ip default-network - Again

kpjunglekpjungle Posts: 426Member
Hi,

I read most of the posts concerning this hard-to-understand command, but im still a bit shaky on it.

I have tried with both Packet tracer and real equipment to try and comprehend the meaning of the command, and want to run it by someone else to verify it :)

I understand that the ip default-network command is used to propagate a default route through routing protocols, but how is it actually accomplished. I have followed http://www.thebryantadvantage.com/CCNACCNPICNDipdefaultnetworkcommand.htm to get the same results.

I have configured a "fake" ISP router with a loopback address, and hooked this ISP router up to another router (border router). the loopback IP on the ISP router is 13.13.13.1, and the network between the ISP router and the border router is 172.16.1.0. I can ping the 172.16.1.x addresses between the two routers just fine. I then have a third router (Sales router) connected to the Border router. By using the "ip default-network 172.16.1.0" command on the border router, this default route also become a default route on the Sales-router.

My expectation to begin with was that you would now from both the border router AND the sales-router be able to ping 13.13.13.1, but no such luck. I am however, able to ping the 172.16.1.x addresses from the sales router even though i didnt propagate those directly through the routing protocol. These are also the results I saw in the link above, but should you be able to ping "into the cloud" as well?

In order to make it work, I will on the border router, have to add a static default route (0.0.0.0) to point to the ISP, then it will work, but doesnt this defeat the purpose somehow?

Layout:

ISP <-- 172.16.1.0 --> Border <---> Sales

A bit lost at the moment, even after reading all the other explanations :)
Studying for CCNP (All done)

Comments

  • nicklauscombsnicklauscombs Posts: 885Member
    if im reading correctly it sounds like the routers dont have a route to the 13.0.0.0 network which would be why you cant ping the loopback

    but the ip default-network did work if it propagated to the sales router
    WIP: IPS exam
  • networker050184networker050184 Posts: 11,962Mod Mod
    Just because your router has a default route into the "cloud" doesn't mean your ISP router has a route back. You may want to check that out. Also remember that is is a classful command...
    An expert is a man who has made all the mistakes which can be made.
  • kpjunglekpjungle Posts: 426Member
    Just because your router has a default route into the "cloud" doesn't mean your ISP router has a route back. You may want to check that out. Also remember that is is a classful command...

    Yeah, i have a static 0.0.0.0 route pointing back into my border router.. so i know that part works fine..
    Studying for CCNP (All done)
  • networker050184networker050184 Posts: 11,962Mod Mod
    Does your border router have a route to the 13 network? If not it isn't going to work because once the border router recieves the packet destined for an unknown network it will drop the packet.
    An expert is a man who has made all the mistakes which can be made.
  • kpjunglekpjungle Posts: 426Member
    Does your border router have a route to the 13 network? If not it isn't going to work because once the border router recieves the packet destined for an unknown network it will drop the packet.

    No.. thats the entire problem. If i do generate a route from my border router to the cloud, it will work. But everywhere ive read, this step is left out.. including the one i linked to originally.

    So in the end, the border router DOES need a specific route to the "cloud", and the only reason you use ip default-network is to propagate it to the other routers?

    and ive read on Cisco Docs that the behavior of ip default-network is not the same on all routing protocols?
    Studying for CCNP (All done)
  • nicklauscombsnicklauscombs Posts: 885Member
    kpjungle wrote:
    So in the end, the border router DOES need a specific route to the "cloud", and the only reason you use ip default-network is to propagate it to the other routers?
    That is what I understood from reading it, you would use this to propegate it and use static route if you wanted only one router to know it
    WIP: IPS exam
  • kpjunglekpjungle Posts: 426Member
    kpjungle wrote:
    So in the end, the border router DOES need a specific route to the "cloud", and the only reason you use ip default-network is to propagate it to the other routers?
    That is what I understood from reading it, you would use this to propegate it and use static route if you wanted only one router to know it

    So to make all routers able to ping into the cloud, you would configure a static route on the border router. Then you can ping from the border router into the cloud. After that you use ip default-network to push our this information to other routers? After those two steps, you should all be able to ping into the cloud?
    Studying for CCNP (All done)
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