BGP sub-AS prefix.

[SergeevMS]

Hi everyone!

I have BGP AS with /23 prefix(AS-100 in scheme). Now i need divide the AS on two. I plan to use BGP confederation for this purpose. What minimal prefix should be on sub-AS(101 or 102) in BGP confederation? Can I use /28 prefix on sub-AS 102?

[pman]

Hi,

If I'm not wrong most of ISPs not accept prefixes larger than /24 for IPv4 (/25-/32)

You should check with the ISP if they will allow you to publications higher than /24

I have some questions about the architecture you decided to use:

Why did you decide to use confederation?

What exactly is the goal you are trying to achieve?

What is the exact purpose for which you want to split the advertising?
Why not split into two classes of /24?
Why not advertising to both ISPs /23 and in addition, to one of the two ISPs advertising more specific like a /24?

[SergeevMS]

Thank You for answer

What is the exact purpose for which you want to split the advertising?

Our department has become a separate business unit in a large IT company (owner of AS 100). We have our own services that we provide to our customers. I am not satisfied of the quality of the company's IT service. I want to be as independent of them as possible.

Why not split into two classes of /24?

Yes, I thought about it. But it's may be problem. For many reasons. One of them is that the parent company will not give half of its own AS

Why not advertising to both ISPs /23 and in addition, to one of the two ISPs advertising more specific like a /24?

This solution will not work. Since I will still be dependent on the poor service of the parent company.

I think that our company needs to buy own AS

[Giuseppe Larosa · Accepted solution]

Hello @SergeevMS ,

>> I think that our company needs to buy own AS

Now that I have seen your answer I agree on this to be noted BGP confederation does not provide you this.

You will need to fill an application form with the RIR  of your area RIPE for Europe, ARIN for North America and so on.

a new BGP AS needs to be multihomed = served by two ISPs and you can also ask for your own public address block but this resource is becoming the most precious in IPv4.

If you can manage the parent company to give you  the /23 address block you are probably fine.

Hope to help

Giuseppe

[Giuseppe Larosa]

Hello @SergeevMS ,

internally inside the BGP confederation you are free to use whatever prefix length you want. However, when going to outer world to the ISPs the usual limit is /24 as noted by @pman .

As noted by our colleague usually the /23 public address block is split in two /24 subnets and the trick is to advertise the /23 and one more specific /24 out to each ISP in order to achieve some form of load balancing / sharing. And this can be achieved without using BGP confederations, that can be seen as an an overkilling weapon / tool for this kind of scenario.

Even the need of further subnetting using /28 subnets inside your network is allowed without introducing BGP confederations, of course the /24 and the /23 must be created as aggregate addresses in this case to be sent to ISPs.

Hope to help

Giuseppe