[arin-ppml] Rationale for /22

William Herrin bill at herrin.us
Wed Jul 29 11:35:39 EDT 2009

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.


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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