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We have equipment stored in two data centres. Currently we have a layer 2 network that spans both centres. This layer 2 network is provided to us by our ISP and they also run transit on top, a single IP range that also spans both centres. This forms essentially an active/active connection

Both sites have the same default gateway

This means I can use

a.b.c.1/24 on site A
a.b.c.2/24 on site B

and if I have any issues on site A I can move a.b.c.1/24 to a device on site B and everything carries on working ok.

I don't know 100% what our provider uses to achieve this apart from a VRRP instance to provide some resilience for gateway address.

Now I am looking to install an exact copy of this setup as a backup connection. For now all I wanted was for another provider to do exactly the same thing just with a different IP range (we don't have a PI range).

This time round I have been told that the only way this can be done is if we establish a BGP peering session to them (the backup provider) and advertise the (new) IP addresses we are using at each site back to them.

To me this didn't sound right but then I haven't done a lot of work with BGP. Is it possible that you can advertise the same IP range to a single ISP from two different sites and be able to achieve what I described above?

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Yes you can advertise the same range (technically, an IP prefix) from two locations. You will have to choose one as the primary and one as the backup connection. This will operate somewhat differently from your current connection.

I'm reluctant to give you a sample configuration, since there are a lot of variables. If you want to continue this in chat, we can work through it.

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  • Would you be able to explain what you mean by Primary and Backup please
    – Gareth Hastings
    Jan 26, 2016 at 15:19
  • I mean, one site will receive all your traffic (primary). If that site were to go down, then all your traffic will go to the other (backup) site.
    – Ron Trunk
    Jan 26, 2016 at 15:22
  • ok thanks, that isn't what I want. So does that mean that BGP can't do active/active
    – Gareth Hastings
    Jan 26, 2016 at 15:28
  • exactly what do you mean by active/active? You can have your outbound traffic go to the closest gateway, but inbound traffic is harder to control. I doubt you can do that now. To achieve this you will still need a layer two connection between the sites.
    – Ron Trunk
    Jan 26, 2016 at 15:31
  • it can be done but it must be discussed with the ISP itself, only him can control how the incoming traffic for your AS / Prefix will be routed.
    – JFL
    Jan 26, 2016 at 15:47
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It is possible to announce a prefix in BGP to the same provider from different locations and it would work. This should be coordinated in advance with the ISP in order to accept the same prefixes from 2 locations and to manage the Inbound routing policies.

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Yes you can advertise the same prefix from multiple locations to one ISP.

Unless you take steps to deprioritize one of the links then which site the traffic arrives at will depend on the internal routing in the ISPs network. While both sites can be active at the same time you may well find that when they are both active the vast majority of traffic arrives at one of the two sites.

If traffic arrives at the wrong site you will need to send it over your private interconnect to the correct site.

Something to think about is the possibility of a "split brain" scenario, where both sites are individually up but the private interconnect between them is down. In this case some clients will be unable to reach the servers at site A while other clients will be unable to reach the servers at site B.

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