Information Technology Reference
In-Depth Information
Figure 6-8
Prefix Advertisement for a Multihomed Enterprise
Internet
100.16.0.0/16
100.16.0.0/20
Provider B (AS 200)
Provider A (AS 100)
100.16.0.0/20
100.16.0.0/20
Enterprise (AS 300)
Peering Filters
To reduce their routing tables, almost all service providers perform filtering at public and
private peering points. The filtering is to disallow prefixes that are too specific, because
reachability can still be achieved through the use of shorter summary routes. A common
peering filter might look like this:
•
In the traditional Class A address space, allow prefixes that are of /21 or shorter.
•
In the traditional Class B address space, allow prefixes that are of /22 or shorter.
•
In the traditional Class C address space, allow prefixes that are of /24 or shorter.
It is common for peering filters to be based on the allocation sizes that the registries use.
In the traditional Class A address space, the longest prefix allocations are /20 networks. In
the traditional Class B address space, the longest prefix assignments are /21 networks. In the
traditional Class C address space, the longest assignments are /24 networks.
Assume that the enterprise has been assigned 100.16.0.0/24 by Provider A. The enterprise
is multihomed to Provider A and Provider B. The enterprise advertises its /24 network to
both providers. Provider A summarizes the prefix and advertises 100.16.0.0/16. Provider B
is unable to summarize and advertises the prefix 100.16.0.0/24.
