BGP Internet ASN

[imran.moulvi]

Hi Experts,

What can be the possible reasons why an enterprize would need an Internet BGP Autonomous no?

We have 2 ISPs in our company...do we need a BGP ASN and how can that benefit us?

Thanks.

[Peter Paluch]

Hi Imran,

I am sure that other friends here will add their own views to my answer, and I am looking forward to it

In essence, a unique public ASN would be needed if your enterprise needed to advertise its own networks assigned from a provider-independent IP space through multiple ISPs into the internet backbone. In such a case, the reachability of those networks can be achieved through possibly several ISPs and the networks themselves have to be identified by their originating AS in BGP, which would obviously be your own AS number.

Another need for an own AS number would be if your enterprise actually served as a transit AS to other ASes.

Best regards,

Peter

[milan.kulik]

Hi Peter,

I found an interesting document here http://www.menog.net/menog-meetings/menog1/presentations/Multihoming-noconf.pdf

saying BGP inconsistent-as is not bad, nor illegal.

So it seems to be possible to survive without your own public AS number while peering to multiple ISPs (I agree it's easier if you have your public AS number, of course).

On the other hand CYMRU is watching those suspicious subnets:

http://www.cymru.com/BGP/incon_asn_list.html

BR,

Milan

[Edison Ortiz]

Similar reason as to why you need an unique public IP addressing at the edge: To participate in the internet routing.

BGP has a default loop prevention to drop incoming route updates with same origin AS.

Say, you wanted to reach cisco.com and cisco.com announced its public subnet using the same BGP AS you are using.

cisco.com won't be able to reach your IP subnet because your routes were dropped on the BGP incoming update and you won't be able to get cisco.com IP subnet as well.

Yes, you may say the router will fall back to 0.0.0.0 but most internet routers with full BGP table rarely have a 0.0.0.0 from BGP.

If they have a 0.0.0.0, it's done statically.

Regards,

Edison

[Mahesh Gohil]

Hello Imran,

Another reason could be most of the provider denying to advertise customer owned IP

over internet with their AS number.

The major reason is blacklisting of AS. You can refer few site www.uceprotect.net and in level-3 section they show how much percentage

of their AS is blacklisted.

Something like below

AS	Status	Provider has total IP's	Level 1 listed spammers within the last 7 days	Level 3 Escalation limit by Level 1 records	Optional express delisting WARNING! PROBLEM MUST BE FIXED FIRST TO PREVENT NEW LISTINGS
24560	LISTED	1276160	52207 (4.091 %)	2552	No longer available. Provider is part of the TOP 20 Spamhosters.

Hope this is helpful to you

Regards

Mahesh

[imran.moulvi]

Thank You Very much everyone...Please keep it coming...:)