[vc_row][vc_column][vc_column_text]Ideally, you’ll get new IP addresses for running BGP and you’ll have some time to set up BGP and test everything before these addresses are given to servers and other systems. A slightly more complex situation is the one where you’ll be shooting holes in an ISP’s address block. In our example, you’ll be advertising 10.0.16.0/22, while your ISP advertises 10.0.0.0/8. To switch over, two things need to happen:
- You need to start advertising 10.0.16.0/22
- Your ISP needs to stop statically routing 10.0.16.0/22 to you
The good thing is that both these steps can happen independently. You can set up the BGP configuration towards your ISP but without advertising your prefix (i.e., leaving out the network statement) beforehand. This shouldn’t have any impact, but it’s still a good idea to do this during a maintenance window outside business / busy hours. See if the BGP session comes up. Then, add the network statement and determine if your prefix propagates to the rest of the world using the monitoring tools mentioned below. If all of this works, your ISP can remove their static route, which will otherwise interfere with BGP in some situations. Again, this shouldn’t have any impact, but it’s best done during a maintenance window and you should be on the phone with your ISP so you can ask them to roll back the change immediately if there’s any impact on your network.
The most complex situation is the one where you have a prefix that is currently advertised by your ISP, but you’re going to advertise that prefix yourself. You could use the procedure discussed above, but the problem is that as long as your ISP advertises your prefix, they won’t be propagating your advertisement of that same prefix. You can’t have a “make before break” switchover—at least, not for the connection through that ISP. What you can do is advertise the prefix to a second ISP and then monitor if the prefix propagates to at least part of the rest of the world. (Some networks will prefer the path over your first ISP; this is normal.) Then ask your first ISP to stop advertising the prefix, and make sure that they propagate your own advertisement through them.
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