[arin-ppml] Rationale for /22

Chris Grundemann cgrundemann at gmail.com
Wed Jul 29 12:08:16 EDT 2009

On Wed, Jul 29, 2009 at 09:35, William Herrin<bill at herrin.us> wrote:
> On Wed, Jul 29, 2009 at 11:22 AM, Kevin Kargel<kkargel at polartel.com> wrote:
>>> In the multivendor multihomed case, each of the customers ISPs
>>> connects to the rest of the Internet differently. The network topology
>>> for the multihomed customer generally differs from the network
>>> topology of any of the ISPs serving the customer. As a result, his
>>> route is not aggregable.
>> I don't remember a topology discriminator in the BGP algorithm.
> Hi Kevin,
> There isn't one. When the topology differs, you have to announce a
> separate route. BGP offers no way to differentiate between multiple
> topologies within a single route.
>> I have dual
>> homed customers who use IP's from me.  When I advertise those out my BGP
>> portal they are aggregated.
>> For example if I advertise in my BGP and I have allocated
>> to one of my customers his network will be aggregated to be
>> within my BGP advertisements without adding a specific route.
> Then you would be making a mistake. His /23 announcement via the other
> ISP will override your covering route, preventing him from receiving
> any service from you whenever his other link is operational. The
> more-specific route always overrides the less specific route.

Saves on bandwidth costs though I guess ;)

> Regards,
> Bill
> --
> William D. Herrin ................ herrin at dirtside.com  bill at herrin.us
> 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
> Falls Church, VA 22042-3004
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Chris Grundemann

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