EIGRP manual route summarization

Posted: January 23, 2016 in CISCO

Why use route summarization:

Summarization would decrease the number of routes advertised by EIGRP. Decreasing the number of routes causes less bandwidth utilization,smaller IP routing tables, and smaller EIGRP topology tables. This can also result in less CPU utilization and less memory usage.
Summarization could prevent updates regarding flapping routes from being propagated throughout the EIGRP domain if those flapping routes fall within a summary address placed at a critical point in the network (usually as close to the source as possible).
Summarization limits the depth of the network into which a query is propagated. Because upstream routers know only about a summary route and not about its individual components, they immediately respond with an infinite metric to any query about component routes without propagating the query further. This helps to limit the scope of diffusing computation and prevent the stuck-in-active states.

Summarization,however,can cause suboptimal route (when router don’t take shortest route to destination).Summarization can also cause that packet destined for inaccessible destination will flow to the summarizing router when it will be discarded.

Untitled

R1:

!
interface Serial1/0
ip address 192.168.100.1 255.255.255.248

!
interface Serial1/1
no ip address

!
router eigrp 100
network 172.31.1.0 0.0.0.255
network 192.168.100.0

 

R2:

interface Loopback1
ip address 192.168.200.1 255.255.255.252
!
interface Loopback5
ip address 192.168.200.5 255.255.255.252
!
interface Loopback9
ip address 192.168.200.9 255.255.255.252
!
interface Loopback13
ip address 192.168.200.13 255.255.255.252
!
interface Loopback17
ip address 192.168.200.17 255.255.255.252
!
interface Loopback21
ip address 192.168.200.21 255.255.255.252
ip summary-address eigrp 100 192.168.200.0 255.255.255.224
!
interface Loopback25
ip address 192.168.200.25 255.255.255.252
!
interface Serial1/0
ip address 192.168.100.2 255.255.255.248
!
interface Serial1/1
ip address 10.1.1.2 255.255.255.248
!
router eigrp 100
network 10.1.1.0 0.0.0.255
network 192.168.100.0
network 192.168.200.0

 

R3:

!
interface Loopback1
ip address 192.168.1.1 255.255.254.0
!
interface Loopback5
ip address 192.168.5.5 255.255.254.0
!
interface Loopback9
ip address 192.168.9.9 255.255.254.0
!
interface Loopback13
ip address 192.168.13.13 255.255.254.0
!
interface Loopback17
ip address 192.168.17.17 255.255.254.0
!
interface Loopback21
ip address 192.168.21.21 255.255.254.0
!
interface Loopback25
ip address 192.168.25.25 255.255.254.0
!
interface Loopback100
ip address 10.1.3.1 255.255.255.252
!
interface Loopback172
ip address 172.16.1.1 255.255.255.0
!
interface Serial1/0
ip address 10.1.1.3 255.255.255.248
!
router eigrp 100
network 10.1.1.0 0.0.0.255
network 10.1.3.0 0.0.0.255
network 172.16.0.0
network 192.168.0.0 0.0.31.255

 

Routing table on R1 before summarization

R1#sh ip route
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

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.1.1.0/29 [90/2681856] via 192.168.100.2, 02:08:59, Serial1/0
D 10.1.3.0/30 [90/2809856] via 192.168.100.2, 02:08:59, Serial1/0
172.16.0.0/24 is subnetted, 1 subnets
D 172.16.1.0 [90/2809856] via 192.168.100.2, 02:10:12, Serial1/0
172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.31.1.0/24 is directly connected, Loopback0
L 172.31.1.1/32 is directly connected, Loopback0
D 192.168.0.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.4.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.8.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.12.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.16.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.20.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
D 192.168.24.0/23 [90/2809856] via 192.168.100.2, 00:01:52, Serial1/0
192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.100.0/29 is directly connected, Serial1/0
L 192.168.100.1/32 is directly connected, Serial1/0
192.168.200.0/30 is subnetted, 7 subnets
D 192.168.200.0 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.4 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.8 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.12 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.16 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.20 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0
D 192.168.200.24 [90/2297856] via 192.168.100.2, 00:00:15, Serial1/0

In this example we’ll sumarize 192.168. network in R3

first 16 bits are the same for all networks.3rd octet is not the same (1,5,9,13,17,21,25)

128      64       32       16       8        4         2        1           1

0          0         0         0       0        0        0        1           5

0          0         0         0       1         0         0        1           9

0          0         0         0       1          1         0        1           13

0          0         0         1        0         0         0       1            17

0          0         0         1        0          1         0        1           21

0          0         0         1        1          0         0         1           25

3 bits in 3rd octets are the same,adding this 3 bits to first two octets gives 19 (8+8+3=19)

Faster method:

larger number in 3rd octet is 27,starting from left to right,the larger number just before 27 is 16,so 3 bits lefts off (128,64,32)

128      64       32       16       8        4         2        1

1           1         1          0        0        0         0       0

128+64+32=224  so summary subnet mask will be 255.255.224.0   (first two octets are the same (255.255) and third is 224 (128+64+32)

R3(config-router)#int s1/0
R3(config-if)#ip summary-address eigrp 100 192.168.0.0 255.255.224.0

 

In the same way,we can summarize 192.168.200.0 network on R2

This time,first three octets are the same (192.168.200),because subnet numbers in 4-th octets are the same (1,5,9,13,17,21,25) subnet mask would be 255.255.255.224 (8+8+8+3=27)

Because R2 has 2 interfaces,summarization should be set on s1/0 and s1/0

R4(config-router)#int s1/0
R3(config-if)#ip summary-address eigrp 100 192.168.200.0 255.255.255.224

R4(config-router)#int s1/1
R3(config-if)#ip summary-address eigrp 100 192.168.200.0 255.255.255.224

 

 

Routing table after summarization:

R1(config-router)#do sh ip route
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

10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D 10.1.1.0/29 [90/2681856] via 192.168.100.2, 02:05:56, Serial1/0
D 10.1.3.0/30 [90/2809856] via 192.168.100.2, 02:05:56, Serial1/0
172.16.0.0/24 is subnetted, 1 subnets
D 172.16.1.0 [90/2809856] via 192.168.100.2, 02:07:09, Serial1/0
172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.31.1.0/24 is directly connected, Loopback0
L 172.31.1.1/32 is directly connected, Loopback0
D 192.168.0.0/19 [90/2809856] via 192.168.100.2, 01:02:50, Serial1/0
192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.100.0/29 is directly connected, Serial1/0
192.168.200.0/24 is variably subnetted, 2 subnets, 2 masks
D 192.168.200.0/24 [90/2297856] via 192.168.100.2, 00:09:44, Serial1/0

We can see that routing table is now significant smaller,and summary routes are adverised

 

 

 

 

 

 

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