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Default-network with or without default route ?

Answered Question
Jan 11th, 2009
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Is it needed to set a default route, i.e:

ip route 0.0.0.0 0.0.0.0 ISP_Address or Exit_interface


before issuing a default network command:

ip default-network subnet_number_of_connection_to_ISP


Or it is sufficient enough to let only the "ip default-network ..."?

Correct Answer by Edison Ortiz about 8 years 7 months ago

Acknowledged...


I wanted to expand on what you wrote.


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Edison.

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badalam_nt Sun, 01/11/2009 - 16:34
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Could you give me some more details, as I miss the difference?

Ex:

1) If a router has an interface declared with IP@ 10.10.0.2/16 connected to another router with [email protected], what is the difference between:

a)ip route 0.0.0.0 0.0.0.0 10.10.0.1

ip default-network 10.10.0.0

and

b)ip default-network 10.10.0.0


2) If a router has no directly connected interface in the 10.10.0.0 subnet, what is the difference between the 2 cases listed above?

Edison Ortiz Sun, 01/11/2009 - 17:16
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As per your example, the second option will only generate a default route when 10.10.0.0/16 is in the routing table.


The first option is generating a default route with the Quad 0 statement (first line of the command). The second line will only generate a default route if 10.10.0.0/16 is in the routing table so you need ip route 10.10.0.0 255.255.0.0 ISP_Gateway for the second line to work as intended.


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badalam_nt Mon, 01/12/2009 - 02:09
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I hope I get it right, please correct me if I'm wrong:

1) There is no difference between the cases a) and b), the same router behavior will be observed in both cases

2) Case b) won't work as intended, i.e the default network won't be advertised nor used as a default route. However, it could work but only in case additionally there is a static route (not a default route) towards that default network defined. That is to say for point 2) if we use a default route or a static route together with the "ip default-network" won't make any difference.

Edison Ortiz Mon, 01/12/2009 - 06:10
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Basically a conditional advertisement of a default route, the default network command - that is.


It will advertise the Quad 0 to the rest of devices only when the default network is on the router's routing table.


Inserting a Quad 0 statement won't force a default network Quad 0 origination but the static Quad 0 on itself is doing it.


Keep in mind, the default network send the default route advertisement by using an IGP while the static route needs to be manually redistributed into the IGP for advertisement.


HTH,


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Edison.



badalam_nt Mon, 01/12/2009 - 10:23
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Edison, sorry I didn't really catch the exact meaning...my level is quite basic, just started to read the CCNA books.

Could you please acknowledge/deny directly the statements I made in my previous post?

Thanks.

Correct Answer
Edison Ortiz Mon, 01/12/2009 - 10:29
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Acknowledged...


I wanted to expand on what you wrote.


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Edison.

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