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Moving BGP to a new ISP

hi all,

I need to move a AS to another ISP, although this involves moving several class-C's.

Presently we advertise 3 C classes with a specific carrier. We will be moving all 3 of them to a new carrier, but before this can be accomplished, I need to setup a new router with this new carrier.

The issue I'm having is understanding the impact this will have on the current traffic.

I need to make this as seemless as possible. We have a window of about 1 hour to move production traffic from the current provider to a new provider.

Out of theses 16 c'classes, we will be using 1 of them for the LAN side ou a new 3845. The WAN side will be the peering adress (/30) with the new carrier which we already have.

I need to know what do I have to look for and program in advance (making sure this does not affect current traffic on the current provider) in order to make this seemless as possible.

Thanks for the input.

1 ACCEPTED SOLUTION

Accepted Solutions
Gold

Re: Moving BGP to a new ISP

It will in most cases propagate within minutes.

You could to a point advertise it to your new provider with a very large AS-path prepend. This is how its is done when you run redundant and want to prefer one path over the other.

The risk you take is that you get most the traffic but not all to follow the other path. No matter how hard I have tried I always seem to get some traffic on the so called backup path.

If you work with your new ISP they could tell you how long the AS-path needed to be. You want your new ISP to send all traffic to your old ISP rather than directly to you.

If you get it working this way then you could just turn off the old connection and the new one would take over in less than a minute or in seconds if you get lucky. You could then go back and remove the AS-PATH prepend when you have time to make it more clean.

You have a similar issue on your lan side but since you control all that it is not too hard to adjust the metric to prefer the old path.

5 REPLIES
Gold

Re: Moving BGP to a new ISP

In effect you could turn them both up at the same time. This would be the same configuration as if you were going to keep both as redundant connections.

To do it as a hot cut all you need to do it create the BGP neighbor to the new carrier but filter all prefixes incoming and outgoing. This would allow you to test the basic BGP connectivity. When you are ready to go apply similar filter to your current router and remove them from the new one. You just clear both BGP sessions and all the routes will be relearned and your traffic should move.

New Member

Re: Moving BGP to a new ISP

What do you mean filtering incomming and outgoing prefixes?

The 3 classes on the old router are production inbound and outbound traffic.

On the new router, we would like to advertise the same 3 networks, although only be able to test outbound and NOT receive inbound as the production inbound will be the part we do the hot cut on.

Gold

Re: Moving BGP to a new ISP

Am somewhat confused I guess. The direction you send/filter the routes is the reverse of the data itself.

As soon as you send the 3 prefixes to the new neighbor it will start to forward traffic to you. You cannot advertise these prefixes until you are ready to do your cut.

If you are very careful you could allow the router to receive prefixes from the new BGP neighbor . You just have to make sure these prefixes do not get advertised to any other router. Still there is little you can do with this. The only source address you have if you were to ping one of these addresses you learned is the point to point to the bgp neighbor.

New Member

Re: Moving BGP to a new ISP

so your basically saying I cannot do anything until i'm ready to do the cut right?

normally, how long does the New BGP prefixes take to propagate through the root routers?

Is there an option to do Prefered route to a network (the current router), and the have the new router setup as a secondary route for inbound? Does the carrier have the ability to somehow dope the route making it less favorable to inbound traffic?

Gold

Re: Moving BGP to a new ISP

It will in most cases propagate within minutes.

You could to a point advertise it to your new provider with a very large AS-path prepend. This is how its is done when you run redundant and want to prefer one path over the other.

The risk you take is that you get most the traffic but not all to follow the other path. No matter how hard I have tried I always seem to get some traffic on the so called backup path.

If you work with your new ISP they could tell you how long the AS-path needed to be. You want your new ISP to send all traffic to your old ISP rather than directly to you.

If you get it working this way then you could just turn off the old connection and the new one would take over in less than a minute or in seconds if you get lucky. You could then go back and remove the AS-PATH prepend when you have time to make it more clean.

You have a similar issue on your lan side but since you control all that it is not too hard to adjust the metric to prefer the old path.

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