Information Technology Reference
In-Depth Information
Additional Connectivity Concerns
Different factors that cannot be directly controlled by the engineers operating the enterprise
network can affect connectivity. This section looks at two primary issues:
•
Provider-based summarization
•
Prefix filtering at private peering points
Provider-Based Summarization
Provider-based summarization
is when the provider assigns address space to a number of
customers and then aggregates those address blocks into a single summary that it announc-
es to its upstream transit providers and peers. In general, it is considered good practice for
providers to perform this summarization, because it reduces the size of the global routing
table. However, in some instances it is beneficial to advertise some of the more-specific pre-
fixes in addition to the summary route.
In Figure 6-8, for example, the enterprise has two upstream providers. Provider A assigns
100.16.0.0/20 to the enterprise. The enterprise advertises this prefix via BGP to both Pro-
vider A and Provider B. Provider A summarizes the prefix into 100.16.0.0/16 and advertises
only the summary to its peers and upstream transit. Provider B does not own the address space,
so it does not perform any summarization, but it advertises the 100.16.0.0/20 directed to its
peers and upstream transit providers.
The result of these advertisements is that all route lookups, except in Provider A's network,
prefer the path through Provider B because of the longest match. This means that the enter-
prise sees the vast majority of traffic inbound on the link to Provider B. The only traffic that
is inbound from Provider A is traffic from Provider A itself and its customers. In fact, it is
very possible that traffic from any of Provider A's multihomed customers would use Pro-
vider B to reach the enterprise.
The solution to this problem is for Provider A to advertise the prefix 100.16.0.0/20 in addi-
tion to 100.16.0.0/16. This provides a much closer balance for the inbound traffic flow to
the enterprise. It is important that you communicate with your upstream providers about
your intentions to multihome. This allows them to make the necessary changes for your
multihoming to be successful.
