(640-901) cannot set subnetmask

MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
Hi all.
This is a read question on the examen
at the 640-901 (routing) ccnp examen i was to do a cisco router config (simulator)

[NOTE: question removed by Webmaster, you cannot post actual questions in these forums, we can get into major trouble for you doing so. Always try to reword the part you have a question about in a regular technical question/concept.]

I need to configure the interface loopback 0 with mask ( /32)

I tried:

inter loop 0
ip addr

not possible: got the ^mark at 255.255.255.^ why ?

any suggestion ?


  • MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
    sorry by the question. (the original one)
  • WebmasterWebmaster Admin Posts: 10,292 Admin
    No problem...

    Unfortunately, I don't know the answer to your question. I tried to simulate it on a 2500 but it works when I do so... Hopefuly sikdogg, our CCNA moderator who passed many Cisco exams, can help you further.
  • MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
    I belive there must bee some smart persons in this forum.
    If you don't know the answer then pick a guees. Everything is better than nothing.

    # better to die on your feet than live on your kness!
  • sikdoggsikdogg Member Posts: 43 ■■□□□□□□□□
    It should work as you described. The only thing that I can think of that may cause problems with using this IP/mask is if the address that you're using for the loopback is overlapping with an IP segment that you have previously assigned to another interface.

    My suggestion is to carefully look over all the IP segments assigned to all the other interfaces on this router to see if there is any overlap.
  • RussSRussS Member Posts: 2,068 ■■■□□□□□□□
    Me - I'm just a dummy, but I was always of the opinion that was invalid. The old can't be all 1s or 0s thing?
    FIM website of the year 2007
  • MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
    I know the situation, but not in this case. No conflict. i've posted the question to cisco. I will dorp there reply on this site.

    Know what u think, (broadcast addr) but think of it as a net with only 1 ip addr.
    this is also possible to do a route with a specific route (ip route Dialer122) where dialer 122 is a dialer interface on the Local router.
  • sikdoggsikdogg Member Posts: 43 ■■□□□□□□□□
    Can I ask what version of IOS you're running on your router?
  • MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
    Its was on the ccnp (640-901) test i got the question to config the router.
    simulator stuff.
  • MrCiscoMrCisco Member Posts: 6 ■□□□□□□□□□
    icon_rolleyes.gif I got the very late reply from cisco.

    but here it is. (Great answer)

    Thank you for contacting Cisco Learning Support.

    We appreciate your feedback regarding the BSCI Exam. Cisco Career Certification exams are reviewed and updated periodically based on customer feedback and technology updates.
    We were unable to find any errors with your exam. Our records indicate that the simulation questions functioned properly. Unfortunately, it is not the policy of Cisco to discuss any of the specific items that are included in an exam, or if any was answered correctly or incorrectly.

    Exam scores are based on a point scaling system that is widely used in the industry. Your score can fall within a point scale of 300 to 1000 points. You are automatically awarded 300 points for beginning the test and can score up to 1000 points if you answer every question correctly on the exam. Please note that the specific number of points awarded to a given question varies on each of our different exams.

    We appreciate your feedback on the test questions. Cisco Systems is continually improving their tests based on customer feedback. Cisco's policy is never to change test scores; however, your comments have been forwarded to the test developer for review. Cisco tests are very difficult to ensure a high standard for those people who do achieve certification. We hope that you will continue to pursue your goal. To help you prepare for the exam, please refer to the exam outlines found under the Exam Information option at www.cisco.com/go/certifications. We also recommend the training as listed on our web site at the same location. (Please note that our exams are based on the recommended training.)

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    Kind regards,

    Cisco Learning Support
  • Flyboy004Flyboy004 Member Posts: 1 ■□□□□□□□□□
    Maybe your dyslexic like me and typed it wrong. I have a bad time with 255 cause half the time it comes out 225 by mistake. hehe
  • techman-aka-Formattechman-aka-Format Inactive Imported Users Posts: 59 ■■□□□□□□□□
    interesting they would make you put a 32 mask on a loopback with a class A address? are you sure you have the correct mask? otherwise it should work, unless it is overlapping with another IP, like sikdogg said. or maybe it is a typo. I am dreading taking this exam next. I think Ill take the remote access test first.
    information belongs to the public! hack the planet!
  • SartanSartan Inactive Imported Users Posts: 152
    Probably completely unrelated to the entire thread, but loopback addresses can also be used to raise the priority of an OSPF router.

    When the OSPF process starts, the cisco uses the highest local IP address as it's OSPF router ID. This is good, in most cases. By default it requires no configuration on your part. However, if you want your router to *always* have a specific ip address in order to pick a designated router, you must configure it with a loopback address. You can pick a DR using the priority, but in the case that they are the same the routers exchange information and pick the router with the highest ID, or IP.

    For those of you here who are still doubting the /32 netmask, this specifies a 1 host network, with 1 network ID and 1 broadcast.

    It's configured in exactly the way MrCisco posted.

    Loopback addresses can also take predecence over existing addresses.
    They are still routable, as you can use them if you want to lazily null route an entire network or simulate an external IP. (quad zero routes are handy, send them to the loopback)


    Configuring a Loopback Interface
    You can specify a software-only interface called a loopback interface to emulate an interface. Loopback interfaces are supported on all platforms. A loopback interface is a virtual interface that is always up and allows Border Gateway Protocol (BGP) and remote source-route bridging (RSRB) sessions to stay up even if the outbound interface is down.

    You can use the loopback interface as the termination address for BGP sessions, for RSRB connections, or to establish a Telnet session from the device's console to its auxiliary port when all other interfaces are down. You can also use a loopback interface to configure IPX-PPP on asynchronous interfaces. To do so, you must associate an asynchronous interface with a loopback interface configured to run IPX. In applications in which other routers or access servers attempt to reach this loopback interface, you should configure a routing protocol to distribute the subnet assigned to the loopback address.

    Packets routed to the loopback interface are rerouted back to the router or access server and processed locally. IP packets routed out the loopback interface but not destined to the loopback interface are dropped. This means that the loopback interface serves as the Null 0 interface also.
    Network Tech student, actively learning Windows 2000, Linux, Cisco, Cabling & Internet Security.
  • roXetroXet Member Posts: 1 ■□□□□□□□□□
    need to enter "ip subnet 0" at the exec prompt I think. I ran into this problem when I was trying to configure at router at work once. It would not let me give my s0 interface a mask of until I entered that command.
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