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Auto Summarization in EIGRP

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Auto summarization is a feature which allows Enhanced Interior Gateway Routing Protocol (EIGRP) 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 OmniSecuR1 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 OmniSecuR2 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 Enhanced Interior Gateway Routing Protocol (EIGRP), view the routing table in OmniSecuR1, using the show command "show ip route" command. The output of the show command "show ip route" in OmniSecuR1 is copied below.

OmniSecuR1#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
D       172.16.0.0/16 is a summary, 00:00:32, Null0
C       172.16.96.0/19 is directly connected, Loopback3
C       172.16.64.0/19 is directly connected, Loopback2

The output of the show command "show ip route" in OmniSecuR2 is copied below.

OmniSecuR2#sh 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
D       172.16.0.0/16 is a summary, 00:01:27, Null0 

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 OmniSecuR1 and OmniSecuR2 are advertising the same Class B network 172.16.0.0/16. Each router is advertising each other about the same network.

To turn off Auto summarization feature in Enhanced Interior Gateway Routing Protocol (EIGRP), we can use the "no auto-summary" command from the Router Configuration mode.

Use the following steps in OmniSecuR1 to disable Auto summarization in OmniSecuR1 using "no auto-summary" command.

OmniSecuR1#configure terminal
OmniSecuR1(config)#router eigrp 65500
OmniSecuR1(config-router)#no auto-summary
OmniSecuR1(config-router)#exit
OmniSecuR1(config)#exit
OmniSecuR1#

Use the following steps in OmniSecuR2 to disable Auto summarization in OmniSecuR2 using "no auto-summary" command.

OmniSecuR2#configure terminal
OmniSecuR2(config)#router eigrp 65500
OmniSecuR2(config-router)#no auto-summary
OmniSecuR2(config-router)#exit
OmniSecuR2(config)#exit
OmniSecuR2#

After disabling Auto Summarization using "no auto-summary" command, again check the routing tables in OmniSecuR1 and OmniSecuR2 to confirn the routes are advertised individually as separate subnets, using the show command "show ip route" command as shown below.

OmniSecuR1#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
     172.16.0.0/19 is subnetted, 8 subnets
D       172.16.160.0 [90/2297856] via 192.168.10.2, 00:03:09, Serial1/0
D       172.16.128.0 [90/2297856] via 192.168.10.2, 00:03:09, Serial1/0
D       172.16.224.0 [90/2297856] via 192.168.10.2, 00:03:09, Serial1/0
D       172.16.192.0 [90/2297856] via 192.168.10.2, 00:03:09, Serial1/0
C       172.16.32.0 is directly connected, Loopback1
C       172.16.0.0 is directly connected, Loopback0
C       172.16.96.0 is directly connected, Loopback3
C       172.16.64.0 is directly connected, Loopback2

 

OmniSecuR2#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
     172.16.0.0/19 is subnetted, 8 subnets
C       172.16.160.0 is directly connected, Loopback1
C       172.16.128.0 is directly connected, Loopback0
C       172.16.224.0 is directly connected, Loopback3
C       172.16.192.0 is directly connected, Loopback2
D       172.16.32.0 [90/2297856] via 192.168.10.1, 00:00:05, Serial1/0
D       172.16.0.0 [90/2297856] via 192.168.10.1, 00:19:14, Serial1/0
D       172.16.96.0 [90/2297856] via 192.168.10.1, 00:19:14, Serial1/0
D       172.16.64.0 [90/2297856] via 192.168.10.1, 00:00:05, Serial1/0
              Jajish Thomason Google+
Related Topics
• Introduction to Static Routes and Default Routes
• How to configure Static Routes and Default Routes
• What is Dynamic Routing and different types of Dynamic Routing
• What is Routing Metric Value
• What is Convergence of Routing Tables
• Introduction to Distance Vector Routing Protocols
• Introduction to Routing Information Protocol (RIP)
• How to configure Routing Information Protocol (RIP)
• Introduction to Interior Gateway Routing Protocol (IGRP)
• How to configure Interior Gateway Routing Protocol (IGRP)
• What is Routing Loop and how to avoid Routing Loop
• Introduction to Link State Routing Protocols
• Introduction to Open Shortest Path First (OSPF) Protocol
• How to configure Open Shortest Path First (OSPF)
• Introduction to Hybrid Routing Protocols
• Introduction to Enhanced Interior Gateway Routing Protocol (EIGRP)
• Auto summarization in EIGRP
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