Do I need my own AS number? [BGP Internet Edge Design]

[Bilal Nawaz]

Hello. We own address space, but do not have our own AS number, currently we are using one ISP but looking to introduce another ISP for resiliency. If we take the multihoming scenario, will we need our own AS number to advertise our prefix? Or will both ISP's advertise out our prefix's? or can we use an internal AS number and advertise to our ISP's our address range? What is common practice around multihoming BGP?

Thank you

[Richard Burts]

There are multiple alternatives that could work well for you. Some of the alternatives involve running BGP and some do not. If you are running BGP to two service providers it is common to apply for your own AS number. But it is not required.

HTH

Rick

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[Aileron88]

If you already own provider independent address space then you should get your own AS. You might find that you will only be offered a 4-byte AS, so make sure your devices support this. You will need to have discussions with your providers though, because depending on the size of you're address range and the policy you want to implement you might well need the providers to work together.

Thanks,

Adam

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[sean_evershed]

Hi,

In this design you also need to consider the size of the PI prefix that you want to advertise to the second ISP.

Most ISPs have a miminal length prefix that they will accept from their customers.

For example where I live the longest prefix that one of the larger ISP's will accept is a /24.

Concerning the multi-homing question see this presentation:

https://supportforums.cisco.com/videos/1787

Don't forget to rate all posts that are helpful.

Cheers

Sean

[Bilal Nawaz]

Hey guys,

Very informative, so thanks for your input. Just out of curiosity - what happens when the address space is owned by an ISP and is a PA...

You go with two ISP's how would that work?

Thank you

[Aileron88]

You'd need an agreement between the two ISPs saying that whoever doesn't own the address space is willing to advertise another providers address space. This will mean punching a hole in their summarisation to the rest of the 'Internet'. This isn't good practice. You should have your own AS and PI address space. This will make enforcing policy far easier from your perspective and won't involve any complex peering agreements.

Cheers,

Adam

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