No Auto-Summary Command on CISCO Router/Switch

Command

No Auto-Summary

Use

Prevents RIP from advertising networks as a classful boundary.

Syntax

Router(config-router)#no auto-summary

Example

Here’s the route table from R2 with auto-summary enabled.

R2#show ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route
Gateway of last resort is not set

R 1.0.0.0/8 [120/1] via 10.1.1.1, 00:00:03, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet1/0
C 10.1.1.0 is directly connected, FastEthernet0/0

Heres what happens to the routing tables when no auto-summary is enabled.

R1#conf t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#router rip
R1(config-router)#no auto-summary
R2#show ip route
Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP
D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area
N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2
E1 – OSPF external type 1, E2 – OSPF external type 2
i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2
ia – IS-IS inter area, * – candidate default, U – per-user static route
o – ODR, P – periodic downloaded static route
Gateway of last resort is not set

1.0.0.0/32 is subnetted, 3 subnets
R 1.1.1.1 [120/1] via 10.1.1.1, 00:00:02, FastEthernet0/0
R 1.3.3.3 [120/1] via 10.1.1.1, 00:00:02, FastEthernet0/0
R 1.2.2.2 [120/1] via 10.1.1.1, 00:00:02, FastEthernet0/0
2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 2 subnets
C 10.2.2.0 is directly connected, FastEthernet1/0
C 10.1.1.0 is directly connected, FastEthernet0/0

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