How to generate default routes in EIGRP

There are several ways to generate a default route as an EIGRP route and advertise it to other routers, as follows.

  • Redistribute the static default route to EIGRP
  • Route summarization
  • ip default-network command

In the following sections, we will explain the configuration example of route summarization.

Route summarization

【Network diagram】

Fig Network diagram
Fig Network diagram

【Condition】

  • Allow R2 to advertise the default route to R3 via EIGRP to ensure connectivity for all interfaces.
  • The default route generation is done by route summarization.

【Initial Configuration】

R1 initial configuration

interface Loopback0
 ip address 172.16.2.1 255.255.255.0 secondary
 ip address 172.16.1.1 255.255.255.0
!
interface Ethernet0/0
 ip address 192.168.12.1 255.255.255.0
!
ip route 192.168.0.0 255.255.0.0 192.168.12.2

R2 initial configuration

interface Ethernet0/0
 ip address 192.168.12.2 255.255.255.0
!
interface Ethernet0/1
 ip address 192.168.23.2 255.255.255.0
!
router eigrp 100
 network 192.168.23.0
 no auto-summary

R3 initial configuration

interface Loopback0
 ip address 192.168.3.3 255.255.255.0
!
interface Ethernet0/0
 ip address 192.168.23.3 255.255.255.0
!
router eigrp 100
 network 192.168.3.0
 network 192.168.23.0
 no auto-summary

【Step1:Advertise EIGRP route to R3】

If R2 has not advertised any routes to R3, it will not be able to generate summary routes, so include 192.168.12.0/24 in the EIGRP process on R2 so that it can advertise routes to R3. Also, designate E0/0 as a passive interface so that it will not send EIGRP packets to R1.

R2 

router eigrp 100
 network 192.168.12.0
 passive-interface Ethernet0/0

【Step2:Generate default route as summary route】

Generate a summary route on E0/1 of R2. Summarize 192.168.12.0/24 to 0.0.0.0/0 and advertise it to R3.

R2 

interface Ethernet0/1
 ip summary-address eigrp 100 0.0.0.0 0.0.0.0 5

【Step3:Verifying the default route】

Look at the routing table of R3 and verify that the default route is advertised from R2.

R3 show ip route

R3#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 192.168.23.2 to network 0.0.0.0

C    192.168.23.0/24 is directly connected, Ethernet0/0
C    192.168.3.0/24 is directly connected, Loopback0
D*   0.0.0.0/0 [90/307200] via 192.168.23.2, 00:01:06, Ethernet0/0

【Step4:Vefifying communication】

If you execute a ping from R3 to R1, you will see that no response is returned.

R3 

R3#ping 172.16.1.1 source 192.168.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.3.3 
U.U.U
Success rate is 0 percent (0/5)
R3#ping 172.16.2.1 source 192.168.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.3.3 
U.U.U
Success rate is 0 percent (0/5)

The reason why the ping from R3 to R1 fails is that the necessary route information does not exist in R2’s routing table. R3 forwards packets destined for R1 to R2 using the default route. R3 will forward them to R2 using the default route, but the packets will be discarded because the route information to route to R1 does not exist in R2’s routing table.

【Step5:Configuring default route on R2】

Configure the static default route as the route information for routing to R1 on R2.

R2 configuring default route

ip route 0.0.0.0 0.0.0.0 192.168.12.1

【Step6:Vefifying communication】

When we ping R3 to R1 again, we can confirm that the response is normal.

R3

R3#ping 172.16.1.1 source 192.168.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.1.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.3.3 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/11/24 ms
R3#ping 172.16.2.1 source 192.168.3.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.2.1, timeout is 2 seconds:
Packet sent with a source address of 192.168.3.3 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/10/16 ms

How the EIGRP works