When a customer can connect to the Internet through either a single connection to a service provider or multiple connections to the same Internet service provider (ISP), static routing is the simplest routing approach to implement between customer and provider. When network administrators are implementing customer-to-provider connectivity with static routes, knowledge of static routing implementation guidelines will aid in successfully deploying static routing network configurations.
This lesson discusses static routing as a solution for connecting customer networks to service providers. Also included in this lesson is a discussion of when static routing should be used between a customer and a provider and information on how to configure static routing in nonredundant, backup, and load-sharing configurations.
Upon completing this lesson, you will be able to implement customer connectivity by using static routing in a service provider network. This ability includes being able to meet these objectives:
■ Identify when to use static routing between a customer and a service provider in a BGP environment
■ Describe the characteristics of static routing between a customer and a service provider in a BGP environment
■ Identify design considerations for propagating static routes in a service provider network
■ Configure static route propagation in a BGP environment with different service levels
■ Configure a typical backup setup that uses static routing between a customer and a service provider in a BGP environment
■ Describe the limitations of floating static routes when they are used in typical backup static routing scenarios and the corrective actions to overcome these limitations
■ Describe the characteristics of load sharing when you are configuring static routing between a customer and a service provider
5-22 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.
Why Use Static Routing?
This topic identifies when to use static routing between a customer and a service provider in a Border Gateway Protocol (BGP) environment.
Static routing is the best solution to implement when there is no redundancy in the network topology. A single connection between the customer network and the service provider network does not provide any redundancy. If the link goes down, the connection is lost regardless of which routing protocol is configured in the customer or provider network. When there are redundant connections between the customer network and the network of a single service provider, static routing can be used under specific circumstances.
A static default route must be conditionally announced by the customer edge routers that are using an Interior Gateway Protocol (IGP). If the link to one of the customer edge routers goes down, then the router must be able to detect the failure and invalidate the static default route. Announcement of this router as a default gateway that is using an IGP must now cease. Likewise, on the service provider edge routers, the static routes that are pointing to the customer networks must be invalidated if the link between them goes down, and redistribution to BGP is therefore stopped.
If link-level procedures cannot detect a link failure, the interface remains in the up state. The static routes are not invalidated, and packets are forwarded into a "black hole." In such cases, because the router cannot detect a failure at the link level, BGP must be used between the customer and the provider.
BGP must also be used between the customer and the service provider networks when the customer is multihomed. This is the case regardless of which link failure detection mechanisms are in use.
© 2005, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP 5-23
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