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I have an ASN and IPv4 block, let's say AS 65001 and 10.0.0.0/24.

The ISP, AS 65000, has two data centers, locate in Asia and North America.

I want to break the /24 into two /25, to use it in Asia(10.0.0.1/25) and North America(10.0.0.129/25).

ISP won't accept any annoucement smaller then /24, so I advertised /24 on both sites, and created a GRE tunnel between two sites, to establishe a iBGP peering session.

65001(Asia)                65001(North America)   
  \                               /                 
   +-- 65000 =---gre---= 65000 --+
       (Asia)        (North America)

With in AS65001, two /25 subnets are able to reach each other. However, if someone from North America try to access Asia part (10.0.0.1), the traffic won't be able to come back to North America. The traffic from Asia won't be able to reach North America too.

I can see traffic coming from North America destinated Asia, but Asia is trying to respond it using its default gateway, which is the Asia router of ISP, thus it won't work.

I can make Asia reachable from North America by adding a routing policy:

ip rule add from 10.0.0.0/24 lookup gre_peer_as_gw
ip route add default via 10.0.0.129 dev gre0 table gre_peer_as_gw

But that's obveriosuly not what I want. User in Asia area won't be able to reach anything with that being set.

How could this be done?

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Feb 21 '18 at 16:39
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Of course you can't reach your goal like this.
Both NICs are in a different subnet so you need to install a router between them which routes the traffic between the virtual NICs. But that's not how it works.
You normally set up a tunnel between A and B with e.g. openvpn.
The ends of the tunnel normally share one subnet e.g. 10.0.0.0/24.
Within that tunnel you are then able to perform IBGP without beeing connected directly.
Furthermore you do not need

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One way to do this is to take the full BGP routing table. That way outbound packets will follow the shortest route back out and the more specific route will override the normal default route.

The other issue is that you can only have one default route inserted into the BGP table. So either North America or Asia will be the default route out but not both.

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