Question about BGP route exchange between ISP

[mahesh18]

Hi All,

I was reading the book on BGP  and it says

AS 2  Smaller ISP

The BGP table for AS 2 is a bit more complex than the one for AS 1. AS 2 relays

the customer routes 164.0.0.0 and 165.0.0.0 that it receives from ASes 4 and 5

to AS 1, so the rest of the world knows how to reach them. The peering link

between AS 2 and AS 3 is used to exchange traffic to (and thus routes from) each

other's customers. So AS 2 sends the routes it received from ASes 4 and 5 to AS

3, but not the routes received from AS 1.

Note *********Diagram is under attachment.

My question is that why ISP  AS  2  will not send routes which it receives from bigger ISP  AS 1  to smaller ISP  AS  3???

Many thanks

MAhesh

[milan.kulik · Accepted solution]

Hi,

because in that case ISP3 could use ISP2 as a transit to the whole Internet.

The sense of the peering between that two small ISP2 and ISP3 is to let only their customers speak each to the others directly.

But not to let the other provider (ISP3) customers to access all Internet destinations THROUGH ISP2 consuming his uplink line to ISP1 so.

HTH,

Milan

[mahesh18]

Hi Milan,

Many thanks for answering my question.

Regards

Mahesh