I am starting to see the following scenario more and more as IPv4 space is hard to get, but isn’t.
With ARIN it is still possible to get an IPv4 allotment. Many smaller ISPs qualify for a /24 and can get one if they wait long enough on the ARIN waiting list. a /24 of IPv4 space is the smallest block that 99% of the Internet allows to be advertised on the Capital I Internet. There are filter rules in place that drop smaller prefixes because that is the agreed upon norm.
So what happens if you are an ISP and you have a shiny new /24 but you have two networks which are not connected. Let’s look at our scenario.
The above network have no connectivity between the two of them on the internal side. These could be half way across the world or next door. If they were half way across the world it would make sense to try and get another /24. Maybe they are either side of a big mountain or one is down in a valley and there is no way to get a decent link between the two networks.
So what is a way you can use this /24 and still be able to assign IP addresses to both sides of the network? One way is to use a tunnel between your two edge routers.
Without the tunnel the scenario is traffic could come into network1, but if the IP is assigned on network 2 it will come back as unreachable. BGP is all about networks finding the shortest path to other networks. You don’t have much control over how networks find your public IP space if you have two providers advertising the same information. Some of the Internet will come in Network2 and some will come in Network1.
By running a tunnel between the two you can now subnet out that /24 into two eqal /25s and assign one /25 Network1 and one /25 to Network2 or however you want to. You can make the tunnel a GRE, EOIP, or other tunnel type. If I am using Mikrotik I prefer to use EOIP. If it’s another vendor I tend to use GRE.
Once the tunnel is established you can use static routing, OSPF, or your favorite IGP (interior Gateway Protocol) to “tell” one side about the routes on the other side. Let’s look at a fictional use.
In the above example our fictional ISP has an IPv4 block of 184.108.40.206/24. They have two networks separated by a tall mountain range in the center. It’s too cost prohibitive to run fiber or a wireless backhaul between the two networks so they have two different upstream providers. The ISP is advertising this /24 via BGP to Upstream1 from the Network 1 router. Network 2 router is also advertising the same /24 via BGP to Upstream 2.
We now create a Tunnel between the Mikrotiks. As mentioned before this can be EOIP, GRE, etc. We won’t go into the details of the tunnel but let’s assume the ISP is using Mikrotik. We create an EOIP tunnel (tons of tutorials out there) between Network 1 router and Network 2 router. Once this is established we will use 172.16.200.0/30 as our “Glue” on our tunnel interfaces at each side. Network 1 router gets 172.16.200.1/30. Network 2 router gets 172.16.200.2/30
To keep it simple we have a static route statement on the Network 1 Mikrotik router that looks like this:
/ip route add dst-address=220.127.116.11/25 gateway=172.16.200.2
This statement routes any traffic that comes in for 18.104.22.168/25 via ISP 1 to network1 across the tunnel to the Network 2 router. The Network 2 router then send it to the destination inside that side of the network.
Conversely, we have a similar statement in the Network 2 Mikrotik router
/ip route add dst-address=22.214.171.124/25 gateway=172.16.200.1
This statement routes any traffic that comes in for 126.96.36.199/25 via ISP 2 to network2 across the tunnel to the Network 2 router. The Network 2 router then send it to the destination inside that side of the network.
It’s as simple as that. You can apply this to any other vendor such as Cisco, Juniper, PFSense, etc. You also do not have to split the network into even /25’s like I did. You can choose to have os of the ips available on one side and route a /29 or something to the other side.
The major drawback of this scenario is you will takef a speed hit because if the traffic comes in one side and has to route across the tunnel it will have to go back out to the public internet and over to the other ISP.
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