Failed 640-801 again!

Can't believe it. I was going ok. Answering questions fairly quickly. The I hit the dreaded SIM (OSPF) the one I hate the most. What was throwing me was it stated I needed to setup OSPF for SPECIFIC subnets or interfaces (don't remember which). There were 2 interfaces, one was

192.168.0.1/24 and the other was 192.168.101.1/30 I tried all sorts of stuff like network 192.168.0.0 0.0.0.255 network 192.168.0.0 0.0.0.3 network 192.168.0.1 0.0.0.0 area 0 network 192.168.101.1 0.0.0.0 area 0 plus many other variations. At one stage my show ip route actually showed some ospf networks but I still coldn't ping to the remote interfaces.

I failed with 801. I know it was the sim that screwed me. Pissed off, but taking exam again on Friday. I'll be sure to review this sim and all the others again.

Any idea what they were wanting?

RB

Reply to
rb33339
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Hi RB,

network 192.168.0.0 0.0.0.255 ------Lacks Area 0 or it would be correct network 192.168.0.0 0.0.0.3 ---- Lacks .101. and Area 0 also network 192.168.0.1 0.0.0.0 area 0 ---- Not a network network 192.168.101.1 0.0.0.0 area 0 ---- Not a network either

The last 2 might work (I don't know) in a real world, but this is a sim. Don't even bother typing an ospf network statement without an area.

The correct statement for 192.168.0.1/24 would be

network 192.168.0.0 0.0.0.255 area 0

The correct statement for 192.168.101.1/30 would be

network 192.168.101.0 0.0.0.3 area 0

If the serial ip was 192.168.101.5 /30, it would be

network 192.168.101.4 0.0.0.3 area 0

This statement tells the router to advertise everything from

192.168.101.4 to 192.168.101.4+3 (or 7) is in this network and nothing else is.

Viewing the routing table on the router containing this network statement will show these routes as directly connected and not as ospf.

Hope this helps.

Bill

Reply to
Bill Sanders

I know about the area 0 bit. I just forgot to put it in this post. I'll just need to do some reviewing on wildcard masks

I'll get it next time.

Thanks

RB

Reply to
rb33339

In OSPF, the network statement tells the router what interfaces you want to participate in the OSPF process. Thus, you can either specify a host mask (/32) or a network mask. Both will work in real life, but for the sim, I would go with using the network mask.

network 192.168.101.5 0.0.0.0 area 0 network 192.168.101.4 0.0.0.3 area 0

Use this command:

show ip ospf database

Doan

Reply to
Doan

Hi Marc,

192.168.0.0 0.0.0.3 area 0 is advertising the ip range

192.168.0.0 through 192.168.0.3 as being in area 0

This fails to advertise most of the lan and advertises none of the wan.

192.168.0.0 0.0.255.255 area 0 would advertise everything 192.168 (and screw things up good if 192.168. anything else was in use.

Here is how I learned it.

-------------------------------

The wan network 192.168.101.1/30 has 4 ip's.

192.168.101.0 --- network 192.168.101.1 --- this host 192.168.101.2 --- other end host 192.168.101.3 --- broadcast

The last octet of the above addresses in binary are

00000000 00000001 00000010 00000011

strike a vertical line to the right of the last identical column

000000|00 < this is your network (all 8 bits) 000000|01 000000|10 000000|11 < add up the bits to the right of the vertical line. That is your wildcard mask. It is 3

thus Network 192.168.101.0 0.0.0.3 area 0

-------------------------------------

Here is another example

wan 192.168.101.25 /30

192.168.101.24 --- network 192.168.101.25 --- this host 192.168.101.26 --- other end host 192.168.101.27 --- broadcast

The last octet of the above addresses in binary are

00011000 00011001 00011010 00011011

strike a vertical line to the right of the last identical column

Reply to
Bill Sanders

ive had this one, failed with an 821! the correct answer is network

192.168.0.0 0.0.0.3 area 0 according to my buddy who passed the same day

Reply to
marc

This is very useful and I kinda understnad the logic. But what in the case of the defualt subnet mask of /24 for a class C adx? Too much stuff to write down and calculate. Can you always assume with default subnetmasks that you can use the classfull address (ie 10.0.0.0

172.16.0.0 192.168.101.0) for the first part of the network statement and use the "subtract the mask from 222.255.255.255" rule to sort out te remainder of the network statement?

