ip default-network

Unanswered Question
Jan 15th, 2009

hi folks,

I've setup a lab with 3 routers in a hub and spoke design. R1 is the hub, R2 and R3 are spokes. IP addresses are..

X = number of router

all are /24

R1 LAN to R2 (10.0.1.X)

R1 LAN to R3 (10.0.2.X)

R1 Lo0

R1 Lo1

R2 Lo0

R3 Lo0

RIP config


router rip

version 2


no auto-summary


router rip

version 2



no auto-summary


router rip

version 2



no auto-summary

If I type in the command...

ip default-network

then R2 and R3 recieve this as their default route and on R1 it's marked as a candidate.

If I type in the command

ip default-network

then R2 and R3 don't install this as a default route and on R1 it isn't even a candidate. also on R1 the conf shows "ip route"

Can someone please tell me why I can't use Lo0 on R1 as a default route?

many thanks


I have this problem too.
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Paolo Bevilacqua Thu, 01/15/2009 - 04:39

Loopback is never a valid route, and ip default-network has no practical use in real networking.

What are you trying to accomplish ?

d.hodgson Thu, 01/15/2009 - 16:11

This is for my BSCI exam only and not for production. I'm just trying to understand why Lo1 works and Lo0 doesn't work.

d.hodgson Thu, 01/15/2009 - 17:56

sorry that's a typo in the post only, the router was configured with the correct IP's.

Paolo Bevilacqua Fri, 01/16/2009 - 03:54

R1 has no rip network statement for hence that is not included in rip updates.

d.hodgson Fri, 01/16/2009 - 06:25

yeah but R1 RIP doesn't contain either and it works fine.

Paolo Bevilacqua Fri, 01/16/2009 - 06:37

Which Lo masks are you using all over ? R2 and 3 have in rip database - network (rip) statement. Therefore they won't overwrite it when coming as default. They don't know about and are fine taking it.

d.hodgson Sat, 01/17/2009 - 02:08

they're all /24. I thought that RIP v2 used subnet masks so that R2 would know it only had and R3 would know it only had And why does it put that statement into the running config (ip route




gabrielshorn Tue, 01/20/2009 - 08:27

It doesn't put the "ip route" statement in. That had to be added by the administrator.

d.hodgson Tue, 01/20/2009 - 15:05

I'm the administrator and it did insert that statement after I added ip default-network

Paolo Bevilacqua Tue, 01/20/2009 - 15:08

Please change the mask on loopback to /32. With /24 makes routers believe they all can address it natively, and generate confusion.

gabrielshorn Fri, 01/16/2009 - 10:13

Could this be a subnet-zero issue?

Also, your next hop in the "ip route 172..." statement needs to be an interface address, not a network. It needs to have either the LAN address to R3 or R2 as the third address statement--"ip route 10.0.1.x".


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