Runs one instance of Igp Ospf or Isis Igp used for internal routes only

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.

IBGP is required in the provider network because all EBGP-speaking routers in an 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.

Copyright © 2004, Cisco Systems, Inc. Scaling Service Provider Networks 6-5

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

© 2004 Cisco Systems,

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.

6-6 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

POP3

Networks are divided into POPs.

Different types of media are concentrated at the POP.

Optimal routing between POPs is desired.

POP3

Networks are divided into POPs.

Different types of media are concentrated at the POP.

Optimal routing between POPs is desired.

© 2004 Cisco Systems, Inc. All rights re

Service provider devices that connect access links to customers are physically located in groups that are called "points of presence" (POPs). In general, the POP is a group of routers where access links are terminated. The edge routers that peer with other service providers can in this sense be considered a POP.

Service providers use different types of access links with different types of customers and usually mix access links in the same POP. Some customers use leased lines, others use xDSL, and still others use dial-in access or any other access that the provider can support.

POP routers connect to the network core using a layer of concentration routers at the POP. The network core forwards packets between POPs, different customer access points, or peering points with other service providers. Optimal routing between POPs is a desirable feature.

Copyright © 2004, Cisco Systems, Inc. Scaling Service Provider Networks

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

POP routers use BGP or static routing with customer routers.

Provider core IGP is a single instance of IS-IS or OSPF.

The core IGP is used only within the service provider backbone.

© 2004 Cisco Systems, Inc. All rights re

Customer access lines terminate in the POP edge routers. In many cases, the POP edge routers use static routing to customer networks. The POP edge routers advertise static routes to the rest of the service provider network and to other autonomous systems using BGP.

Service providers use BGP routing with the customer when redundancy requires the use of a routing protocol.

The service provider backbone typically uses a single instance of either IS-IS or OSPF as its IGP. The IGP is used within the provider backbone only. The provider backbone exchanges no IGP routing information with customer routers or with routers in other autonomous systems.

6-8 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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