 10.1.34.0/30 [90/10537472] via 10.1.2.3, 01:08:12, Ethernet0/0
D* 192.168.1.0/24 [90/10537472] via 10.1.2.3, 01:08:09, Ethernet0/0
BB-Router(config)#int ser0/0
BB-Router(config-if)#ip summary-address eigrp 90 172.30.0.0 255.255.248.0
R2>show ip route
Gateway of last resort is 10.1.2.3 to network 192.168.1.0
172.30.0.0/16 is variably subnetted, 10 subnets, 2 masks
D 172.30.2.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.3.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.0.0/21 [90/20640000] via 10.1.24.1, 00:00:15, Serial0/0 <---- there it is
D 172.30.0.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.1.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.6.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.7.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.4.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.5.0/24 [90/10665472] via 10.1.2.3, 01:10:11, Ethernet0/0
D 172.30.8.0/24 [90/10665472] via 10.1.2.3, 00:00:15, Ethernet0/0
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
C 10.1.2.0/24 is directly connected, Ethernet0/0
C 10.1.24.1/32 is directly connected, Serial0/0
C 10.1.25.0/24 is directly connected, Loopback2
C 10.1.24.0/30 is directly connected, Serial0/0
D 10.1.34.0/30 [90/10537472] via 10.1.2.3, 00:00:15, Ethernet0/0
D* 192.168.1.0/24 [90/10537472] via 10.1.2.3, 00:00:15, Ethernet0/0
We can see the new summary route taking affect, but R2 is still learning about this network from 10.1.2.3 (R3) REMEMBER regardless of Admin distance or speed of link, if a router has a more SPECIFIC mask for a network, it will use that route!!!
lets fix that;
BB-Router(config)#int serial0/1
BB-Router(config-if)#ip summary-address eigrp 90 172.30.0.0 255.255.248.0
R2>show ip route
172.30.0.0/16 is variably subnetted, 2 subnets, 2 masks
D 172.30.0.0/21 [90/10665472] via 10.1.2.3, 00:14:38, Ethernet0/0 <-- summary
D 172.30.8.0/24 [90/10665472] via 10.1.2.3, 00:14:38, Ethernet0/0 <--
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
C 10.1.2.0/24 is directly connected, Ethernet0/0
C 10.1.24.1/32 is directly connected, Serial0/0
C 10.1.25.0/24 is directly connected, Loopback2
C 10.1.24.0/30 is directly connected, Serial0/0
D 10.1.34.0/30 [90/10537472] via 10.1.2.3, 00:16:07, Ethernet0/0
D* 192.168.1.0/24 [90/10537472] via 10.1.2.3, 00:14:38, Ethernet0/0
Excellent! notice we have the 172.30.8.0/24 route too, this is becuase the summary is only advertising; 172.30.0.0 --> 172.30.7.255 and the network statement on EIGRP catches the rest of the 172 network;
router eigrp 90
passive-interface default
no passive-interface Serial0/0
no passive-interface Serial0/1
network 10.1.0.0 0.0.255.255
network 172.30.0.0
network 192.168.1.0
no auto-summary
Job done
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