RB

Reply to
rb33339

Hi RB,

Yes, subtracting from 255.255.255.255 will always give you the wildcard mask.

If this is already familiar to you, excuse me for going on about it. This sort of interaction is helpful to me as well and someone else may be able to use it.

Finding the correct network is important and easy too

192.168.101.17 255.255.255.252

get the mask using subtraction as usual

255.255.255.255

-255.255.255.252 =0.0.0.3

The network can be found by finding the block size. The block size is just the significant octet in the wildcard mask plus 1. In this case, it's 4. Here are the nets

0 4 8 12 16 20 24

17 comes after 16, so 16 is the network to advertise

network 192.168.101.16 0.0.0.3 area 0

The method I was talking about earlier is really handy when summarizing routes

If you are asked to summarize

172.16.48.0 /24 172.16.49.0 /24 172.16.50.0 /24 172.16.51.0 /24

It is easy using the earlier method. The 3rd octet is the only one different, so I write them out in binary

48 = 00110000 49 = 00110001 50 = 00110010 51 = 00110011

Draw the line where things become different

Since each one begins 001100, the line is after that

001100|00 001100|01 001100|10 001100|11

The line is after bit 22, so /22 is the cidr mask. Take the lowest network in the bunch and /22 and you have

172.16.48.0 /22 is the summary

Bill

Reply to
Bill Sanders

Excellent! So to summarize (no pun intended), to figure out the correct network statement for OSPF when you need to set OSPF to subnets of interfaces as opposed to the specific interface:

The network will be the subnet id for the ip address in question (bearing in mind the block size) The wildcard mask is 255.255.255.255 - the subnet mask

In the case of an interface of 192.168.33.1/24, the block size is 1, so the subnet is 192.168.33.0 and the wildcard mask is 0.0.0.255 using the subtraction rule. So the correct statement is network 192.168.33.0 0.0.0.255 area 0 yeah?

You have been a tremendous help with this. I do appreciate it.

BTW I'm ok with summarizing (suprisingly) Just these wildcard masks and network statements in OSPF gets me. But not anymore!

Thanks again!!

RB

Reply to
rb33339

RB -

That's a real shit. I hope you get this cert soon!

(consider the below for the benefit of anyone reading this)

As per what Bill mentioned, I also find it easier to think of the masks in their binary form:

/26 = /24 with two extra bits, so last octet is 11000000, the decimal value of the last octet is 128+64 = 192 so the subnet mask would be

255.255.255.192

Inverted for the wildcard mark, the last octet is 00111111 so the wildcard mask would be 0.0.0.63

Additionally, with OSPF, contrary to what some others have said (though some did state this correctly) and what some documentation seems to indicate, the network statement definess which interfaces the OSPF process will operate on.

For example:

router ospf 1 network 10.0.0.0 0.255.255.255 area 0

will tell the router to run OSPF process #1 on all interfaces which have an address starting with 10. - if as a result of this, a neighbor router advertises the subnet 139.134.0.0/16 to yours, it /will/ be redistributed by your OSPF process.

I personally don't like the syntax of this - I think a more logical syntax would be something like "interface eth0 area 0", though I can see some reason for it - i.e. if an organisation owns a block of addresses, a single network statement can be configured to run OSPF on all interfaces with an address in their own block. But then I think much IOS syntax is illogical.

~Brad

Reply to
Brad H

I hope I get it too soon. I am SO over studying for it. I won't go into it yet again cause I don't want the usual debate, but I do think its unfair that they can be so pedantic on OSPF configs. Especially, given the 3 router network in question, other statements are perfectly valid (and recommended). Why would you bother mapping OSPF to non-existent interfaces? This is only done to make the question more difficult. For it to be weighed so heavily in a sim is nuts.

I had a look in an old Sybex 640-507 (old ccna) book for info on OSPF. Guess what; not even covered! Nor was EIGRP for that matter. Don't get me wrong. I know there was plenty in the old exam we don't have to worry about like IPX, but it just goes to show you how much they've wound this thing up a notch.

Reply to
rb33339

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