Now that we are familiar with the options that are available when connecting the Internet, the next step is to determine what routing information you want to receive from the Internet Service Provider(s). There are three options available for receiving routes from the ISP. These three options, which are described in the following sections, are:
- Default Route Only
- Default Route plus a Partial Internet Routing Table
- Full Internet Routing Table
Default Route Only
This option can be used with or without BGP. When BGP is used, the ISP simply advertises only a default route to the enterprise edge routers. This route is then redistributed into the IGP and propagated to downstream internetwork devices allowing for connectivity to the Internet. When BGP is not used, the enterprise simple configures one or more static default routes that point to the provider edge router. This static default route can also be redistributed into the IGP and propagated to downstream internetwork devices.
The primary advantage of using a default route only is that it is not resource intensive, i.e. does not greatly consume router resources such as memory. In addition to this, this is an easy solution to implement. However, when BGP is used and only a default route is received, no path manipulation is possible, i.e. there is a loss of BGP policy control. Additionally, this solution can also lead to suboptimal routing as was described in previous chapters in this guide.
Default Route plus a Partial Internet Routing Table
This is the most commonly implemented solution by enterprises connected to Internet Service Providers. This solution uses more bandwidth and router resources than the default route solution, but less bandwidth and router resources than the full Internet routing table solution which is described in the following section.
There are two methods this solution can be implemented. Using the first method, the enterprise can request the ISP to send full Internet routing tables and then use inbound route filtering to accept only certain prefixes for example. With the second method, the ISP can advertise the enterprise prefixes for its local networks, as well as those for other customers of the same ISP and then advertise the default route to allow reachability to networks belonging to upstream ISPs and the general Internet community.
Full Internet Routing Table
Receiving a full or complete Internet routing table is performed by only very large enterprise customers and service providers. With this solution, the complete Internet routing table is advertised to the customer edge routers. This solution consumes the most router and bandwidth resources. However, the advantage of this solution is that it provides specific information for all reachable prefixes and allows for the greatest amount of route optimization.