© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Implementing BGP Explaining BGP Concepts and Terminology

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Using BGP to Connect to the Internet

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v What Is Multihoming? Connecting to two or more ISPs to increase the following: Reliability: If one ISP or connection fails, there is still Internet access. Performance: Path selection to common Internet destinations is better.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Example: Default Routes from All Providers

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Default Routes from All Providers and Partial Table

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Example: Full Routes from All Providers

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Autonomous Systems An AS is a collection of networks under a single technical administration. IGPs operate within an AS. BGP is used between autonomous systems. Exchange of loop-free routing information is guaranteed.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Path-Vector Routing IGPs announce networks and describe the metric to reach those networks. BGP announces paths and the networks that are reachable at the end of the path. BGP describes the path by using attributes, which are similar to metrics. BGP allows administrators to define policies or rules for how data will flow through the autonomous systems.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Routing Policies BGP can support any policy conforming to the hop- by-hop (AS-by-AS) routing paradigm.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Characteristics BGP is most appropriate when at least one of the following conditions exists: –An AS allows packets to transit through it to reach other autonomous systems (for example, it is a service provider). –An AS has multiple connections to other autonomous systems. –Routing policy and route selection for traffic entering and leaving the AS must be manipulated. BGP is not always appropriate. You do not have to use BGP if you have one of the following conditions: –Limited understanding of route filtering and BGP path- selection process –A single connection to the Internet or another AS –Lack of memory or processor power to handle constant updates on BGP routers

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Characteristics (Cont.) BGP is a path-vector protocol with the following enhancements over distance vector protocols: Reliable updates: runs on top of TCP (port 179) Incremental, triggered updates only Periodic keepalive messages to verify TCP connectivity Rich metrics (called path vectors or attributes) Designed to scale to huge internetworks (for example, the Internet)

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Databases Neighbor table –List of BGP neighbors BGP table (forwarding database) –List of all networks learned from each neighbor –Can contain multiple paths to destination networks –Contains BGP attributes for each path IP routing table –List of best paths to destination networks

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v BGP Message Types BGP defines the following message types: Open –Includes hold time and BGP router ID Keepalive Update –Information for one path only (could be to multiple networks) –Includes path attributes and networks Notification –When error is detected –BGP connection closed after message is sent

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary If your network is multihomedhas more than one connection to the Internetthen using BGP to connect to your ISPs may be appropriate. Multihoming options include having each ISP pass these: –Only a default route –A default route and provider-owned specific routes –All routes BGP is the external routing protocol used between autonomous systems. Forwarding is based on policy and not on best path.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v Summary (Cont.) BGP routers exchange network reachability information called path vectors, made up of path attributes. The path- vector information includes a list of the full path of BGP AS numbers necessary to reach a destination network. A router running BGP keeps its own tables to store BGP information that it receives from and sends to other routers, including a neighbor table, a BGP table (also called a forwarding database or topology database), and an IP routing table. There are four BGP message types: open, keepalive, update, and notification.

© 2006 Cisco Systems, Inc. All rights reserved. BSCI v