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Community Member

ip default-network

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 172.16.0.1

R1 Lo1 192.168.0.1

R2 Lo0 172.16.1.2

R3 Lo0 172.16.2.3

RIP config

R1

router rip

version 2

network 10.0.0.0

no auto-summary

R2

router rip

version 2

network 10.0.0.0

network 172.16.0.0

no auto-summary

R3

router rip

version 2

network 10.0.0.0

network 172.16.0.0

no auto-summary

If I type in the command...

ip default-network 192.168.0.0

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 172.168.0.0

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 172.16.0.0 255.255.0.0 172.16.2.0"

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

many thanks

Dave

13 REPLIES
Hall of Fame Super Gold

Re: ip default-network

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

What are you trying to accomplish ?

Community Member

Re: ip default-network

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.

Hall of Fame Super Gold

Re: ip default-network

You typed 172.168.0.0 instead of 172.16.0.0 perhaps ?

Community Member

Re: ip default-network

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

Hall of Fame Super Gold

Re: ip default-network

R1 has no rip network statement for 172.16.0.0 hence that is not included in rip updates.

Community Member

Re: ip default-network

yeah but R1 RIP doesn't contain 192.168.0.0 either and it works fine.

Hall of Fame Super Gold

Re: ip default-network

Which Lo masks are you using all over ? R2 and 3 have 172.16.0.0 in rip database - network (rip) statement. Therefore they won't overwrite it when coming as default. They don't know about 192.168.0.0 and are fine taking it.

Community Member

Re: ip default-network

they're all /24. I thought that RIP v2 used subnet masks so that R2 would know it only had 172.16.1.0 and R3 would know it only had 172.16.2.0? And why does it put that statement into the running config (ip route 172.16.0.0 255.255.0.0 172.16.2.0

)

thanks

Dave

Community Member

Re: ip default-network

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

Community Member

Re: ip default-network

I'm the administrator and it did insert that statement after I added ip default-network 172.16.2.0

Hall of Fame Super Gold

Re: ip default-network

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

Hall of Fame Super Gold

Re: ip default-network

I stand corrected on the above. The mask on lb shouldn't be causing it.

Community Member

Re: ip default-network

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 172.16.0.0 255.255.0.0 10.0.1.x".

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