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Sharing an ARIN block amongst a couple regional sites

[ Edited ]
‎09-30-2016 03:14 PM

As we all know ipv4 exhaustion is a real thing. Our company has already acquired a /24 subnet from ARIN. We are now opening a new datacenter to distribute load across a regional area. The fight with ARIN has suggested that they may not assign us additional IP space since we aren't using our current block fully. We could easily split our /24 into 2 x /25, but none of our ISP's will allow us to advertise anything less than /24.

 

How do other companies do this? It seems to me that on the ISP side(s), I advertise my /24.  And then amongst all the edge routers I could do some iBGP but advertise just the local part of the  subnet for that site (a /25).  Like the image. 

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Re: Sharing an ARIN block amongst a couple regional sites

‎10-01-2016 03:57 AM

Hi,

 

If the ISPs are common on both sites, you could announce separate /25  with no-export community or both /25s with different MED + no-export and the aggregate /24 as default. This would allow routing of the 2x /25 prefixes within the ISP ASN to different eBGP neighbors, i.e site A & B, if the ISP is agreeable.

 

As per diagram, if route for only 1x /25 is present on one site edge routers and no next-hop available for /24 aggregate, this could result in blackholing the other part of the /25 at that site.

 

Cheers,

Ashvin

 

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Re: Sharing an ARIN block amongst a couple regional sites

‎10-02-2016 04:14 PM

Since you indicate you have two different ISP at each site, you must have a full /24 to advertise.  We all only accept that as the minimum size for upstream advertisement.

 

As Ashvin notes, if you have the SAME ISP at both sites you can split the advertisement on the ISP network and we would advertise the full 24 up from there.  But from your diagram this does not seem to be what you have.

 

You could advertise the full /24 from both sites.  Then use your L2 links to pass the traffic for site A that arrives at site B and the reverse.  This will add some amount of traffic to the links.  And you would need to segregate this to arrive at the correct upstream side of your firewalls.

Steve Puluka BSEET - Juniper Ambassador
IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP)
http://puluka.com/home
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Re: Sharing an ARIN block amongst a couple regional sites

‎10-03-2016 01:41 PM

Yeah we definitely have 2 different ISP's at each site. There might be something like, say, CenturyLink for one of the ISP's at each site but then it would be someone else for the other ISP link.

 

I should have commented in the original post that be splitting the /25 between two sites, we would manually keep the assocated IP blocks separate by site. So if a site did go down, we would only lose half those IP's in terms of accessibility and we could manage that via our GLB solution.  But routing the /25's between the two sites via private L2 links seems reasonable right? 

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Re: Sharing an ARIN block amongst a couple regional sites

‎10-04-2016 08:59 AM

Hi, 

 

If the ISPs are different, I believe routing the /25s over the L2 links is the only option, otherwise traffic can be blackholed as mentioned previously. You could leverage the no-export option on the common ISP, at least for traffic coming via this ISP would not need to use the L2 links, except on eBGP failure.

 

Cheers,

Ashvin

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