(640-901) cannot set 255.255.255.255 subnetmask

MrCisco

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 255.255.255.255 (19.90.20.10 /32)

I tried:

inter loop 0
ip addr 19.90.20.10 255.255.255.255

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

any suggestion ?

MrCisco

sorry by the question. (the original one)

Webmaster

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.

MrCisco

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!

sikdogg

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.

RussS

Me - I'm just a dummy, but I was always of the opinion that 255.255.255.255 was invalid. The old can't be all 1s or 0s thing?

MrCisco

#sikdogg
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.



#RussS
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 192.168.100.1 255.255.255.255 Dialer122) where dialer 122 is a dialer interface on the Local router.

sikdogg

Can I ask what version of IOS you're running on your router?

MrCisco

Hehe
Its was on the ccnp (640-901) test i got the question to config the router.
simulator stuff.

MrCisco

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.

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Flyboy004

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-Format

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.

Sartan

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 255.255.255.255 /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)
</babble>

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a0080087093.html#1012547

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.

roXet

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 255.255.255.252 until I entered that command.