Information Technology Reference
In-Depth Information
Default Plus Partial Routes
The use of partial routes is the most common. It provides a fair amount of specific informa-
tion to allow routing optimization but has lower resource requirements than full tables.
Partial tables can be sent by the provider or defined by the enterprise. When partial tables
are defined by the enterprise, full tables are requested from the provider, and inbound fil-
tering is applied. When partial tables are sent by the provider, it is often the provider's local
routes and their customers, filtering out prefixes received from peers or upstream transit
providers. When partial routes are used, the default route is used in conjunction to ensure
full reachability.
Full Internet Tables
The enterprise accepts full Internet routing tables from the provider(s). This method
provides the most specific information, with the trade-off of being the most resource-
intensive. The availability of specific information for every reachable prefix allows for the
greatest amount of routing optimization. The use of default routing is not required when full
tables are used, because any destination without a specific prefix is not reachable.
Multihoming
The term
multihoming
has become quite common. So what does it mean to be multihomed
with respect to Internet connectivity? A network is multihomed when it has more than one
path to reach the Internet. This might be multiple paths to a single provider or multiple
paths to different providers. There are two primary reasons for multihoming:
•
Reliability
—Internet connectivity has become a mission-critical service in many
environments. Multihoming when done correctly provides the redundancy needed to
ensure reliable service.
•
Optimal routing
—The performance of Internet connectivity can be enhanced
through multihoming. This is commonly done through the use of different providers
to offer a more diverse selection of paths to reach destinations.
The following sections examine methods to provide Internet connectivity for an enterprise:
•
Stub network single-homed
•
Stub network multihomed with single or multiple border routers
•
Standard multihomed network with single or multiple border routers
Stub network design scenarios present options where there is a single upstream provider. A
single-homed network is by definition a stub network. Nonstub network design scenarios
present options where there are multiple upstream providers. The discussion about multiple
sessions to the ISP focuses on when multiple links are used between two routers to provide
additional bandwidth.
