Routing

Auto Summarization in RIP

Auto summarization is a feature which allows Routing Information Protocol (RIP) to summarize its routes to their classful networks automatically.

For example, consider we are planning to use eight subnets of class B default network 172.16.0.0/16, subnetted using a three-bit subnetting as shown below. If we do a three-bit subnetting (Cick the following link if you want to learn more on Class B Subnetting) on default Class B network 172.16.0.0/16, we will get eight subnets as shown below.

• 172.16.0.0/19
• 172.16.32.0/19
• 172.16.64.0/19
• 172.16.96.0/19
• 172.16.128.0/19
• 172.16.160.0/19
• 172.16.192.0/19
• 172.16.224.0/19

Auto Summarization in RIP

Router R1 has four subnets 172.16.0.0/19, 172.16.32.0/19, 172.16.64.0/19, and 172.16.96.0/19 and Router R2 has remaining four networks 172.16.128.0/19, 172.16.160.0/19, 172.16.192.0/19 and 172.16.224.0/19 respectivly.

After configuring Routing Information Protocol (RIP), view the routing table in R1, using the show command “show ip route” command. The output of the show command “show ip route” in R1 is copied below.

R1>enable
R1#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

C     192.168.10.0/24 is directly connected, Serial1/0
      172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C     172.16.32.0/19 is directly connected, Loopback1
C     172.16.0.0/19 is directly connected, Loopback0
R     172.16.0.0/16 [120/1] via 192.168.10.2, 00:00:05, Serial1/0
C     172.16.96.0/19 is directly connected, Loopback3
C     172.16.64.0/19 is directly connected, Loopback2
R1#

The output of the show command “show ip route” in R2 is copied below.

R2>enable
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

C    192.168.10.0/24 is directly connected, Serial1/0
     172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
C    172.16.160.0/19 is directly connected, Loopback1
C    172.16.128.0/19 is directly connected, Loopback0
C    172.16.224.0/19 is directly connected, Loopback3
C    172.16.192.0/19 is directly connected, Loopback2
R    172.16.0.0/16 [120/1] via 192.168.10.1, 00:00:02, Serial1/0
R2#

Now we can see from the Routing Table of the routers that both the routers are summarizing its subnets to default Class B unsubnetted network 172.16.0.0/16. This will create confusion in network, because both the routers R1 and R2 are advertising the same Class B network 172.16.0.0/16. Each router is advertising each other about same network.

To turn off Auto summarization feature in RIP, we can use the “no auto-summary” command from the Router Configuration mode.

Use the following steps in R1 to disable Auto summarization in R1 using “no auto-summary” command.

R1>enable
R1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R1(config)#router rip
R1(config-router)#no auto-summary
R1(config-router)#exit
R1(config)#exit
R1#

Use the following steps in R2 to disable Auto summarization in R2 using “no auto-summary” command.

R2>
R2>enable
R2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)#router rip
R2(config-router)#no auto-summary
R2(config-router)#exit
R2(config)#exit
R2#sh

After disabling Auto Summarization using “no auto-summary” command, again check the routing tables in R1 and R2 to confirn the routes are advertised individually as separate subnets, using the show command “show ip route” command as shown below.

R1#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

C    192.168.10.0/24 is directly connected, Serial1/0
     172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
R    172.16.128.0/19 [120/1] via 192.168.10.2, 00:00:00, Serial1/0
R    172.16.160.0/19 [120/1] via 192.168.10.2, 00:00:00, Serial1/0
R    172.16.192.0/19 [120/1] via 192.168.10.2, 00:00:00, Serial1/0
R    172.16.224.0/19 [120/1] via 192.168.10.2, 00:00:00, Serial1/0
C    172.16.32.0/19 is directly connected, Loopback1
C    172.16.0.0/19 is directly connected, Loopback0
C    172.16.96.0/19 is directly connected, Loopback3
C    172.16.64.0/19 is directly connected, Loopback2

 

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
C    192.168.10.0/24 is directly connected, Serial1/0
     172.16.0.0/16 is variably subnetted, 9 subnets, 2 masks
C    172.16.160.0/19 is directly connected, Loopback1
C    172.16.128.0/19 is directly connected, Loopback0
C    172.16.224.0/19 is directly connected, Loopback3
C    172.16.192.0/19 is directly connected, Loopback2
R    172.16.0.0/19 [120/1] via 192.168.10.1, 00:00:05, Serial1/0
R    172.16.32.0/19 [120/1] via 192.168.10.1, 00:00:05, Serial1/0
R    172.16.64.0/19 [120/1] via 192.168.10.1, 00:00:05, Serial1/0
R    172.16.96.0/19 [120/1] via 192.168.10.1, 00:00:05, Serial1/0
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