Route Redistribution Direction

Redistribution is often applied between the Campus Core and Enterprise Edge protocols. As shown in Figure 7-16, redistribution is possible in two ways:

■ One-way route redistribution: Routing information is redistributed from one routing protocol or domain to another, but not vice versa. Static or default routes are required in the opposite direction to provide connectivity.

■ Two-way route redistribution: Routing information is redistributed from one routing protocol or domain to another, and vice versa. Static or default routes are not required because all routing information is passed between two entities.

Figure 7-16 Route Redistribution Can Be One-Way or Two-Way

Specific routes can be filtered, and the administrative distance of redistributed routes can be changed in either of these cases to reduce the possibility of routing loops and ensure that traffic is routed optimally.

Route Redistribution Planning

When deciding where and how to use route redistribution, determine the following:

■ The routing protocols and domains that will be used in the network

■ The routing protocol and domain boundaries (the boundary routers)

■ The direction of route redistribution (one-way or two-way)

If route redistribution is not carefully designed, suboptimal routing and routing loops can be introduced into the network when routes are redistributed in a network with redundant paths between dissimilar routing protocols or domains. Route filtering (as described in the "Route Filtering" section of this chapter) helps solve this problem.

Route Redistribution in the Enterprise Architecture

Redistribution is needed in the Building Distribution layer when different routing protocols or domains exist in the Building Access layer and Campus Core. Redistribution might also be needed between the Campus Core and the Enterprise Edge, including to and from WAN module routers, from static or BGP routes in the Internet Connectivity module, and from static routes in the

Remote Access and VPN module. Figure 7-17 shows a sample enterprise network with redistribution points throughout.

Figure 7-17 Route Redistribution in the Enterprise Architecture

Servers

Enterprise Edge

Figure 7-17 Route Redistribution in the Enterprise Architecture

Servers

Enterprise Edge

In this example, some remote sites require connectivity to the Server Farm; therefore, one-way redistribution is performed to inject routes from these remote sites into the Campus Core. Some remote sites require connectivity to the entire network; this is provided by two-way redistribution (otherwise, static routes would have to be configured in the Campus Core). The Building Distribution layer propagates only a default route down to the Building Access layer, whereas the Building Access layer advertises its own subnets to the Building Distribution layer.

Redistribution might also be necessary in the Remote Access and VPN and Internet Connectivity modules. For a Remote Access and VPN module with static routing, static routes are injected into the Campus Core routing protocol. In the opposite direction, default routes provide connectivity for remote users.

In an Internet Connectivity module with only one exit point, that exit point is the default route for traffic destined for the Internet and is propagated through the core routing protocol. If multiple exit points toward multiple ISPs exist, BGP provides Internet connectivity, and redistribution can be used.

KEY POINT

Redistribution with BGP requires careful planning. For more details, see Authorized Self-Study Guide: Building Scalable Cisco Internetworks (BSCI), Third Edition, by Diane Teare and Catherine Paquet, Cisco Press, 2006.

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