Same public BGP on two routers on two ISP

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Sep 8th, 2009
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Hi, I have two routers R1 and R2. Router R1 is on AT&T and R2 is on Verizon. ISP has given same BGP AS 1700 on both routers. Is it possible to give SAME public AS on two clouds ?

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Giuseppe Larosa Tue, 09/08/2009 - 23:37
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Hello Rupesh,

ISPs cannot give public BGP AS numbers.

This BGP AS 1700 should be that of your customer.

if it is so you should be fine.

To make the two routers able to exchange routes you need


neigh att-ip-addr allowas-in N

where N is the number of times AS 1700 can appear in an AS path before the BGP advertisement is ignored.

default value for N is 1 but you need a different value if you are using AS path prepending towards one ISP.

on R2:

neigh verizon-ip-addr allowas-in N

This kind of setup is becoming common and there have been some threads in last mounths about this.

Hope to help


Rupesh Kashyap Tue, 09/08/2009 - 23:53
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Allow-in is ok, I know it. My question is how AT&T & Verizon have givne us same AS 1700 ? Is it legal and would not create any loop ?

Giuseppe Larosa Wed, 09/09/2009 - 00:07
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Hello Rupesh,

public AS numbers like AS 1700 are assigned by regional internet registries.

Currently AS1700 is associated with ARIN

if you go here

and you use the search righmost box you find the following information:

OrgName: Sesquinet


Address: Rice University

Address: P.O. Box 1892

City: Houston

StateProv: TX

PostalCode: 77251-1892

Country: US

ASNumber: 1700


ASHandle: AS1700


RegDate: 1992-02-13

Updated: 1994-09-28

So if your customer is not Rice University of Houston you are in trouble.

You may have used AS1700 as a way to hide your real AS number check with the regional registries.

to search use AS+number

AS1700 to search AS 1700

Hope to help



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