Upon completing this lesson, you will be able to describe common routing scalability issues in service provider networks. This ability includes being able to meet these objectives:

■ Describe the basic structure of service provider networks

■ Describe the propagation of internal and customer routes in service provider networks

■ Describe proper scaling of IGPs and BGP in service provider networks

■ Describe scaling issues that are relevant to IP addressing in ISP networks

■ Describe the function of BGP policy accounting in relation to BGP scaling

Common Service Provider Network

This topic describes the basic structure of service provider networks.

This topic describes the basic structure of service provider networks.

The common service provider network runs External Border Gateway Protocol (EBGP) or static routing with customers. EBGP is always used as the routing protocol between different service providers.

Internal Border Gateway Protocol (IBGP) is required in the provider network because all EBGP-speaking routers in an autonomous system (AS) must exchange external routes via IBGP. Also, non-EBGP speakers are required to take part in the IBGP exchange if they are in a transit path and forward packets based on destination IP addresses.

The service provider network also runs an IGP. The protocols of choice are Open Shortest Path First (OSPF) and Intermediate System-to-Intermediate System (IS-IS). The IGP is used for two purposes:

■ Provides IP connectivity between all IBGP speakers so that TCP sessions for IBGP can be established between BGP-speaking routers

■ Provides optimal routing to the BGP next-hop address

A single IGP should be used within the entire AS. This setup facilitates effective packet forwarding from the ingress router to egress routers. The IGP is configured to carry internal routes only, including internal links and loopback addresses of the routers. For performance and scalability reasons, no customer routes or external routes should be injected into the IGP.

6-4 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

Peer Service Providers

Customer o Q-

Peer Service Providers

Customer o Q-

AS 400

AS 400

The typical service provider network consists of a network core that connects various edge devices. Some of the edge devices connect customers; others connect to other service providers.

The edge devices that connect to other service providers use EBGP to exchange routing information. The edge devices that connect customers use either static routing or EBGP.

Unless Multiprotocol Label Switching (MPLS) is configured on the service provider backbone, routers in a transit path are also required to have full routing information. Therefore, these routers take part in the IBGP routing exchange.

An IGP is also required within the service provider network. The IGP is used to carry internal routes, including the loopback interface addresses of IBGP-speaking routers. The IGP provides reachability information to establish IBGP sessions and to perform the recursive routing lookup for the BGP next hop.

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