EIGRP: ip default-network command

This is usually one of the more confusing routing protocol commands supported by Cisco. Every couple of months, there is a post about it on GroupStudy or I get a question about it. Hopefully these two scenarios answer most of your questions and show you just exactly how it works!

This is our topology:

Scenario #1: R3 advertises a loopback 3.3.3.3 with a /8 mask

R2 learns 3.0.0.0/8 via EIGRP:

R2#sho ip route | be Gat
Gateway of last resort is not set
C    192.168.12.0/24 is directly connected, Serial1/0
3.0.0.0/8 is subnetted, 1 subnets
D       3.0.0.0 [90/2297856] via 192.168.23.3, 00:07:40, Serial1/1
C    192.168.23.0/24 is directly connected, Serial1/1

R2 uses the ip default-network 3.0.0.0 command to mark this as a default route. This now shows up as a “Gateway of last resort” on R2 and R1:

R2(config)#ip default-network 3.0.0.0
R2(config)#^Z
R2#sho ip route | be Gat
Gateway of last resort is 192.168.23.3 to network 3.0.0.0
C    192.168.12.0/24 is directly connected, Serial1/0
*   3.0.0.0/8 is subnetted, 1 subnets
D*      3.0.0.0 [90/2297856] via 192.168.23.3, 00:08:31, Serial1/1
C    192.168.23.0/24 is directly connected, Serial1/1

R1:

R1#sho ip route | be Gat
Gateway of last resort is 192.168.12.2 to network 3.0.0.0
C    192.168.12.0/24 is directly connected, Serial1/0
D*   3.0.0.0/8 [90/2809856] via 192.168.12.2, 00:00:19, Serial1/0
D    192.168.23.0/24 [90/2681856] via 192.168.12.2, 00:12:07, Serial1/0

When a classful network is used as the default, nothing special needs to happen to advertise the network as a candidate default. Let’s see what happens when the network specified is not classful.

Scenario #2: R3 advertises a loopback 3.3.3.3 with a /32 mask.

R2 is learning 3.3.3.3 as a host route. We try to mark it as a candidate default.

R2#sho ip route | be Gat
Gateway of last resort is not set
C    192.168.12.0/24 is directly connected, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
D       3.3.3.3 [90/2297856] via 192.168.23.3, 00:00:03, Serial1/1
C    192.168.23.0/24 is directly connected, Serial1/1
R2(config)#ip default-network 3.3.3.3

A static route for 3.0.0.0 has been automatically entered into the routing table towards 3.3.3.3!

R2#sho ip route | be Gat
Gateway of last resort is not set
C    192.168.12.0/24 is directly connected, Serial1/0
3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       3.3.3.3/32 [90/2297856] via 192.168.23.3, 00:03:06, Serial1/1
S       3.0.0.0/8 [1/0] via 3.3.3.3
C    192.168.23.0/24 is directly connected, Serial1/1

Let’s redistribute this into EIGRP.

R2:

R2(config)#router eigrp 10
R2(config-router)#red static

R1:

R1#sho ip route | be Gat
Gateway of last resort is not set
C    192.168.12.0/24 is directly connected, Serial1/0
3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       3.3.3.3/32 [90/2809856] via 192.168.12.2, 00:04:02, Serial1/0
D EX    3.0.0.0/8 [170/2681856] via 192.168.12.2, 00:02:13, Serial1/0
D    192.168.23.0/24 [90/2681856] via 192.168.12.2, 00:17:10, Serial1/0

Notice that none of the routes are marked as candidate default yet. Now we can use ip default-network 3.0.0.0 on R2:

R2(config)#ip default-network 3.0.0.0
R2#sho ip route | be Gat
Gateway of last resort is 3.3.3.3 to network 3.0.0.0
C    192.168.12.0/24 is directly connected, Serial1/0
*   3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       3.3.3.3/32 [90/2297856] via 192.168.23.3, 00:05:07, Serial1/1
S*      3.0.0.0/8 [1/0] via 3.3.3.3
C    192.168.23.0/24 is directly connected, Serial1/1

R1:

R1#sho ip route | be Gat
Gateway of last resort is 192.168.12.2 to network 3.0.0.0
C    192.168.12.0/24 is directly connected, Serial1/0
3.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
D       3.3.3.3/32 [90/2809856] via 192.168.12.2, 00:05:18, Serial1/0
D*EX    3.0.0.0/8 [170/2681856] via 192.168.12.2, 00:01:01, Serial1/0
D    192.168.23.0/24 [90/2681856] via 192.168.12.2, 00:18:27, Serial1/0

The ip default-network 3.0.0.0 had no impact on the gateway of last resort initially. We first specified 3.3.3.3 as the gateway of last resort. This command entered 3.0.0.0/8 as a static route into the route table, which was then redistributed into EIGRP on R2. We then were able to mark 3.0.0.0 as the default with ip default-network.

Summary:

The ip default-network command will only mark a classful route as a default route. If you need to mark a subnet route (3.3.3.3/32 for example) as a default, the command generates a static route for the classful network. You can then redistribute that static route into EIGRP and mark that as a default with ip default-network command.

Regards,

Bryan Bartik
CCIE #23707 (R&S, SP), CCNP
Sr. Support Engineer – IPexpert, Inc.
URL: http://www.IPexpert.com

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Tags: CCIE Routing and Switching, eigrp, eigrp topology