Solved: Question regarding ip default-network

s.quirion · ‎10-07-2012

I am studying route summarization and am having an issue with my remote router automatically marking a route as default. Thanks in advance for any advice! Please advise if you require more settings from either router.

Local Router:

First I created a loopback with the ip 192.31.7.1

Second I added it to my EIGRP AS

Third I added the command: ip default-network 192.31.7.0

As you can see, my loopback IP is marked as exterior

sho ip eigrp top 192.31.7.0 255.255.255.0:

IP-EIGRP (AS 1): Topology entry for 192.31.7.0/24

State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128256

Routing Descriptor Blocks:

0.0.0.0 (Loopback8), from Connected, Send flag is 0x0

Composite metric is (128256/0), Route is Internal

Vector metric:

Minimum bandwidth is 10000000 Kbit

Total delay is 5000 microseconds

Reliability is 255/255

Load is 1/255

Minimum MTU is 1514

Hop count is 0

Exterior flag is set

Remote router:

show ip route:

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 5 subnets

D 172.16.104.0 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

D 172.16.105.0 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

D 172.16.106.0 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

D 172.16.102.0 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

D 172.16.103.0 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

10.0.0.0/30 is subnetted, 1 subnets

C 10.0.0.0 is directly connected, FastEthernet4

D 192.31.7.0/24 [90/156160] via 10.0.0.1, 00:10:20, FastEthernet4

D 192.168.2.0/24 [90/30720] via 10.0.0.1, 00:10:20, FastEthernet4

D 192.168.100.0/24 [90/28416] via 10.0.0.1, 00:10:20, FastEthernet4

The second router HAS learned the route via EIGRP. Am I incorrect in thinking that when the router that is advertising the default network does so, the remote router should automatically mark this route as default? As you can see there is no asterisk next to any route. Of course if I add the ip default-network command to the second router as well, it then sets the gateway of last resort. But shouldn't this happen automatically?

Thanks very much!

Steve

Peter Paluch · ‎10-07-2012

Hello Steve,

I have observed a similar behavior myself when experimenting with the ip default-network command on 15.x IOSes. While the EIGRP-learned route had the Exterior flag set, it was not marked as candidate default in the router's routing table. This is exactly what you observe here. Are you perhaps running some 15.x IOS here?

To be honest, I have never found a solution to this. In my personal experience, this is either a bug, or Cisco is silently dropping the support for ip default-network command that is itself an unfortunate remainder of past IGRP and has a number of shortcomings. Whenever I was able to, I discouraged people from using the ip default-network command - but I understand your desire to learn about its behavior so this is not the kind of advice I should be giving to you Unfortunately, I cannot give you any other advice, either. The recent IOSes simply seem to take the external flag on EIGRP-learned routes into account.

Best regards,

Peter

View solution in original post

Peter Paluch · ‎10-07-2012

Hello Steve,

I have observed a similar behavior myself when experimenting with the ip default-network command on 15.x IOSes. While the EIGRP-learned route had the Exterior flag set, it was not marked as candidate default in the router's routing table. This is exactly what you observe here. Are you perhaps running some 15.x IOS here?

To be honest, I have never found a solution to this. In my personal experience, this is either a bug, or Cisco is silently dropping the support for ip default-network command that is itself an unfortunate remainder of past IGRP and has a number of shortcomings. Whenever I was able to, I discouraged people from using the ip default-network command - but I understand your desire to learn about its behavior so this is not the kind of advice I should be giving to you Unfortunately, I cannot give you any other advice, either. The recent IOSes simply seem to take the external flag on EIGRP-learned routes into account.

Best regards,

Peter

s.quirion · ‎10-07-2012

Peter,

Thanks so much for your reply. It's nice to know I had the config correct, and it was an issue of out my hands.

In regards to my IOS I am not running:

Local: C3560 Software (C3560-ADVIPSERVICESK9-M), Version 12.2(37)SE

Remote: C870 Software (C870-ADVIPSERVICESK9-M), Version 12.4(15)T7

So not 15.x, but I am not terribly concerned with getting it to work as its just my lab. However for future reference, in your opinion, what is the best way to accomplish the task of telling other routers about default routes, without using the ip default-network command?

Thanks again,

Steve

Peter Paluch · ‎10-07-2012

Hi Steve,

You could have a look at using the default-information in EIGRP-level command that should control which neighbors will EIGRP accept the default networks from. Still, the default setting of this command should be that there are no limitations on which neighbors are allowed to send us the default routing information.

in your opinion, what is the best way to accomplish the task of telling  other routers about default routes, without using the ip default-network  command?

The best way in my opinion is to redistribute a default route 0.0.0.0/0 into EIGRP. The default route 0.0.0.0/0 is so or so the only universal way of having a gateway of last resort propagated through any routing protocol, be it RIP, EIGRP, OSPF, IS-IS or BGP. The concept of "default network" was originally invented for IGRP that was, by design, unable to carry the 0.0.0.0/0 network. However, IGRP is not supported anymore, and the 0.0.0.0/0 route is universally recognized.

Injecting a default route into EIGRP can either be done by redistributing it from a different source (a static route, for example), or in selected topologies, you may use the ip summary-address eigrp command to summarize all advertised routes into a 0.0.0.0/0 network.

Best regards,

Peter