route summarization - could this be wrong ?

[SJ K]

Hi,

I am reading up about CIDR and came across this diagram as below

If ISP1 router advertise 192.168.0.0/20 to ISP2,  what if ISP3 owned 192.168.16.0/20  ?  Wouldn't end-user of ISP2 go to ISP1 and found nothing ?

It is then assumed that ISP3 have to advertise the route to ISP2 or ISP1 or both then.

I am seeing that 192.168.16.0 still falls under the /20 network and hence my assumption above.

==============================================================================

But somehow I feel that I might be wrong as

.16 is  192.168 | 0001 0000 | 0000 0000  whereas for the earlier networks advertise by ISP1,  all of them have the 1st 4 bits in the 3rd octet as 0000.

==============================================================================

q1) which of my assumption is correct; How should route summarization be calculated ?

Thanks for reading.

Regards,

Confused Noob.

[SJ K]

Hi Jon,

Duly noted. Thanks!

Regards,

Noob

[SJ K]

Hi Jon,

I am sorry it has to this way again. I have really no understanding of why my posts are not showing. I have tried different timing to post, different format. different topic, contents etc but i just dont know why.

Can you let me know if you are able to see this post below

https://supportforums.cisco.com/discussion/12486151/line-console-password-vs-privilege-mode-enable-secret

Thanks.

[Jon Marshall]

There is definitely something funny going on with your account because I can't see this post ie. the one I am posting into now,  in this forum any more.

I can only get to this post by using my notifications.

Same error as before with the other post ie. I am not authorised to access it.

If I get time later today I'll drop a line to the CSC support team and see if they can look at your account but it could just be the site as it has quite a few issues at the moment and this could just be another one of them.

Jon

[SJ K]

Hi Jon,

I have also dropped a message to cisco forum support. Hope they can do something about it.

I will try the posting again later, if it is still not showing, do you mind if i paste my questions here instead ?

Regards,
Noob

[Jon Marshall]

Okay, if you have sent a message then i'll leave it with you at the moment.

You can paste it here if you want although even this thread isn't visible any more to the forum, or at least to me.

I should be able to get to it via my notification tab and if/when they get you sorted out you could always move the new part to a new thread so others can benefit if they are looking for similar information.

Like I say it's difficult to know if this is specific to you or it is a symptom of the more general problems we are having with the site.

Jon

[SJ K]

Hi Jon,

Its showing now, over here- >

https://supportforums.cisco.com/discussion/12485881/line-console-password-vs-privilege-mode-enable-secret

Regards,

Noob

[Jon Marshall]

An 8 bit value = 2^8 which is 256 values ie. 0 - 255.

Jon

[SJ K]

Hi Jon,

Thanks for the answer.
I guess i am doing VLSM and subnetting whole day, my brain has gone haywire.

When i mentioned 192.168.16.0, i see it as it is eligible to be another subnet under the /20 network, but it isn't in the 192.168.0.0/20 subnet / subnet range.  So sorry for the confusion.

===================

Back to what you have mentioned earlier, if I do a route summary but there is 1 particular subnet that is within the range but isnt assigned to me nor under my network.  I am then not suppose to use route summary on my router already right ?

Regards,
Noob

[Bilal Nawaz · Accepted solution]

Yes you're not meant to advertise what doesn't belong to you. Though there is a degree of summation that you can do.

Lets say 192.168.4.0/24 didn't belong to you

Then routes can be summarized like this

192.168.0.0/22 [ 192.168.0.0 - 192.168.3.255]

192.168.5.0/24

192.168.6.0/23 [ 192.168.6.0 - 192.168.7.255]

192.168.8.0/20 [ 192.168.8.0 - 192.168.15.255]

As long as there is no overlap with whatever doesn't belong to you, it should be fine.

Bilal

[SJ K]

Thanks Bilal.