EIGRP Default Network and Auto Summary relation

Hi Guys,

I was practising EIGRP default-network command in my lab today and i'm stuck!

>After executing the command ip-default network my understanding was the network will be advertised to all eigrp neighbours to use as last gateway of resort.
>However when i execute the command and check for external network pings it doesn't work unless i use auto-summary command on the router executing the default network command.

I so doesn't understand this behaviour,shouldn't it be working just after we execute the ip-default network X.x.x.x[ classful] command?

Please find the logs below :

Before configuring Auto summary on R1 :

R1#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

2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback2
* 172.31.0.0/24 is subnetted, 1 subnets
C 172.31.1.0 is directly connected, Loopback1
192.168.200.0/27 is subnetted, 1 subnets
D 192.168.200.0 [90/2297856] via 192.168.100.2, 01:13:40, Serial0/0
10.0.0.0/29 is subnetted, 1 subnets
D 10.1.1.0 [90/2681856] via 192.168.100.2, 01:30:35, Serial0/0
192.168.100.0/29 is subnetted, 1 subnets
C 192.168.100.0 is directly connected, Serial0/0
D 192.168.0.0/19 [90/2809856] via 192.168.100.2, 01:15:15, Serial0/0
R1#sh run
R1#sh running-config | s eig
R1#sh running-config | s eig
router eigrp 1
network 172.31.0.0
network 192.168.0.0 0.0.255.255
no auto-summary
R1#

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

172.31.0.0/24 is subnetted, 1 subnets
D 172.31.1.0 [90/2297856] via 192.168.100.1, 00:00:46, Serial0/0
192.168.200.0/24 is variably subnetted, 6 subnets, 2 masks
C 192.168.200.0/30 is directly connected, Loopback1
D 192.168.200.0/27 is a summary, 01:14:24, Null0
C 192.168.200.4/30 is directly connected, Loopback2
C 192.168.200.8/30 is directly connected, Loopback3
C 192.168.200.12/30 is directly connected, Loopback4
C 192.168.200.16/30 is directly connected, Loopback5
10.0.0.0/29 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Serial0/1
192.168.100.0/29 is subnetted, 1 subnets
C 192.168.100.0 is directly connected, Serial0/0
D 192.168.0.0/19 [90/2297856] via 10.1.1.3, 01:15:58, Serial0/1
R2#pi 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R2#

Same goes to R3 as well like R2

After cofiguring Auto-summary on R1

R1#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 0.0.0.0 to network 172.31.0.0

2.0.0.0/32 is subnetted, 1 subnets
C 2.2.2.2 is directly connected, Loopback2
* 172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.31.1.0/24 is directly connected, Loopback1
D* 172.31.0.0/16 is a summary, 00:00:05, Null0
192.168.200.0/27 is subnetted, 1 subnets
D 192.168.200.0 [90/2297856] via 192.168.100.2, 01:16:11, Serial0/0
10.0.0.0/29 is subnetted, 1 subnets
D 10.1.1.0 [90/2681856] via 192.168.100.2, 01:33:05, Serial0/0
192.168.100.0/24 is variably subnetted, 2 subnets, 2 masks
C 192.168.100.0/29 is directly connected, Serial0/0
D 192.168.100.0/24 is a summary, 00:00:06, Null0
D 192.168.0.0/19 [90/2809856] via 192.168.100.2, 01:17:46, Serial0/0
R1#sh run
R1#sh running-config | s ei
R1#sh running-config | s ei
router eigrp 1
network 172.31.0.0
network 192.168.0.0 0.0.255.255
auto-summary
R1#

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 192.168.100.1 to network 172.31.0.0

D* 172.31.0.0/16 [90/2297856] via 192.168.100.1, 00:00:34, Serial0/0
192.168.200.0/24 is variably subnetted, 6 subnets, 2 masks
C 192.168.200.0/30 is directly connected, Loopback1
D 192.168.200.0/27 is a summary, 01:16:40, Null0
C 192.168.200.4/30 is directly connected, Loopback2
C 192.168.200.8/30 is directly connected, Loopback3
C 192.168.200.12/30 is directly connected, Loopback4
C 192.168.200.16/30 is directly connected, Loopback5
10.0.0.0/29 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, Serial0/1
192.168.100.0/29 is subnetted, 1 subnets
C 192.168.100.0 is directly connected, Serial0/0
D 192.168.0.0/19 [90/2297856] via 10.1.1.3, 01:18:15, Serial0/1
R2#pi 2.2.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 20/32/44 ms
R2#

Thanks

Default-network.jpg

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Comments

fredrikjj

It's a legacy command that's difficult to work with, but one issue might be that the prefix you are trying to use as the default network isn't in its classful form in the routing table of R1 in the first scenario. So, most likely, it has nothing to do with the auto summary per se, but rather the classful nature of the command. Auto summary simply results in a classful /16 prefix appearing the routing table of R1 in this case. So what will probably work is disabling auto-summary, but making sure that 172.31.0.0/16 is in the routing table by some other means.

PS.
I did a test:

R1 -p2p- R2. 10.0.0.0/24 between the routers. 172.31.0.0/24 as a loopback on R1.

With auto summary enabled, the routing table of R1 looks like this:

R1#
Gateway of last resort is not set

172.31.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.31.0.0/24 is directly connected, Loopback0
D 172.31.0.0/16 is a summary, 00:00:06, Null0
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.0.0.0/24 is directly connected, Serial0/0
D 10.0.0.0/8 is a summary, 00:00:06, Null0

If I issue the default network command on R1 it works:

R2#show ip route
Gateway of last resort is 10.0.0.1 to network 172.31.0.0

D* 172.31.0.0/16 [90/2297856] via 10.0.0.1, 00:00:10, Serial0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial0/0

However, if I disable auto-summary, it stops working because the /16 classful summary to null0 no longer exists in R1's routing table.

Now, if I change the subnet mask on the loopback to /16, it should work again.

R1(config-if)#do show ip route
C 172.31.0.0/16 is directly connected, Loopback
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial0/0

And it does:

R2#show ip route
Gateway of last resort is 10.0.0.1 to network 172.31.0.0

D* 172.31.0.0/16 [90/2297856] via 10.0.0.1, 00:00:54, Serial0/0
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0 is directly connected, Serial0/0

-DeXteR-

Yes! I went back to the lab and tried that and it works .

Thanks!

stlsmoore

Would it be fixed by just creating a route 172.16.0.0 /16 to null0 on R1?

-DeXteR-

Yes that will fix as well