ip default-network command

Hi,

Im a bit baffled by the use of the "ip default-network <network>" command.
I know it sets the gateway of last resort, and that different routing protocols treat it differently (propagates it, versus not propagating it). But what im a bit confused about is Cisco's documentation on using this command on a non-classful network.

Suppose I have this scenario:

ISP <-- 172.16.1.0/24 --> Border-Router <-- --> Internal-Router.

According to Cisco's documentation, to accomplish a default route on the Border-Router, i must:

ip default-network 172.16.1.0

This will create a static route: 172.16.0.0/16 to 172.16.1.0

Then i must do an: ip default-network 172.16.0.0

This will work fine on the border-router, but it wont propagate (Ive tried with RIP and EIGRP).

I can do a static route and distribute it, or i can summarize on the interface, but can you not get this command to work on non-classful networks?

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Comments

gojericho0

jungle,

have you checked out this link?

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

Plazma

I believe a static 0.0.0.0 route will trump an ip default-network route. Essentially all ip default-network does is allow you to pick a default route to an already learned route in the routing table.

More info here:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

telco1

gateway of last resorts are configured differently depending on the routing protocol.

the configs must be put on the ASBR in order to propagate down to the other devices.

with EIGRP you would use three statements.

the ip default-network statement (for devices with Ip routing already enabled)
an ip route statement.
and the third statement is a redistribute static.

with OSPF you use two statements.

an ip route statement.

and a default information originate always.

for ISIS-
one ip route statement
and one default information originate statement.

this will propagate a candidate default route AKA gateway of last resort.

kpjungle

gojericho0 wrote:

jungle,

have you checked out this link?

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094374.shtml

Yeah, thats the link i used as my primary source.

Im just wondering how much use this command is for propagating a default route, as compared to other ways mentioned, since it doesnt seem possible to propagate a non-classful route, no matter what IGP you use.

networker050184

kpjungle wrote:

Im just wondering how much use this command is for propagating a default route, as compared to other ways mentioned, since it doesnt seem possible to propagate a non-classful route, no matter what IGP you use.

I have only actually seen this used once in a live network. Most people just use a static default route. Its much easier to administer a network with a recognizable default route such as a static default.

LBC90805

networker050184 wrote:

kpjungle wrote:

Im just wondering how much use this command is for propagating a default route, as compared to other ways mentioned, since it doesnt seem possible to propagate a non-classful route, no matter what IGP you use.

I have only actually seen this used once in a live network. Most people just use a static default route. Its much easier to administer a network with a recognizable default route such as a static default.

What if you have thousands of Subnets and must summarize routes across the entire U eS of eH?

networker050184

That is what summarization is for. It has nothing to do with a default route.....