Redistributing default route with EIGRP

[vault]

Hello
so here's something that doesn't work I can't understand why:

I have several(6) routers that share routing topologies through eigrp. Router 1 is connected to ISP(7th) router on subnet 85.0.0.0/30 and has a static default route (0.0.0.0 0.0.0.0 85.0.0.1) to it that I want redistribute to other routers.

there're several ways to do it - redistributing connected, adding network 0.0.0.0 to eigrp config and etc.
But there is a method shown in official cert guide with ip default-network [classful network] command and if got it right it should be:

ip route 0.0.0.0 0.0.0.0 85.0.0.1

int loo0
ip address 172.16.100.1 255.255.255.0

ip default-network 172.16.100.0

router eigrp 100
network 172.16.100.0

the result is that the route to 172.16.100.0 is added into routing tables of other routers but is not set there as a default route and gateway of last resort isn't set

I know that it's my fail because it doesn't work on a real equipment

[Richard Burts]

Note that the default network says that you should specify a classful network. You have specified 172.16.100.0 which is a subnet and not a classful network. what happens if you change your default network to specify 172.16.0.0?

HTH

Rick

[vault]

here's the result of sh ip route on my gateway router and one of lan routers

Gateway of last resort is not set

S* 0.0.0.0/0 [0/0], Loopback0

* 172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C* 172.16.0.0/16 is directly connected, Loopback0
L 172.16.0.100/32 is directly connected, Loopback0

and

Gateway of last resort is not set

D* 172.16.0.0/16 [90/2810368] via 172.16.0.1, 00:00:56, Serial0/0/0

[Richard Burts]

I notice that the addressing used here is significantly different from what you had in the original post. The original post had the loopback address as 172.16.100.1 while the current post has it as 172.16.0.100. It is not clear what else has changed between the two posts, especially not clear what you now have configured for your default network statement. Please clarify what you have changed.

HTH

Rick

[vault]

yeah I have changed the address to make it classful

I used 172.16.0.0/16 instead of 172.16.100.0/24 and configuration became this:

int loo0
ip address 172.16.0.100 255.255.0.0

ip default-network 172.16.0.0

router eigrp 100
network 172.16.0.0

but it doesn't work still 

[Richard Burts]

Changing the loopback address from a subnet address to a network address does not necessarily make it more classful. Changing the default network command from 172.16.100.0 to 172.16.0.0 does make it more classful.

Here is a link to a discussion about default network. I hope you find it helpful

https://www.cisco.com/c/en/us/support/docs/ip/routing-information-protocol-rip/16448-default.html

If you read carefully you will see two important things:

1) it discusses the difference in default network command of specifying a subnet vs specifying the classful network.

2) it also shows that the default network command is pointing to a network that is not on the router where the default network command is configured. So you need to point your default network to something on another router rather than to your own loopback address.

HTH

Rick

[vault]

I really appreciate your participation in my studying and don't want to bother you but if we got here then here's what I got

6 routers connected to each other using some redundant links (gigabitethernet and serial) and sharing subnets of 172.16.0.0/16, 192.168.0.0/24 and 10.0.0.0/8 networks. My 6th router (also called edge/gateway) is connected to so-called ISP router (no routing protocol configured on it) on network 85.0.0.0

So I want my routers dynamically learn a default route to my router, not to the ISP. I don't want them to know what is the network's public address so they just need to send their packets destined for the outside host to my gateway router and then it will route them to the ISP

I've figured out the problem with default-network (for some reason I needed to set the ip default-network command on each inner router). But I can't configure a default-network to a loopback interface and a default-static route to the ISP at the same time. So packets destined to the unknown address are delivered to my edge router and stop there. 

So I need to configure and advertise a default route to ISP but don't want my routers to know that address

Thanks in advance

[Richard Burts · Accepted solution]

You seem to think that what you are doing with the default network command will be advertised to the neighbor routers. But that is not the case. If you look carefully at the documentation for the command or at the link that I sent, you will see that the default network command is only acting to select a default gateway on the local router, but that result is not advertised just by using default network.

Look at what this article says about using default network with EIGRP:

Gateways of last resort selected using the ip default-network command are propagated differently depending on which routing protocol is propagating the default route. For IGRP and EIGRP to propagate the route, the network specified by the ip default-network command must be known to IGRP or EIGRP. This means the network must be an IGRP- or EIGRP-derived network in the routing table, or the static route used to generate the route to the network must be redistributed into IGRP or EIGRP, or advertised into these protocols using the network command.

So if you want this to work I would suggest that you do something like this:

- on your gateway router configure a static route for some network and specify the address of the ISP router as the next hop.

- use the default network command to identify that network reached by the static route is to be the candidate default network.

- under router eigrp either configure a network statement for the network in the static route, or redistribute static into EIGRP.

Since the original post says that the gateway router already has a static default route configured the easy thing would be to use 0.0.0.0 in the default network command and to either redistribute static or use network 0.0.0.0 in EIGRP.

I will add the observation that I extremely rarely see anyone use default network other than when they are studying networking or trying to pass some certification exam. default network does work. But it is awkward and the other methods of propagating a default network (which you mention in the original post) are easier and much more commonly used.

HTH

Rick

[vault]

Now I see that I totally missed the point

Yeah it is kind of obvious that another methods are more useful and better but this method had to be understood

Thank you for your time and help

Regards

[Richard Burts]

You are welcome. It has been an interesting discussion about something I have not thought about for quite a while. I am glad that my explanation was helpful. Good luck as you continue to learn about Cisco networking. These forums are an excellent source of information. I hope to see you continue to be active in the forums.

HTH

Rick