[isp-bgp] Re: Discontiguous AS

[Jeffrey Belles <jbelles@...]

Hi John,
How's it going over in UK?

If you are using Cisco, the allowas-in command will do the trick for you.
If you are using a Juniper you can set a 'loop' statement in your BGP
autonomous system config.

I do beleive however that in some cases your upstream (if using Cisco) can
have some problems advertising prefixes to a neighbor with ASx where there
is already ASx in the AS_PATH.
If you read the  RFC1717 or 1654 section 9.1 it sort of implies that it is
meant to be sending these prefixes, but apparently Cisco are/were
interpreting this differently than other (ie. me:-)

peer-groups (on upstream side) can overcome this, but best thing to do is
also speak to your upstream-providers about this.

Cleanest way of course is to get another AS;-)

Cheers,
Jeffrey

---

Jeffrey Belles
IP Core/Transit Engineer    GDNMS-IPC
email: jbelles@...
tel: +31 (0)20 711 6183     VNET: 711 6183

On Apr 2, 2003 Greenhalgh, John filled my Inbox with:

> Hi,
>
> We currently have a single AS and we are starting a new network other side
> of the States. The two networks will be discontiguous. Both networks will be
> multi-homed to Tier 1 providers and we must support BGP to customers.
>
> Any ideas of how we can support two discontiguous ASs but still using the
> same AS number. We foresee the problem where loop avoidance mechanisms will
> stop the ASs learning each other's routes.
>
> Ideas so far are to use statics at our network egress, or to tunnel sessions
> between the two networks, both of which seem messy.
>
> Has anyone got any ideas how we can do this without 'bodging' it?
>
> John
>
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>
>


------------------------ANNOUNCEMENT-----------------------------------
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         ISPCON Spring 2003 April 23-25, Baltimore, MD
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      The Definitive Event for the Wired and Wireless ISP
           Visit www.ispcon.com and register today!
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