Re: IXP BGP routing

[Brian Candler <B.Candler at pobox.com>]

On Tue, May 21, 2002 at 11:49:53PM +0200, Americo F. Muchanga wrote:
> 2. Meet with all ISPs and convince them to use static with TDM and BGP 
> (of course) with IX.

TDM = your telco?

> Info for outsiders, TDM will peer at the IX not only to exchange traffic 
> with those ISPs who do not use TDM as gateway but also because TDM do 
> have users that get Internet Services through TDM. TDM has a network of 
> cybercafes and now are deploying what they call as Digital Agencies 
> (50+). If TDM did not peer, for those users to access websites from, say 
> CIUEM, Teledata, etc, traffic would have to go to US or Europe.

I don't understand what you are trying to say there.

Traffic from a TDM customer (downstream) to another TDM customer will always
go via TDM - it will never go via US/Europe.

Traffic from a TDM downstream to an ISP which has an independent uplink will
take the 'short' route via the IX if TDM peers with that ISP. It is not
necessary for the downstream itself to peer. When TDM peers, it will
announce all the routes of its customers.

Perhaps a diagram will help:

                                        --- ISP2
                  satellite           /
      Internet <----------------- ISP1 ---- ISP3
                                   |
                                  IX
                  satellite        |
      Internet <----------------- TDM ----- ISP4
                                     \
                                       ---- ISP5

At the IX, ISP1 will announce its own routes and those for ISP2/3 [*]. TDM
will announce its own routes and those for ISP4/5.

Traffic from ISP2 to ISP5 will go ISP2->ISP1->TDM->ISP5, no satellite hop is
required, even if ISPs 2-5 are not IX members.

Of course, ISP's 2 to 5 may decide to set up their own private peering with
each other at the IX, to reduce chargeable traffic on their uplinks.

However, in general, ISP1 will not be interested in peering with ISP4 and 5.
That's because:
(a) there is no cost saving - traffic to ISP4 and 5 already goes via the IX
    (via TDM)
(b) there is an additional cost in maintaining these extra peering
    relationships
(c) there is loss of revenue to TDM. Politically you don't want to cut off
    the revenue streams of people at the same "tier" as you in the game,
    because you don't want them to do the same to you.

Within Mozambique, you could call TDM and ISP1 "tier 1" ISPs (those with
their own International links), whilst ISP2-5 would be "tier 2" ISPs.

So, if I understand your paragraph above correctly, "Digital Agencies" =
ISP4/5. For local users of any other ISP to reach those sites using the
'short' route via the IX, TDM only needs to peer with other tier 1 ISPs.

Regards,

Brian.

[*] If the ISP 2 or 3 has their own public AS, then their routes will be
announced with the ISP's AS in the path. If they don't, then the route will
be announced from ISP1's AS. If ISP 2 or 3 does not have provider-
independent address space but gets their IP addresses from ISP 1 then their
route will be aggregated within ISP1's own announcement anyway.

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