© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-1 Course Introduction

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-2 Outline Overview Interdomain Routing BGP Characteristics Single-Homed Customers Multihomed Customers Transit Autonomous Systems BGP Limitations Summary

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-3 Interdomain Routing An AS is a collection of networks under a single technical administration. An IGP is run inside an AS, resulting in optimum intra-AS routing. An EGP is run between autonomous systems to enable routing policies and improve security.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-4 Design Goals for Interdomain Routing Scalability The Internet has more than 140,000 routes and is still growing. Secure routing information exchange Routers from another AS cannot be trusted. Tight filters are required; authentication is desirable. Support for routing policies Routing between autonomous systems might not always follow the optimum path.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-5 Why External Routing Protocols? Q: Assuming standard IGP route selection rules, how will the traffic between AS 1 and AS 20 flow? Q: Will AS 2 allow this traffic? Q: How would you solve this problem with OSPF or EIGRP?

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-6 BGP Characteristics BGP is a distance vector protocol with enhancements: Reliable updates Triggered updates only Rich metrics (called path attributes) Designed to scale to huge internetworks

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-7 BGP Characteristics (Cont.) Reliable updates TCP used as transport protocol No periodic updates Periodic keepalives to verify TCP connectivity Triggered updates batched and rate-limited –Every 5 seconds for internal peer –Every 30 seconds for external peer

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-8 BGP Characteristics (Cont.) Protocol development considerations BGP was designed to perform well in the following areas: –Interdomain routing applications –Huge internetworks with large routing tables –Environments that require complex routing policies Some design tradeoffs were made: –BGP uses TCP for reliable transport— CPU-intensive –Scalability is the top priority—slower convergence

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-9 BGP Characteristics (Cont.) Common BGP uses Customers connected to more than one service provider Service provider networks (transit autonomous systems) Service providers exchanging traffic at an exchange point (CIX, GIX, NAP, …) Network cores of large-enterprise customers

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-10 Single-Homed Customers Large customer or small ISP connecting to the Internet

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-11 Use Guidelines ― Single-Homed Customers Use BGP between the customer and the service provider in these situations: Customers multihomed to the same service provider Customers that need dynamic routing protocol with the service provider to detect failures –Hint: Use private AS number for these customers. Smaller ISPs that need to originate their routes in the Internet Use static routes in all other cases: Static routes always simpler than BGP

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-12 Multihomed Customers Customer connecting to more than one service provider

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-13 User Guidelines ― Multihomed Customers BGP is almost mandatory for multihomed customers. Multihomed customers have to use public AS numbers. Multihomed customers should use a provider-independent address space.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-14 Transit Autonomous Systems Using BGP to exchange routes is mandatory for transit autonomous systems (provider networks carrying customer traffic).

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-15 BGP Limitations BGP and associated tools cannot express all routing policies. You cannot influence the routing policies of downstream autonomous systems. “BGP does not enable one AS to send traffic to a neighbor AS intending that the traffic take a different route from that taken by traffic originating in the neighbor AS.” RFC 1771

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-16 Summary BGP has the right set of functions to support the various interdomain routing policies. Contrary to BGP, interior routing protocols focus only on finding the optimum (usually fastest) route between two points, without respect to routing policies. BGP is an enhanced distance vector protocol with reliable transport provided by TCP, a rich set of metrics called BGP path attributes, and scalability features such as batched updates that make it suitable for very large networks. Customers that plan to connect to more than one ISP, and small ISPs that plan to have multiple Internet connections in the future, usually use BGP with their service provider.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-17 Summary (Cont.) Although there are designs where BGP could be avoided, most multihomed customers use BGP with their service providers. A transit AS is an AS that exchanges BGP routing information with other autonomous systems and forwards information received from one AS to another AS. BGP is bound by IP hop-by-hop, destination-only routing. Routing policies that deviate from this model cannot be implemented with BGP.

© 2005 Cisco Systems, Inc. All rights reserved. BGP v3.2—1-18