Alternative Methods of Connecting to an ISP

If BGP-4 is unnecessary in your network, consider the other possibilities, including the following:

• A default route into the ISP and a static route from the ISP into the organization.

• A routing protocol into the ISP, making the ISP part of your autonomous system. The ISP will be using redistribution within its domain, and it is advisable for the organization to use some form of security, in the form of access lists or a firewall.

Typically the ISP will give you a written sheet explaining the required configuration, or it will request access to your autonomous system boundary router to configure it itself. Either way, it is useful to be cognizant about the procedure.

There are too many variations to be considered in the configuration of an internal routing protocol to detail in this chapter. The main principles are dealt with in Chapter 10, "Controlling Routing Updates Across the Network."

The use of the default and static routes is worth pondering. This is a solution that has been used for years in connecting remote satellite networks, particularly those connected via a dialup link.

The solution is simple: The smaller network defines a default route, which is propagated throughout the domain. The default route points to the exit point of the network into the ISP. The larger network—in this case, the ISP—configures static routes to the organization. These routes are summarized in the master routing table to the Internet or NIC address of the IP address. The static routes must also be propagated throughout the ISP's network so that they can eventually be advertised into the Internet, connecting the smaller organization into the global internetwork. Figure 8-11 illustrates the use of default and static routes and shows how they would be propagated.

Figure 8-11 Default and Static Route Configuration into the Internet

Figure 8-11 Default and Static Route Configuration into the Internet

The command syntax to configure a static route is as follows: ip route prefix mask {interface | ip-address} [distance]

It is possible to specify either the outgoing interface or the IP address of the next logical hop. If the outgoing interface is specified, the link is treated as if it is directly connected. The default administrative distance of a directly connected link is 0. If the IP address is configured, it is given the administrative distance of 1.

In either case, the administrative distance will ensure that this path will be chosen above all others. If this static route is a redundant link into the ISP and will not be used unless there is no alternative, it may be necessary to change the administrative distance.

To propagate the static routes throughout the domain, it is necessary to redistribute them in the appropriate routing protocol.

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