Multihoming to Multiple Autonomous Systems

Figure 2-14 shows a topology in which a subscriber has homed to more than one service provider. In addition to the advantages of multihoming already described, this subscriber is protected from losing Internet connectivity as the result of a single ISP failure.

For a small corporation or a small ISP, there are substantial obstacles to multihoming to multiple service providers. You already have seen the problems involved if the subscriber's address space is a part of one of the service providers' larger address space:

• The originating provider must be persuaded to "punch a hole" in his CIDR block.

• The second provider must be persuaded to advertise an address space that belongs to a different provider.

• Both providers must be willing to closely coordinate the advertisement of the subscriber's address space.

• If the subscriber's address space is smaller than a /19 (which a small subscriber's space is likely to be), some backbone providers might not accept the route.

Figure 2-14 Multihoming to Multiple Autonomous Systems

Figure 2-14 Multihoming to Multiple Autonomous Systems

The best candidates for multihoming to multiple providers are corporations and ISPs that are large enough to qualify for a provider-independent address space (or who already have one) and a public autonomous system number.

The subscriber in Figure 2-14 could still forego BGP. One option is to use one ISP as a primary Internet connection and the other as a backup only; another option is to default route to both providers and let the routing chips fall where they may. If a subscriber has gone to the expense of multihoming and contracting with multiple providers, however, neither of these solutions is likely to be acceptable. BGP is the preferred option in this scenario.

Again, incoming and outgoing traffic should be considered separately. For incoming traffic, the most reliability is realized if all internal routes are advertised to both providers. This setup ensures that all destinations within the subscriber's AS are completely reachable via either ISP. Even though both providers are advertising the same routes, there are cases in which incoming traffic should prefer one path over another. BGP provides the tools for communicating these preferences.

For outgoing traffic, the routes accepted from the providers should be carefully considered. If full routes are accepted from both providers, the best route for every Internet destination is chosen. In some cases, however, one provider might be a preferred for full Internet connectivity, whereas the other provider is preferred for only some destinations. In this case, full routes can be taken from the preferred provider and partial routes can be taken from the other provider. For example, you might want to use the secondary provider, only to reach its other subscribers and for backup to your primary Internet provider (see Figure 2-15). The secondary provider sends its customer routes, and the subscriber configures a default route to the secondary ISP to be used if the connection to the primary ISP fails.

Figure 2-15 ISP1 Is the Preferred Provider for Most Internet Connectivity; ISP2 Is Used Only to Reach Its Other Customers' Internetworks and for Backup Internet Connectivity

Figure 2-15 ISP1 Is the Preferred Provider for Most Internet Connectivity; ISP2 Is Used Only to Reach Its Other Customers' Internetworks and for Backup Internet Connectivity

Notice that the full routes sent by ISP1 probably include the customer routes of ISP2. Because the same routes are received from ISP2, however, the subscriber's routers normally prefer the shorter path through ISP2. If the link to ISP2 fails, the subscriber uses the longer paths through ISP1 and the rest of the Internet to reach ISP2's customers.

Similarly, the subscriber normally uses ISP1 to reach all destinations other than ISP2's customers. If some or all of those more-specific routes from ISP1 are lost, however, the subscriber uses the default route through ISP2.

If router CPU and memory limitations prohibit taking full routes, partial routes from both providers are an option. Each provider might send its own customer routes, and the subscriber points default routes to both providers. In this scenario, some routing accuracy is traded for a savings in router hardware.

In yet another partial-routes scenario, each ISP might send its customer routes and also the customer routes of its upstream provider. In Figure 2-16, for example, ISP1 is connected to Sprint, and ISP2 is connected to MCI. The partial routes sent to the subscriber by ISP1 consist of all of ISP1 's customer routes and all of Sprint's customer routes. The partial routes sent by ISP2 consist of all of ISP2's customer routes and all of MCI's customer routes. The subscriber points to default routes at both providers. Because of the size of the two backbone service providers, the subscriber has enough routes to make efficient routing decisions on a large number of destinations. At the same time, the partial routes are still significantly smaller than a full Internet routing table.

Figure 2-16 The Subscriber Is Taking Partial Routes from Both ISPs, Consisting of Each ISP's Customer Routes and the Customer Routes of Their Respective Upstream Providers

Figure 2-16 The Subscriber Is Taking Partial Routes from Both ISPs, Consisting of Each ISP's Customer Routes and the Customer Routes of Their Respective Upstream Providers

The remainder of this chapter (after two short cautionary sections) examines the operation of BGP and the tools it provides for setting preferences and policies for both incoming and outgoing traffic.

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