Information Technology Reference
In-Depth Information
The 100.16.0.0 network is in the traditional Class A address space. This means that standard
peering filters permit only /21 or shorter advertisements. Provider A announces a /16, which
is accepted. Provider B announces a /24, which is denied. This is shown in Figure 6-9.
What are considered “standard” peering filters changes over time. It is becoming more
common for ISPs to accept any prefix that is a /24 or shorter.
NOTE
Figure 6-9
Prefix Advertisement for a Multihomed Enterprise
Internet
100.16.0.0/16
100.16.0.0/24
Provider B (AS 200)
Provider A (AS 100)
100.16.0.0/24
100.16.0.0/24
Enterprise (AS 300)
The enterprise has connectivity through Provider A. There is very little traffic through Pro-
vider B. Another problem arises if there is a link failure between Provider A and the enter-
prise. The advertisement from Provider B is blocked, which results in loss of connectivity
because the prefix 100.16.0.0/24 is not propagated beyond the network for Provider B.
The solution to this problem is for the enterprise to obtain an address assignment from
Provider A or Provider B that is large enough to fit through standard peering filters. When
multihoming is desired, discussing your intentions with your upstream providers can often
avoid scenarios such as this.
