Re: [RRG] Geographic aggregation-based routing is at odds with reality

[Iljitsch van Beijnum <iljitsch@muada.com>]

On 22 jul 2008, at 17:07, William Herrin wrote:

> > The point is aggregation in the tier-1 service provider networks  
> > where
> > default routes can't be used.

> Then what exactly are you aggregating?

Routes received from peers.

> If you're aggregating routes in a tier-1 then the routes that tier-1
> passes on to its neighbors have been aggregated.

> If aggregation is local then you're not passing aggregated routes to
> your neighbors.

That's right, there's no propagation of aggregates to peers.

> What about your plan modifies these two statements so they're no
> longer mutually exclusive?

Suppose networks 100, 101 and 102 that operate in regions east and  
west. East has prefix 50/8 and west has 60/8. The networks have the  
following customers:

101: east 50.1/16 AS 1001
101: west 60.2/16 AS 1002
102: east 50.3/16 AS 1003
102: west 60.4/16 AS 1004

So 100 aggregates by allowing 50/8 and disallowing 60/8 routes in the  
east and allowing 60/8 and disallowing 50/8 routes in the west. There  
is an aggregate in the east for 60/8 pointing to the west and one in  
the west for 50/8 pointing to the east.

So the 100 router in the east has the following routing table:

prefix   next hop  AS path
50.1/16  101 east  101 1001
50.3/16  102 east  102 1003
60/8     100 west

When 100 east needs to reach 60.4.0.1 then it sends the packet to the  
west, and the router in the west knows that this packet needs to go to  
AS 102.

The aggregates are aggregates and therefore not announced to peers.

The other routes are learned from peers and therefore not announced to  
peers as per Gao/Rexford.

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