Single AS Number for multiple prefixes in different country

Michel de Nostredame d.nostra at
Sun Jan 16 23:32:56 CST 2011

On Fri, Jan 14, 2011 at 12:30 PM, Patrick W. Gilmore <patrick at> wrote:
> On Jan 14, 2011, at 11:03 AM, Michel de Nostredame wrote:
>> On Fri, Jan 14, 2011 at 3:33 AM, Bogdan <shoshon at> wrote:
>>> allowas-in will do the trick
>> Provided your uplink ISP does not filter out that.
> Why would your upstream filter that out?
> I would get a new upstream if they do.

According to Juniper junos document,

"BGP checks whether the neighboring AS matches the AS of the external
peer to which the router is advertising. If there is a match, the
route advertisement is suppressed. Advertisement suppression is
enabled by default for BGP peers configured in non-VPN routing and
forwarding (VRF) instances, including the master instance."

We may not able to assume all ISP willing to or by-default add
"advertise-peer-as" into configuration if they use a Juniper router.
Also we may not able to assume ISP will not put a policy/route-map to
prevent route been advertised to the same AS.

If there is a need to use single-AS in multiple sites and these sites
need to communicate each other via Internet ISP, statically route
traffic to Internet (or a default route to Internet) would be safer.

If there is a need on this kind of communication, or the communication
been done via a tunnel with both sides using ISP's IP (interface IP)
as tunnel source, then there should have no risk to use single-AS in
multiple sites in terms of connectivity.


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