By having ISP1 advertise both its aggregate and the more-specific,
the routing system conceptually will contain:
192.168.0.0/16 ISP1
192.168.2.0/24 ISP1
192.168.2.0/24 ISP2
CL: And two more routes go into the global tables. Wasn't CIDR supposed to
stop this kind of thing? ;->
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
Howard C. Berkowitz
Sent: Friday, January 19, 2001 2:28 PM
To: [EMAIL PROTECTED]
Subject: Re: AS numbers - Is there a global crisis?
>Brian,
>
>Hi! Funny you bring this up, I just got a phone call on it today.
>Basically, you can have two seperate ISPs and have incoming redundant
>connections without using BGP. ISP1 will provide a block of IPs from a
>portion of their CIDR block to the "company." Since this is part of ISP1s
>CIDR block, they already broadcast a route to the rest of the internet
>containing the company's block of IPs.
>
>ISP2 will then also broadcast a route to ISP1's block of IPs (just the
>block!!!). The tricky part comes when you try to do load balancing between
>the two for incoming traffic!!!
>
> I am making several assumptions here (that the ISPs will play nice with
>each other among other things).
>
ISP1, however, MUST advertise not its aggregate alone, but both its
aggregate and the more-specific customer block that also is
advertised by ISP2.
Assume the following:
ISP1 has the block 192.168.0.0/16. This is the only block it advertises.
It delegates 192.168.2.0/24 to the customer.
ISP2 advertises 192.168.2.0/24.
So in the global routing table, there will be two routes:
192.168.0.0/16 ISP1
192.168.2.0/24 ISP2
Since 192.16.2.0/24 is more specific than 192.168.0.0/16, the rest of
the world will send ALL 192.168.2.0/24 traffic to ISP2.
By having ISP1 advertise both its aggregate and the more-specific,
the routing system conceptually will contain:
192.168.0.0/16 ISP1
192.168.2.0/24 ISP1
192.168.2.0/24 ISP2
Other AS will install the ISP1 route to 192.168.2.0/24 if their
connectivity to ISP1 is better than their connectivity to ISP2, and
vice versa.
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