EIGRP Automatic vs. Manual Summarization

Concepts:

EIGRP has two ways of summarizing networks

1. Automatic summarization:

a. Subnets are summarized to the classful network
b. This is the default for EIGRP.

2. Manual summarization.

Command ( Interface Mode): ip summary-address eigrp

Purposed of Routes Summarizations:

1. Reduced the size of routing table
2. Less routing updates

MANILA Router EIGRP Configurations:

R1#config t
Enter configuration commands, one per line. End with CNTL/Z.
R1(config)#hostname MANILA
MANILA(config)#int s3/0
MANILA(config-if)#ip add 192.168.12.1 255.255.255.0
MANILA(config-if)#no shut
MANILA(config-if)#exit

MANILA(config)#int lo1
MANILA(config-if)#ip address 100.100.1.1 255.255.255.0
MANILA(config-if)#int lo2
MANILA(config-if)#ip address 100.100.2.1 255.255.255.0
MANILA(config-if)#int lo3
MANILA(config-if)#ip address 100.100.3.1 255.255.255.0
MANILA(config-if)#int lo4
MANILA(config-if)#ip address 100.100.4.1 255.255.255.0
MANILA(config-if)#^Z

MANILA(config)#router eigrp 12
MANILA(config-router)#network 192.168.12.0
MANILA(config-router)#network 100.100.1.0 0.0.0.255
MANILA(config-router)#network 100.100.2.0 0.0.0.255
MANILA(config-router)#network 100.100.3.0 0.0.0.255
MANILA(config-router)#network 100.100.4.0 0.0.0.255
MANILA(config-router)#no auto-summary
MANILA(config-router)#exit

MELBOURNE Router EIGRP Configurations:

R2#config t
Enter configuration commands, one per line. End with CNTL/Z.
R2(config)#hostname MELBOURNE
MELBOURNE(config)#int s3/0
MELBOURNE(config-if)#ip address 192.168.12.2 255.255.255.0
MELBOURNE(config-if)#no shut
MELBOURNE(config-if)#exit
MELBOURNE(config)#

MELBOURNE(config)#int lo1
MELBOURNE(config-if)#ip address 200.200.1.1 255.255.255.0
MELBOURNE(config-if)#int lo2
MELBOURNE(config-if)#ip address 200.200.2.1 255.255.255.0
MELBOURNE(config-if)#int lo3
MELBOURNE(config-if)#ip address 200.200.3.1 255.255.255.0
MELBOURNE(config-if)#int lo4
MELBOURNE(config-if)#ip address 200.200.4.1 255.255.255.0
MELBOURNE(config-if)#exit

MELBOURNE(config)#router eigrp 12
MELBOURNE(config-router)#network 192.168.12.0
MELBOURNE(config-router)#

As observed the moment, I enabled the network address from which both the routers are using for the exchange of EIGRP Hello packets, EIGRP adjacency were formed.

*Jul 12 11:15:41.023: %DUAL-5-NBRCHANGE: EIGRP-IPv4 12: Neighbor 192.168.12.1 (Serial3/0) is up: new adjacency

Now, adding the remaining segments from the MELBOURNE router.

MELBOURNE(config-router)#network 200.200.1.0 0.0.0.255
MELBOURNE(config-router)#network 200.200.2.0 0.0.0.255
MELBOURNE(config-router)#network 200.200.3.0 0.0.0.255
MELBOURNE(config-router)#network 200.200.4.0 0.0.0.255

VERIFICATIONS OF EIGRP CONFIGURATIONS:

Below is the results showing the classless summarized routes advertise by MANILA Router to MELBOURNE router.

MELBOURNE#sh ip route eigrp
Codes: L – local, 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, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      100.0.0.0/24 is subnetted, 4 subnets
D        100.100.1.0 [90/2297856] via 192.168.12.1, 00:15:52, Serial3/0
D        100.100.2.0 [90/2297856] via 192.168.12.1, 00:15:52, Serial3/0
D        100.100.3.0 [90/2297856] via 192.168.12.1, 00:15:52, Serial3/0
D        100.100.4.0 [90/2297856] via 192.168.12.1, 00:15:52, Serial3/0

Similarly, MELBOURNE is advertising the following summarized classles routes to MANILA router.

MANILA#sh ip route eigrp
Codes: L – local, 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, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

D     200.200.1.0/24 [90/2297856] via 192.168.12.2, 00:18:16, Serial3/0
D     200.200.2.0/24 [90/2297856] via 192.168.12.2, 00:18:12, Serial3/0
D     200.200.3.0/24 [90/2297856] via 192.168.12.2, 00:18:08, Serial3/0
D     200.200.4.0/24 [90/2297856] via 192.168.12.2, 00:18:05, Serial3/0

Now let’s manually summarized the routes advertised by MANILA router towards MELBOURNE Router.

DECIMAL BINARY

                 100.100.1.0/24              01100100.01100100.00000001.00000000
   100.100.2.0/24              01100100.01100100.00000010.00000000
   100.100.3.0/24              01100100.01100100.00000011.00000000

100.100.4.0/24 01100100.01100100.00000100.00000000

As seen above, I have two subnets summary for the routes published by MANILA Router as follows:

100.100.0.0/22 & 100.100.4.0/22

Configuring Manual Summarization on MANILA Router,

MANILA#config t
Enter configuration commands, one per line. End with CNTL/Z.
MANILA(config)#int s3/0
MANILA(config-if)#ip summary-address eigrp 12 100.100.0.0 255.255.252.0

*Jul 12 12:12:31.167: %DUAL-5-NBRCHANGE: EIGRP-IPv4 12: Neighbor 192.168.12.2 (Serial3/0) is resync: summary configured

MANILA(config-if)#ip summary-address eigrp 12 100.100.4.0 255.255.252.0
MANILA(config-if)#^Z

*Jul 12 12:12:39.431: %DUAL-5-NBRCHANGE: EIGRP-IPv4 12: Neighbor 192.168.12.2 (Serial3/0) is resync: summary configured

As observed above, the EIGRP neighbors were syncing the changes on the routes.

From MELBOURNE router. the two network segments advertise from the MANILA router is received.

Gateway of last resort is not set

      100.0.0.0/22 is subnetted, 2 subnets
D        100.100.0.0 [90/2297856] via 192.168.12.1, 00:00:25, Serial3/0
D        100.100.4.0 [90/2297856] via 192.168.12.1, 00:00:17, Serial3/0

The routing table of MANILA router for the two subnets are sent to the Null Interface for such reasons as to to AVOID routing loops.

MANILA#show ip route eigrp
Codes: L – local, 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, H – NHRP, l – LISP
       + – replicated route, % – next hop override

Gateway of last resort is not set

      100.0.0.0/8 is variably subnetted, 10 subnets, 3 masks
D        100.100.0.0/22 is a summary, 00:00:47, Null0
D        100.100.4.0/22 is a summary, 00:00:38, Null0
D     200.200.1.0/24 [90/2297856] via 192.168.12.2, 00:57:24, Serial3/0
D     200.200.2.0/24 [90/2297856] via 192.168.12.2, 00:57:20, Serial3/0
D     200.200.3.0/24 [90/2297856] via 192.168.12.2, 00:57:16, Serial3/0
D     200.200.4.0/24 [90/2297856] via 192.168.12.2, 00:57:13, Serial3/0

NETWORK LESSON