Routing Protocol

Auto Summarization in EIGRP

Auto summarization is a feature which allows Enhanced Interior Gateway Routing Protocol (EIGRP) tosummarize 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 Enhanced Interior Gateway Routing Protocol (EIGRP), 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.

1#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 R2 is copied below.

R2#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 R1 and R2 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 R1 to disable Auto summarization in R1 using “no auto-summary” command.

R1#configure terminal
R1(config)#router eigrp 65500
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#configure terminal
R2(config)#router eigrp 65500
R2(config-router)#no auto-summary
R2(config-router)#exit
R2(config)#exit
R2#

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
     172.16.0.0/19 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

 

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
     172.16.0.0/19 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

Posted By – RamCruiseWalker

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