propagation of ip default-network in eigrp

[stefanmarold]

Hi all,

I'm studying for my Route exam and have big problems with the 'ip default-network' command. I read the following link that describes the command very well: http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

I build the topology shown in the above link and it works well. Then I deleted the static route on router 2513 and removed the 'ip default-network' command. I configured RIP on all routers and issued the 'ip default-network 198.10.1.0' command on the upper right router. After that the default route is installed on the 2513 router.

But when I configure EIGRP instead of RIP, the default route on 2513 is not installed:

Gateway of last resort is not set

      131.108.0.0/16 is variably subnetted, 2 subnets, 2 masks

C        131.108.99.0/24 is directly connected, FastEthernet1/1

L        131.108.99.1/32 is directly connected, FastEthernet1/1

      161.44.0.0/16 is variably subnetted, 2 subnets, 2 masks

C        161.44.192.0/24 is directly connected, FastEthernet1/0

L        161.44.192.1/32 is directly connected, FastEthernet1/0

      171.70.0.0/24 is subnetted, 1 subnets

D        171.70.24.0 [90/156160] via 131.108.99.2, 00:52:02, FastEthernet1/1

D*    198.10.1.0/24 [90/156160] via 161.44.192.2, 00:52:02, FastEthernet1/0

Network 198.10.1.0/24 is flagged as default network. I would expect the router to install a default route now but it does not. Can anyone explain where the problem is? I know the experts in this forum are recommanding to not use the 'ip default-network' command. But it is included in the Route exam so I try to understand it.

regards

Stefan

[Peter Paluch · Accepted solution]

Hello Stefan,

According to your show ip route output, you seem to be running a 15.x version IOS on one of your routers, am I correct? I am judging based on the 'L' entries.

It has been my observation, too, that in these recent IOS versions, the default network may be flagged as a candidate default but it is not really being used as a default network, exactly as you have pointed out here: the router mandates that "Gateway of last resort is not set" although there is a candidate default network present in its routing table. I consider this to be a bug - or perhaps a silent attempt to phase out the concept of the default network entirely. Nevertheless, my first take on this is that this is a bug, and I confirm that I have experienced the same behavior. You do not appear to have anything configured wrong.

Are you able to test this using older IOS versions, preferably 12.4 or 12.4T?

I am, by the way, strongly surprised to see you are talking about 2500 seres routers. I do not believe that the show ip route was produced on such a router - 2500 series routers never had 100Mbps FastEthernet interfaces. You could theoretically try to "reverse" your routers so that it is the older, not the newer, router receiving the candidate default.

Best regards,

Peter

[stefanmarold]

Hello Peter,

thanks a lot for the clarification. You're right, other than in the documentation I used 7200er routers with IOS 15.0(1)M under the GNS3 emulator. I will look for an older IOS and post the results later.

Best regards,

Stefan

[stefanmarold]

Hello Peter,

now I've tested this scenario with IOS 12.4(22)T. With this release it works as expected:

Gateway of last resort is 161.44.192.2 to network 198.10.1.0

D    171.70.0.0/16 [90/156160] via 131.108.99.2, 00:04:34, FastEthernet1/1

     161.44.0.0/16 is variably subnetted, 2 subnets, 2 masks

C       161.44.192.0/24 is directly connected, FastEthernet1/0

D       161.44.0.0/16 is a summary, 00:06:56, Null0

     131.108.0.0/16 is variably subnetted, 2 subnets, 2 masks

C       131.108.99.0/24 is directly connected, FastEthernet1/1

D       131.108.0.0/16 is a summary, 00:06:56, Null0

D*   198.10.1.0/24 [90/156160] via 161.44.192.2, 00:05:35, FastEthernet1/0

Thank you again for posting the solution. You helped me very much.

Best regards,

Stefan