Public AS

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Giuseppe Larosa Sat, 12/12/2009 - 09:05
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Hello Alsayed,

1) An enterprise needs a public AS number when it needs/wants to implement true eBGP multihoming where it wants to announce its own public IP address block.

without a public AS number you can have redundancy using two ISPs as described in the document about NAT and multihoming that I've attached in your other thread.

2) I don't know about the costs, but there are technical requirements one of them is to be truly eBGP multihomed.


this describes current policy for AS number assignment in RIPE the european Regional internet Registry.

There are application forms to fullfill with decription of your network and other questions.

3) For the shortage of public ASNs that have been for many years 16 bits wide (up to 64000 public AS numbers, last 1024 are for private us) a new 4 byte AS number field has been introduced in BGP. Support of this requires recent IOS version in Cisco routers.

Hope to help


Rick Morris Mon, 12/14/2009 - 13:21
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As already indicated if you are multi-homed, running BGP with 2 different ISP's then you must have your own AS.  This is also a good thing even if you are not, you can then control your announcemets rather than the ISP.


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