Can I advertise parts of a provisioned Internet subnet from different locations?

Hi all network wizards,

Let's say I have 7.0.0.0/21 assigned to me by ARIN to my ASN 12345.

Can I advertise 7.0.0.0/22 from one site using ASN 12345
and 7.0.4.0/22 from a second site also using ASN 12345?

ie: Can I subdivide my IPs between multiple sites and advertise them as subsets of the original IP assignment?

Thanks!
Mike SmithAsked:
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NetExpert Network Solutions Pte LtdTechnical SpecialistCommented:
Yes, You can advertise the provided subnets to internet mpls routing table.

are you providing sub-divided ip address to your customers or your company going to use the complete subnet?

if you are providing sub divided ip address to your customers, its not necessary to advertise every subnet to internet routing table
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Mike SmithAuthor Commented:
This is the scenario:

site1: advertise /26 advertise 'network 7.0.0.0/23'
Site2 advertise /26 advertise 'network 7.0.2.0/23'
site3: advertise /22 advertise 'network 7.0.4.0/22'

so out of my ARIN assignment of /21 I am advertising different parts of the /21 at different sites using the same ASN.

this will work?

The IPs at the sites do not need to be available at the other sites, ie: site1 IPs will never be advertised at site 2 or site3 unless something goes very wrong and I decide to reconfigure BGP to failover... this will not be in the default config.
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Mike SmithAuthor Commented:
Here's the million dollar question, currently I advertise the entire /21 at Site1 only.

the first two class C's are in use at that site, none other are in use.

if I change the BGP config on that site to /23 is it going to continue to work as long as the used IPs are in that range?
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NetExpert Network Solutions Pte LtdTechnical SpecialistCommented:
since you are going to advertise different subnet from different sites, the above mentioned scenario will work.

before advertising any subnet in internet, make sure that the advertising subnet is not exist in the internet routing table
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NetExpert Network Solutions Pte LtdTechnical SpecialistCommented:
//the first two class C's are in use at that site, none other are in use.

 if I change the BGP config on that site to /23 is it going to continue to work as long as the used IPs are in that range?//

In current scenario you are advertising the summarized routes from site 1 and now you would like to split the network and advertise from the respective site. am I right to say that?

// if I change the BGP config on that site to /23 is it going to continue to work as long as the used IPs are in that range?//  Yes. this will work; provided your bgp advertisement is right.

Make sure that the BGP advertised network should be present in your local routing table
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Mike SmithAuthor Commented:
Okay, I will give it a try and award you the solution. I am going to change the /21 to /23 and see if all the IPs remain accessible. if not then I will quickly change it back to /21

thanks!
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NetExpert Network Solutions Pte LtdTechnical SpecialistCommented:
No Problem.. Now its morning time here.. you can update the status of your BGP change
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Mike SmithAuthor Commented:
It seems to be working, I changes site1 to /23 instead of /21.

I will do some traces from spring looking glass to see if they are working.
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Mike SmithAuthor Commented:
This appears to have worked, thanks!!!

flags: * = Valid, > = Selected, I = via IBGP, A = Announced, S = Stale
origin: i = IGP, e = EGP, ? = Incomplete

flags destination          gateway          lpref   med aspath origin
*>    0.0.0.0/0            209.***.***.81      100     0 1***9 i
AI*>  162.***.***.0/23     0.0.0.0            100     0 i

I wanted to be sure before I possibly broke some customers...
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Mike SmithAuthor Commented:
Excellent support, he confirmed what I thought was correct and I was able to move forward with my change.
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NetExpert Network Solutions Pte LtdTechnical SpecialistCommented:
Nice to know :)
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